國立中山大學材料科學研究所 博士論文
以摩擦旋轉攪拌製程製作奈米細晶鎂基合金與複材之研發
Achieving Ultrafine Nano Grains in AZ31 Mg Based Alloys and Composites by Friction Stir Processing
研究生：張志溢 撰 指導教授：黃志青 博士
中華民國 九十六 年 十 月
Table of Content
Table of Content... i
Lists of Tables ... vi
Lists of Figures ... viii
Abstract ... xvi
中文摘要 ... xviii
謝誌 ...xx
Chapter 1 Introduction...1
1.1 The developments and applications of magnesium alloys ...3
1.2 Properties of magnesium alloys...4
1.2.1 The classification of magnesium alloys ...4
1.2.2 The characteristics of magnesium alloys ...5
1.3 Grain refinements ...8
1.3.1 Grain size refinement techniques ...10
1.4 Metal matrix composites ...14
1.4.1 Processing of metal matrix composites and magnesium matrix composites...17
1.4.1.1 Liquid-state methods...17
1.4.1.2 Solid-state methods...18
1.5 Friction stir welding and friction stir processing...19
1.5.1 Introduction of friction stir welding (FSW) ...19
1.5.2 Characteristics of friction stir welding ...20
1.5.2.1 Microstructure of welding zone in friction stir welding ...20
1.5.2.2 Recrystallization mechanisms...22
1.5.2.3 Onion rings in nugget zone ...24
1.5.2.4 Materials flow behavior in nugget zone...25
1.5.2.5 Hardness variation in the weld zone ...26
1.5.3 Influence of welding parameters ...28
1.5.4 Advantages and disadvantages of friction stir welding...29
1.5.5 Friction stir processing (FSP)...30
1.5.6 Application of friction stir processing...31
1.5.6.1 Friction stir processing for grain refinement ...32
1.5.6.2 Friction stir processing for superplasticity...33
1.5.6.3 Friction stir processing for fabrication of metal matrix composites...34
1.6 Motives of the research...35
Chapter 2 Experimental methods...38
2.1 Materials ...38
2.1.1 The materials for intrinsic reinforced Mg-Al-Zn alloys...38
2.1.2 The extrinsic reinforcements for the Mg-AZ31 based composites ...39
2.2 The set-up of friction stir processing...39
2.2.1 The design of tool and fixture ...39
2.2.2 The special cooling condition during friction stir processing ...40
2.2.2.1 Newly designed effective cooling system...40
2.2.3 The methods of adding nano-sized powders into AZ31 alloys ...41
2.2.4 The parameters of friction stir processing...41
2.2.4.1 FSP parameters for modified AZ31 alloys...41
2.2.4.2 FSP parameters for fabricating intrinsic reinforced Mg-Al-Zn alloys ...42 2.2.4.3 FSP parameters for fabricating extrinsic reinforced Mg-AZ31
based composites ...42
2.2.4.4 FSP parameters for producing the ultrafine grained AZ31 alloys 43 2.3 Microhardness measurements...43
2.4 Mechanical tests ...43
2.5 The analysis of X-ray diffration ...44
2.6 Microstructure observations ...44
2.6.1 Optical microscopy (OM) ...44
2.6.2 Scanning electron microscopy (SEM)...45
2.6.3 Transmission electron microscopy (TEM)...45
Chapter 3 Experimental results...48
3.1 Basic AZ31 alloy FSP trials...48
3.1.1 The appearance of the FSPed pure AZ31 alloy specimens ...48
3.1.2 The microstructure of the modified AZ31 alloy made by FSP ...49
3.1.3 The temperature of the stirred zone of modified alloys ...50
3.1.4 Hardness measurements ...51
3.1.5 Grain orientations ...52
3.1.6 Brief conclusion of basic AZ31 alloy FSP trials ...53
3.2 With reinforcements to enhance higher hardness values and finer grains...54
3.2.1 Intrinsic reinforcements for obtaining finer grains or higher hardness ...54
3.2.1.1 The appearance of the FSPed specimens ...54
3.2.1.2 Microstructure...55
3.2.1.3 X-ray diffraction ...56
3.2.1.4 Hardness measurement ...56
3.2.1.5 TEM examination ...57
3.2.1.6 Brief conclusions of in-situ formed intermetallic compounds reinforced Mg-Al-Zn alloys made by FSP...58
3.2.2 Extrinsic reinforcements for obtaining finer grains or higher hardness....59
3.2.2.1 The appearance of the FSPed composite specimens...59
3.2.1.2 Microstructure of Mg matric composites made by FSP ...59
3.2.2.3 XRD results...62
3.2.2.4 Hardness measurements...63
3.2.2.5 Mechanical properties...63
3.2.2.6 Mg based composites with tetragonal phase nano-ZrO2 particles fabricated by FSP...64
3.2.2.7 The XRD and hardness analysis for the Mg/tetragonal phase ZrO2 composites after subsequent compression ...65
3.2.2.8 Brief conclusion for Mg-AZ31 based composites with nano-ZrO2 and nano-SiO2 particles ...66
3.3 Using lower heat generation for obtaining finer grain size and higher hardness ..67
3.3.1 The effects of tool size, plate thickness, and cooling method...67
3.3.2 The combination of composite and liquid nitrogen cooling methods ...69
3.3.3 Brief conclusions...70
3.4 Ultrafine grained AZ31 Mg alloy made by FSP with new designed cooling system ...71
3.4.1 Microstructure of ultrafine grained AZ31 alloy made by FSP...71
3.4.2 Hardness measurement...72
3.4.3 Brief conclusions...72
3.5 Nanocrystalline AZ31 Mg alloy made by new designed cooling system and subsequent second pass with lower heat input ...73
Chapter 4 Discussion ...76
4.1 Strain rates and temperatures during FSP...76 4.2 Relationship between grain size and Zener-Holloman parameter of FSP Mg alloy
...77
4.3 The hardening mechanism of Mg based nano-ZrO2 and nano-SiO2 particles composites fabricated by friction stir processing ...78
4.4 Mechanisms for forming ultrafine grain in AZ31 Mg alloy made by FSP...82
4.5 Capability for further grain refining with subsequent second pass with lower heatinputs...84
4.6 The mechanism of the nanocrystalline structure evolution for AZ31 Mg base alloyduring two-pass FSP...86
Chapter 5 Conclusions...92
References ...95
Tables...106
Figures ...126
Lists of Tables
Table 1-1 Comparison among Mg alloy, Al alloy, Ti alloy, steel and plastics ...106
Table 1-2 The standard four-part ASTM designation system of alloy and temper for the magnesium alloy. ...107
Table 1-3 The effect of separate solute addition on the mechanical properties ...108
Table 1-4 Mechanical properties of magnesium matrix composites by various processing means. ...109
Table 1-5 Microstructure-mechanical property and fracture correlations for metal matrix composites...111
Table 1-6 The key benefits of friction stir welding...112
Table 1-7 A summary of grain size in nugget zone of FSP aluminum alloys...113
Table 1-8 A summary of ultrafine grained microstructures produced via FSP m aluminum alloys. ...115
Table 1-9 A summary of grain size in nugget zone of FSP magnesium alloys...116
Table 2-1 Chemical composition of the AZ31 (in wt%)...117
Table 2-2 The dimensions of the tools. ...117
Table 3-1 The recrystallized grain size of the modified AZ31 Mg alloy made by FSP...118
Table 3-2 Summary of the measured temperature during FSP ...119
Table 3-3 Summary of the Hv hardness in the 1P FSP processed AZ31 alloys ...119
Table 3-4 Summary of the Hv hardness in the FSP processed intermetallic alloys after multi-pass under different cooling methods. The melt spun alloy is also included for comparison ...120
Table 3-5 Summary of the average cluster size of nano-particles and the average grain size of AZ31 matrix in the 4 passes FSP composites...121
Table 3-6 Comparison of the mechanical properties of AZ31 alloy and AZ31-based composites...121 Table 3-7 A summary of the recrystallized pure AZ31 grain size for the different FSP
parameters ...122 Table 3-8 Grain size and hardness at the FSP nugget bottom...123 Table 4-1 The experimental hardness and predicted hardness used by the iso-stress model
in the present composites. The initial hardness for the AZ31 billet is~50...125
Lists of Figures
Figure 1-1 Schematic illustration of the ARB facility. ...126
Figure 1-2 Schematic illustration of the ECA pressing facility. ...127
Figure 1-3 Schematic illustration of the spray forming facility. ...128
Figure 1-4 Schematic diagram of friction stir welding...129
Figure 1-5 Schematic diagram of entire friction stir welding...130
Figure 1-6 Schematic illustration of the welding zone in friction stir welding. ...131
Figure 1-7 Microstructure of thermo-mechanically affected zone in FSP 7075Al...132
Figure 1-8 Illustration of advancing side and retreating side. ...133
Figure 1-9 Typical onion ring in the nugget zone...134
Figure 1-10 Three-dimensional drawing of the onion rings in the nugget zone...134
Figure 1-11 Schematic illustration of (a) trace surface of basal plane produced below the borderline with an onion shape and (b) its transverse cross section. ...135
Figure 1-12 Micro hardness and strain field map in the banded microstructure. ...136
Figure 1-13 (a) Metal flow patterns and (b) metallurgical processing zones developed during friction stir welding. ...137
Figure 1-14 Schematic drawing of the FSW tool. ...138
Figure 1-15 WorlTM and MX TrifluteTM tools developed by The Welding Institute (TWI), UK...138
Figure 1-16 Illustration of the different probes, (a) smaller contact area of headpin and (b) flared-triflute type probes. ...139
Figure 2-1 Theflow chart for progressive improvement of grain size and mechanical properties...140
Figure 2-2 The microstructure of the as-received AZ31 billet. ...142
Figure 2-3 Schematic illustration of (a)(b) the stacked AZ31 Mg alloy sheets with pure Al and pure Zn foils, and (c) the fixture used for liquid N2 cooling...143 Figure 2-4 The XRD patterns and TEM micrographs of (a) the monoclinic ZrO2 particles
and (b) the amorphous SiO2 particles, both with an average diameter ~20 nm.144 Figure 2-5 The appearance of the horizontal-type miller...145 Figure 2-6 Schematic illustration of the entire fixture design. ...146 Figure 2-7 Schematic drawing of the newly designed cooling system...146 Figure 2-8 Schematic drawings of the friction stir processing in fabricating the
Mg-AZ31/nano-particles composites: (a) appearance of cut deep grooves and (b) cutting groove(s) and inserting nano particles and (c) conducting multiple FSP to fabricate composites...147 Figure 2-9 Schematic illustration for the surface repair method for fabricating Mg base
composites by FSP...148 Figure 2-10 Schematic illustration of the position for the K-type thermocouple inserted in
the sample. ...149 Figure 2-11 (a) Schematic diagram of the chill block melt spinning. (b) Photography of the
melt spinning device. ...150 Figure 2-12 The experiment flowchart for this research ...151 Figure 2-13 Schematic illustration for the sampling and dimension of the tensile sample
perpendicular to the pin advancing direction...152 Figure 2-14 Schematic illustrations of (a) Top-view and (b) Side-view of preparation of
TEM-specimen by focus ion beam (FIB) ...153 Figure 3-1 The appearances of the FSPed pure AZ31 alloys...154 Figure 3-2 Schematic illustration for sampling positions. ...155 Figure 3-3 The semicircular appearances of various FSPed specimens with different
processing parameters...156
Figure 3-4 The optical microscopy of the AZ31 alloy made by FSP: (a)-(b)cross-sectional view, (c)-(d) cross-sectional view at a higher magnification. ...157 Figure 3-5 OM micrographs showing the variation of the recrystallized grain size in the
nugget zone for the different rotational speeds under the same 90 mm/min advancing speed. ...159 Figure 3-6 OM micrographs showing the variation of the recrystallized grain size in the
nugget zone for the different advancing speeds under the same 800 rpm rotational speed. ...160 Figure 3-7 Variation of the average grain size as a function of pin rotation rate for the FSP
AZ31 alloys under the same 90 mm/min advancing speed. ...161 Figure 3-8 Variation of the average grain size as a function of advancing speed for the
FSP AZ31 alloys under the same 800 rpm rotation rate. ...161 Figure 3-9 Typical temperature profiles measured by the inserted thermocouple into the
pure AZ31 alloys under the same 90 mm/min advancing speed. ...162 Figure 3-10 Typical temperature profiles measured by the inserted thermocouple into the
pure AZ31 alloys under the same 800 rpm rotation speed...162 Figure 3-11 Typical microhardness variations in the central cross-sectional zones of FSP
AZ31 alloys at 90 mm/min advancing speed...163 Figure 3-12 Plot for the Hall–Petch relationship for the grain size induced by FSP...163 Figure 3-13 X-ray diffraction for (a) random Mg, (b) as-received AZ31 billet...164 Figure 3-14 XRD patterns for modified AZ31 Mg alloys by FSP at 90 mm/min advancing
speed. ...165 Figure 3-15 The appearance of the FSPed specimens and nugget zone of intermetallic
alloys. ...166 Figure 3-16 SEM/BEI micrograph showing the phase dispersion in Mg70Al5Zn25 after three
passes with air cooling. ...167
Figure 3-17 SEM/BEI micrograph showing the phase dispersion in Mg50Al5Zn45 after three passes with air cooling. ...167 Figure 3-18 SEM/BEI micrograph showing the phase dispersion in Mg37.5Al25Zn37.5 after
three passes with air cooling. ...167 Figure 3-19 SEM/BEI micrograph showing the phase dispersion in Mg37.5Al25Zn37.5 after
three passes with water cooling. ...167 Figure 3-20 SEM/BEI micrograph showing the phase dispersion in Mg37.5Al25Zn37.5 after
ten passes with water cooling. ...167 Figure 3-21 SEM/BEI micrograph showing complete amorphous phase in Mg70Al5Zn25
fabricated by melt spinning...167 Figure 3-22 The XRD patterns for the Mg70Al5Zn25 system fabricated by FSP and melt
spinning...168 Figure 3-23 XRD patterns for Mg70Al5Zn25, Mg50Al5Zn45 and Mg37.5Al25Zn37.5 after 3 or 10
passes ...168 Figure 3-24 The variation of Hv along the transverse cross-sectional plane of the
Mg70Al5Zn25, Mg50Al5Zn45 and Mg37.5Al25Zn37.5 alloys after 3 or FSP 10 passes. ...169 Figure 3-25 TEM micrographs showing (a) the nano-sized Mg3Al2Zn3 τ phase in FSP
Mg50Al20Zn30 and (b) the icosahedral τ phase and (c) its diffraction pattern observed in FSP Mg37.5Al25Zn37.5. ...170 Figure 3-26 The appearances of the FSPed specimens under the 800 rpm rotational speed
and 45 mm/min advancing speed...171 Figure 3-27 The OM micrographs of the 1D4P ZrO2 composite sample made by FSP: (a)
cross-sectional view, (b) cross-sectional view at a higher magnification. ...172 Figure 3-28 SEM/SEI images of the AZ31/10vol%ZrO2 FSP composite showing (a)
relatively homogeneous dispersion, (b) local inhomogenization of the
nano-particle clusters within the stirred zone after one-pass FSP, and (c) the improvement of clustered ZrO2 particles after four passes FSP. ...173 Figure 3-29 SEM/SEI images at different magnifications of the SiO2 composite specimens
with different volume fractions. (a), (c) and (e) 1G4P (~5 vol%), (b), (d) and (f) 2G4P (~10 vol%). ...174 Figure 3-30 SEM/SEI images at different magnifications of the ZrO2 composite specimens
with different volume fractions. (a), (c) and (e) 1G4P (~10 vol%), (b), (d) and (f) 2G4P (~20 vol%). ...175 Figure 3-31 SEM/BEI images of ZrO2 composite specimens with different volume
fractions. (a) 1G4P (~10 vol%), (b) 2G4P (~20 vol%). ...176 Figure 3-32 The SEM/EDS analysis of ZrO2 2G4P (~20 vol%) composite specimens
showing the clustered ZrO2 particles. ...177 Figure 3-33 SEM photograph showing the clustered ZrO2 located on grain boundaries or
trip junctions and some ZrO2 embedded into grains of the AZ31 matrix (1G4P)...178 Figure 3-34 X-ray diffraction for modified AZ31 Mg alloy without any particle by FSP. ...179 Figure 3-35 XRD patterns for (a) the Mg-AZ31/ZrO2 and (b) the AZ31/SiO2 composites. .180 Figure 3-36 Typical variations of the microhardness (Hv) distribution in various FSP AZ31
composites and the FSP AZ31 alloy (no particles). ...181 Figure 3-37 SEM/SEI fractographs of tensile samples: (a) the FSP AZ31 alloy, and (b) the
FSP AZ31/10%ZrO2 composite. ...182 Figure 3-38 The XRD patterns for the transverse cross-sectional plane of the tetragonal
phase ZrO2 FSP composites. ...183 Figure 3-39 Vickers hardness variations measured along the central cross-sectional zones of
the FSP samples. ...184 Figure 3-40 Stress-induced transformation of metastable ZrO2 particles in the elastic stress
field of a crack. ...185 Figure 3-41 The XRD diffraction patterns of the 1G4P and 2G4P Mg/tetragonal phase ZrO2
composites after 6% strain of subsequent compression along the normal direction. ...186 Figure 3-42 The hardness profiles for both 1G4P and 2G4P composites after 6%
compression. ...187 Figure 3-43 The stir zone microstructure of AZ31 billet FSPed at 800 rpm 90 mm/min
(a),(c),(e) plate is 10 mm in thickness (ave. 3.1 μm); (b),(d),(f) plate is 7 mm in thickness (ave. 2.7 μm)...188 Figure 3-44 The stir zone microstructure of AZ31 billet FSPed at 800 rpm 400 mm/min
(a),(c),(e) with steel back plate (ave. 2.8 μm) ; (b),(d),(f) with usual cooling facility (ave. 2.3 μm)...189 Figure 3-45 OM micrographs showing the variation of the recrystallized grain size in the
nugget zone under the different cooling conditions as the same 800 rpm rotational speed. (a) and (c) without liquid nitrogen cooling, (b) and (d) with liquid nitrogen cooling...190 Figure 3-46 Hardness profile of AZ31 7 mm plate FSPed at 800 rpm 90 mm/min and
cooled by liquid N2. ...191 Figure 3-47 The SEM/SEI images for the pure AZ31 4 mm plate FSPed one pass with
liquid N2 cooling (~0.45 μm)...192 Figure 3-48 The SEM/SEI image for the AZ31/ZrO2 2G4P FSPed with subsequent cooling
process by the tool which has 3 mm pin in diameter and length and 10 mm shoulder in diameter...193 Figure 3-49 Hardness profiles of Mg-based composites made by FSP. ...194 Figure 3-50 TEM observations for the Mg/ZrO2 composite with subsequent cooling pass,
together with a select area diffraction (SAD) pattern. Arrows indicate the ZrO2
particles as the obstacles for the grain boundary migration...195 Figure 3-51 Macrograph of cross section of the single-pass FS Processed AZ31 alloy at an
advancing speed is 28 mm/min...196 Figure 3-52 SEM micrograph at low magnification showing the uniform ultrafine grained
structure in the AZ31 alloy after one-pass FSP at 28 mm/min with liquid N2 cooling...196 Figure 3-53 SEM micrographs for the FSP AZ31alloy at an advancing speed of 28 mm/min.
...197 Figure 3-54 SEM micrographs for the FSP AZ31alloy at an advancing speed of 33 mm/min.
...197 Figure 3-55 Grain size distribution chart of the ultrafine grained microstructure in FSP
AZ31 alloys...198 Figure 3-56 Microhardness (Hv) profile measured on cross-sectional planes for the FSP
ultrafine grained AZ31 alloy...198 Figure 3-57 The SEM/SEI images of the grain structures in the AZ31 alloy after two passes
FSP with the same advancing speed of 37 mm/min and the rotation rate of 1200 rpm at the first pass and 800 rpm at the second pass. The same pin tool size is used in both FSP passes ...199 Figure 3-58 The grain size distribution of the ultrafine grained microstructure in FSP AZ31
alloys ...200 Figure 3-59 TEM observations for the two-pass FSPed AZ31 alloy in the as processed
condition ...201 Figure 3-60 SEM/SEI images of the grain structures in the AZ31 alloys after two passes
FSP with the same rotation rate and advancing speed of 1000 rpm and 37 mm/min, respectively. Smaller pin tool is used in the second pass ...202 Figure 3-61 The grain size distribution of the nanometer grained microstructure in FSP
AZ31 alloys...203 Figure 3-62 TEM observations for the two-pass FSPed AZ31 alloy in the as processed
condition, together with a select area diffraction (SAD) pattern ...204 Figure 3-63 TEM observations for the two-pass FSPed AZ31 alloy in the as processed
condition, showing the extremely fine nano-grains ...204 Figure 4-1 Variation of the (a) strain rate and (b) temperature as a function of pin rotation
speed ...205 Figure 4-2 Plots for the relationship between the resulting grain size and Z-parameter in
specimens processed by (a) FSP and (b) extrusion or tension. The data on FSP (solid square) are also included in (b) for comparison. ...206 Figure 4-3 Schematic diagrams showing (a) the iso-stress and (b) the iso-strain models..207 Figure 4-4 Schematic drawing of the load transfer direction under the indentation test ....208 Figure 4-5 The TEM observations for the one-pass FSPed AZ31 alloy samples ...209 Figure 4-6 The typical microstructure of the two-pass FSPed AZ31 alloy in the as
processed condition, together with a select area diffraction (SAD) pattern ...209 Figure 4-7 (a), (b), and (c) Strain-free nanometer grains formed along the subboundary
and grain boundary through DRX; (d) equiaxed nano-sized grains formed by DRX within the nugget region...210 Figure 4-8 Schematic illustration of the grain refinement process of the two-pass FSPed
AZ31 alloy. ...211
Abstract
In this study, firstly, in order to achieve fine grains in solid solution strengthened AZ31 magnesium alloy by friction stir processing (FSP), various efforts have been made. It has found that with a newly designed cooling system, the microstructure of commercial AZ31 alloy can be refined dramatically by carefully controlling the FSP parameters. It is of scientific interest that nanometer grains have been observed in the resultant microstructure for the AZ alloy experienced by two-pass FSP. Besides, in order to modify the microstructure and mechanical properties, FSP is also applied to incorporate AZ31 Mg alloy with nano-ZrO2
particles, nano-SiO2 particles and different fractions of Al and Zn elements. The microstructure and mechanical properties of the modified alloy and composite samples are investigated and compared.
By one-pass FSP coupled with rapid heat sink from liquid nitrogen cooling approach, the ultrafine grain size in AZ31 Mg alloy is successfully achieved. The grain boundaries are well defined and the mean grain size can be refined to 100~300 nm from the initial 75 μm of commercial AZ31 Mg alloys sheets. The ultrafine grained structure can drastically increases the microhardness from the initial 50 up to 120 Hv, or an increment factor of 2.4 times.
Furthermore, the nanometer grains can be even achieved by two passes FSP coupled with rapid heat sink. The resulting microstructure exhibits equiaxed grains ranging from 40 nm to 200 nm with an average grain size of less than 100 nm. The nanocrystalline grains can be characterized by the TEM observations and the diffraction rings in SAD patterns. The highest hardness point can reach ~150 Hv which is equal to triple of the AZ31 matrix, and the mean hardness also increases up to around 134 Hv.
Bulk Mg-AZ31 based composites with 10~20 vol% of nano-ZrO2 particles and 5~10 vol% of nano-SiO2 particles are also successfully fabricated by FSP. The average grain size of the resultant composites could be effectively refined to 2~4 μm, and it demonstrates much higher hardness values compared to commercial AZ31 billet. Moreover, for the Mg/ZrO2
composite fabricated by one pass and subsequent cooling pass FSP, the recrystallized grain size could be further refined to 0.4 μm with the hardness value of 135 Hv. As for multi-element Mg base alloys fabricated by FSP, high fractions of Al and Zn elements can result in apparent grain refinement, this can be proved by the broadening of diffraction peaks.
Multi-passes FSP can induce the appearance of intermetallic compounds, however, some of them are quasi-crystals with icosahedral point group symmetry. The average hardness of the resultant alloys reachs nearly 350 in Hv scale due to the generation of intermetallic compounds and grain refinement.
中文摘要
本研究之目標為致力於利用摩擦旋轉攪拌製程(FSP)對於固溶強化AZ31鎂合金進 行改質以製備出奈米級細晶粒組織。在適當的製程參數下，利用本研究所設計出之新冷 卻系統，可有效大幅度的將晶粒細化。而奈米細晶組織可在適當參數及有效新設計冷卻 系統下，經二次道數後得到。此外，為了對鎂合金之微觀組織與機械性質進行改質，亦
利用摩擦旋轉攪拌製程製備出添加奈米級ZrO2與SiO2顆粒之鎂基複合材料及鎂鋁鋅介
金屬化合物合金以進行分析比較。
經由適當之製程參數與有效之液態氮冷卻系統，次微米級之極細晶粒可經一道摩擦
旋轉攪拌製程得到。晶粒大小可從原始母材之 75 μm 大小細化至 100~300 nm，所得之
再結晶粒具有明顯與清楚之晶界。其硬度值亦可提升至 120 Hv，達到原本母材(~50 Hv)
的 2.4 倍。而本研究目標之奈米微細晶粒可在適當參數及有效冷卻系統下經二道次摩擦 旋轉攪拌製程後得到。所得到之微細等軸再結晶粒大小分佈在 20 nm 至 200 nm 的範圍 之間，其平均晶粒小於 100 nm，約為 80 nm。奈米晶粒可經由掃瞄式電子顯微鏡(SEM) 與穿透式電子顯微鏡(TEM)的觀察下得到證明。於本實驗鎂基合金所得之極細晶粒組織
最高硬度值硬度可達約 150 Hv為原本母材之三倍值，而其平均硬度值為可提升至 134
Hv。
藉由摩擦旋轉攪拌製程可以成功製作出塊狀鎂基複合材料，經四道次的製程後，奈
米級 ZrO2與 SiO2顆粒可有效的分散入鎂基材中。所製備之鎂基複合材料晶粒可細化至
2-4 μm，並可提升硬度值至兩倍以上。在更進一步利用已製備之 Mg/ZrO2複合材料，進
行第二道次之 FSP 冷卻製程後，其晶粒大小可進一步細化至 0.4 μm 而其硬度值亦提升
至 135 Hv接近原始母材之三倍值。經摩擦旋轉攪拌製程所製備之鎂鋁鋅介金屬化合物，
具有明顯的晶粒細化效果，並有許多之介金屬化合物產生，可從其 XRD 繞射圖中有許
多的結晶峰產生與繞射峰的寬化得到映證，而其其硬度值更可大幅提升至 350 Hv。
謝誌
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也感謝所上各實驗室的技術員，王良珠學姐、陳貴香女士、王國強先生、江宏達先 生、古錦松學長、林明政先生、李秀月女士、施淑瑛小姐、行政上所辦的朱惠敏女士、
顏秀芳女士、陳秀月小姐及熱心的華大哥。感謝您們在各項實驗及行政事務上的協助與 幫忙。
在黃幫的大家庭裡，有太多人要感謝了，包括已畢業的木城學長、建超學長、凱琳 學姐、佩汝學姐、鉉凱學長、英博學長、敬仁學長，感謝你們在學識與生活上的開導，
博班的育誠學長、子翔同學、友杰、海明、炎暉、鴻昇、浩然、名哲學弟的協助，再次 相見的宇庭、已畢業的致榮，及碩班的振偉、哲男、大豪，大學部的碩陽、逸志、柏佑 學弟們帶給我美好的回憶與實驗室的新氣象。另外，也要感謝高、張幫的金福、文讀、
小珠、小丁、惠君學長姐、宜珊、憲宗學弟，以及無法詳加記錄的諸位學長姐與學弟妹 們，還有何幫的諸位，感謝你們的協助。
另外，也要感謝已經回到大陸的王軼農博士，在實驗上的指導。以及杜興蒿博士在 實驗上的指導與討論，使我獲益良多，謝謝您。
最感謝的，莫過為我親愛的父母與家人，給予我關懷與支持，沒有你們不會有今天 的我，給我全力的支持，使我無後顧之憂，得以完成學業，在此，把本論文獻給我的家 人，感謝你們的支持與鼓勵。
要感謝的人事物太多了，謹以懷抱感恩的心，感謝上天，謝謝諸天地。
張志溢 謹誌 於 中山大學材料科學所 2007.10
Chapter 1
Introduction
Magnesium alloys have gained increasing interest in transportation industries, because they can achieve considerable weight reduction of structures. Magnesium has the lowest density, 1.74 g/cm3, among all light structural metals, such as Al, Mg and Ti. In addition, magnesium also has the advantage of recyclability, further decreasing the squander of natural resources. Based on the lower density and recyclability, magnesium alloys have gradually become the highly potential metallic materials.
Recently, many efforts have been made to produce ultrafine grained structural metallic materials. Fine grained materials are harder and stronger than those are coarsely grained because of having greater total grain boundary areas to impede dislocation motion and also being expected to exhibit superior mechanical properties, including high strength, high toughness and excellent superplasticity at high strain rates and low temperature [1,2]. Besides, fine grained Mg alloy possesses better ductility and formability near room temperature [3].
Metal matrix composites, MMCs, have arisen extensive researches, and numerous applications have been applied in automobiles, aircrafts, aerospace vehicles, and electronic components. Usually, metals or alloys of light weight and good ductility are adopted to serve as matrix, such as aluminum, magnesium, titanium or copper alloys. As for the reinforcement, ceramic materials with high temperature stability, high modulus, and high hardness, such as carbon fiber, glass fiber, the particulate of SiC, SiO2, Al2O3 or TiB2, are commonly introduced into the matrix. These ceramic materials should not dissolve or react with the metallic matrix.
Due to the particular mechanical and/or physical characteristics of the reinforcement, the composite properties may be significantly toughened, strengthened, modified or tailored.
Better strength, stiffness, hardness, corrosion and wear resistance are commonly seen in the reinforced composite. On the other hand, MMCs can maintain the original characteristics of the matrix such as electric and thermal conductivity. Therefore, the composites with improved physical and mechanical properties will be used in higher temperature and stricter environment than that for pure metals or alloys without any reinforcement.
Friction stir welding (FSW) was invented at The Welding Institute (TWI) of UK in 1991 as a solid-state joining technique, and it was initially applied to aluminum alloys [4]. Now, the progress has been extended into Ti, Mg, Cu alloys and steel. Recently, friction stir processing (FSP) was developed by Mishra et al. [5,6] as a generic tool for microstructural modification based on the basic principles of FSW. In this case, a rotating tool is inserted in a monolithic workpiece for localized microstructural modification for specific property enhancement. FSP technique has been used to produce surface composite on aluminum substrate [6], homogenization of powder metallurgy aluminum alloy [7], microstructural modification of metal matrix composites [8] and property enhancement in cast aluminum alloys [9].
The success in fabrication of various nano-sized powders, wires or tubes has provided the new possibility in modifying the existing commercial materials in terms of their functional or structural characteristics. Except for few reports, the majority of achievements were focused on the polymer matrix modified by ceramic nano particles so as to significantly improve its mechanical or physical properties. The addition of nano powders in metallic alloys has been relatively much less addressed. The grain refining on pure solid solution AZ31 Mg alloys by other means are unable to reach nano-scale. Also, the efforts to grain
refinements on Mg alloys by FSP are less reported and the present progresses on pure Mg alloys are only in micro-scale.
In this study, the goals of grain refinement and high hardness value will be carried out by optimizing FSP process parameters and cooling facilities. Intensive efforts are made to refine the grains to nano-scale in Mg base alloys or composites by FSP. Also, the properties and the relationship between resulting grain size and parameters for the friction stir processing in AZ31 Mg alloy are systematically examined. The AZ31 Mg alloy is adopted as the matrix and nano-sized ZrO2 and SiO2 particulates as reinforcement to fabricate the Mg matrix composites. FSP is also applied on the dispersion of particulates by the complex plastic flow of AZ31 Mg matrix during the stirring process. Afterward, the examination and analyses of microstructures and mechanical properties of the resulting composites and modified AZ31 Mg alloy by FSP are conducted.
1.1 The developments and applications of magnesium alloys
After magnesium element was discovered by Davy in 1808, nowadays, magnesium alloys have become one of the most popular materials for structural utilization, especially in lots of areas of transportation vehicles because of the current global tend toward light-weight utilization and environmental consciousness. In order to achieve the light-weight goal, it is necessary to adopt the light metals in structural and non-structural components on the vehicles. Table 1-1 lists the comparison between the Mg alloys with many common materials.
The advantages of magnesium and magnesium alloys are listed as follows, (a) Low density:
Magnesium is the lightest engineering metal, with a density of 1.74, roughly 1/4 that of steel and 2/3 that of aluminum. (b) High specific strength: Mg alloys can reach required strength
with less material which can reduce the product thickness. (c) Good thermal conductivity: Mg has better thermal conductivity than equal weight polymeric materials. (d) Good damping and crash resistance: The damping capability of Mg is better than plastic and aluminum and the crash resistance is much superior to plastics. (e) Thin thickness: The thickness of Mg enclosures used for 3C procducts can be reduced to 0.5 mm (plastic ~ 1 mm) for sufficient strength. (f) Good castability: Mg can be easily die-cast into complicated shapes with minimum wall thickness. (g) Excellent electromagnetic shielding capabilities: A thin Mg wall can effectively shield electromagnetic wave. Mg is a better material than plastic to be used in the computer, consumer and communication (3C) electronic products. (h) Recyclabilty: Mg can be easily and economically recycled through proper re-melting procedures. This imposes a strong benefit over plastics.
Therefore, Mg alloys have potential to replace aluminum alloys or plastics on the application of the electronic products, or be the important structural materials according to the advantages mentioned above. Despite the numerous advantages, magnesium alloys exhibit poor workability due to the hexagonal closed-packed (HCP) crystal structure. Hence, the shaping of magnesium alloys needs to adopt the die casting, thixomolding [10] or thixocasting [11] for the present stage. The way to improve the workability of magnesium alloys will be essential in order to promote mass production of Mg alloys in many different engineering applications.
1.2 Properties of magnesium alloys
1.2.1 The classification of magnesium alloys
Alloying behavior of magnesium is notable for the variety of elements with which it will form solid solutions. Therefore, aluminium, zinc, lithium, cerium, silver, zirconium and thorium are examples of metals that are present in commercial Mg alloys [12]. Magnesium alloys are usually designed by two capital letters followed by two or three numbers. Table 1-2 [13] presents the standard four-digit ASTM designation system for magnesium alloys and their temper treatments. In this system the first two letters indicates the main alloying elements according to the following code: A for aluminium; C for copper; E for rare earths; F for iron; H for thorium; K for zirconium; L for lithium; Q for silver; Z for zinc and so on. The first latter is the element with greater quantity, and if they are equal in quality the letters are listed alphabetically. The two (or one) letters are followed by numbers which represent the nominal compositions of these main alloying elements in weight percent. For example, AZ31B-H24 means the alloy contains a nominal 3 wt% aluminum and 1 wt% zinc and is in the B modification, distinguishing form the same AZ31 that contains the different levels of impurity. The H24 designation indicates that the alloy is strain-hardened and partially annealed. The addition of solute elements in Mg alloys is generally helpful for improving its properties, such as casting capability and corrosion behavior. The effects of the various solute elements on the mechanical, corrosion and casting behavior are listed in Table 1-3 [14].
1.2.2 The characteristics of magnesium alloys
In recent years, an increasing resource shortage and environmental conditions require new ways of engineering. By using consistent lightweight construction within all areas of engineering new potentials can be created. Magnesium is the lightest metallic constructional material with a density approximately 1.74 g/cm3 represents an attractive possibility for the lightweight construction. Therefore, the applications of magnesium and magnesium alloys gradual grow widely. The main applications of magnesium and magnesium alloys can be
divided into two categories: non-structural applications and structural applications. In the applications of structural materials, common cast magnesium alloys can be divided into several systems according to various alloy compositions:
(1) Mg-Al-X systems: The Mg-Al system has been the basis of the most widely used magnesium casting alloys. Most alloys contain 8-9% aluminium with small amounts of zinc.
(a) Mg-Al-Zn series alloys: The addition of zinc causes some strengthening such as AZ91D which has good mechanical property, castability and corrosion resistance.
(b) Mg-Al-Mn series alloys: Such as AM60, AM50 and AM20 which have good ductility.
(c) Mg-Al-Si series alloys: Such as AS41 and AS21.
(d) Mg-Al-RE (rare earth elements): Such as AE42 which has the improvements in creep resistance, ductility, and corrosion resistance.
(2) Mg-Zn-X systems:
(a) Mg-Zn-Zr series alloys: The ability to grain refine Mg-Zn alloys with Zr led to the introduction of alloys, e.g. ZK51 and higher strength ZK61. They have high yield stress and good casting ability.
(b) Mg-Zn-RE series alloys: Te alloys such as ZE33 and ZE41 show good creep resistance and are widely used for casting exposed to elevated temperature.
(3) Mg-Th-X systems: The addition of thorium also confers increased creep resistance to Mg alloys, and the ability for high working temperature.
(a) Mg-Th-Zr series alloys: Such as HK31A and HK32.
(b) Mg-Th-Zn series alloys: Such as HZ32A and ZH62A, the presence of zinc further increases the creep strength. Alloy ZH62A is also noted for its relatively high strength at room temperature.
(4) Mg-Ag-X systems: The alloys in this system possess high strength at elevated temperatures and good casting and welding capabilities.
(a) Mg-Ag-RE series alloys: The most widely used alloy has been QE22 which has been used for a number of aerospace applications.
(b) Mg-Ag-Zr series alloys: QK21A.
Most of all magnesium components are produced by die casting process. This process has high cycle times and good casting properties. Cast magnesium alloys have high strength and high productivity. But cast magnesium alloys common have the problem of the gas porosities which occur in the process and cause low ductility and insufficient fatigue strength.
Wrought magnesium alloys offer crucial advantages like superior mechanical properties, opposite casting components weldability, possibility of a thermal treatment and an increased ductility. Because of high costs, a small deformation ratio and a severely limited alloy spectrum, therefore, there exists a requirement for technical breakthrough in the processings and new alloy systems. Some of the commonly used wrought magnesium alloys are listed below.
(1) Mg-Mn series alloys: Such as MA1.
(2) Mg-Al-Zn series alloys: Such as AZ31, AZ61 and AZ80, these alloys are of major industrial importance because of their combination of light weight, strength, castability and relatively good corrosion resistance for magnesium alloys. The alloys in these series could precipitate the β phase (Mg17Al12) in the matrix or on the boundaries to strengthen the alloys after solution and aging treatments.
(3) Mg-Zn-Zr series alloys: Such as ZK40A and ZK60A, these alloys have the characteristic of higher yielding strength and good castability, especially for the increased strength with about 0.7% zirconium for grain refinement.
The properties of different alloy systems can change by adding different alloying elements in Mg alloys according to different application demands, such as:
(1) Zinc (Zn) additions can improve castability and mechanical properties.
(2) The addition of yttrium (Y) and rare earth element into the magnesium alloys would impose superior creep resistance to the QE series alloys after fully hardened through the T6 thermal treatment [15].
(3) Silver (Ag) additions to magnesium alloys results in improving high temperature and creep properties.
(4) A small amount of Ca added into AZ series alloys could produce relatively smaller grain t to improve yield strength due to the presence of Al2Ca and β-Mg17Al12 phases on the grain boundaries [16,17].
(5) Scandium (Sc) is a potential alloying element for improving the high temperature properties of magnesium alloys because of the high melting point of Sc (Tm = 1541oC) or a lower diffusivity in Mg. Von Buch et al. [18] have developed the Mg-Sc-Mn alloys showing good creep resistance.
(6) Mg-Mn alloys system could add Sc, Gd, Y and Zr so as to develop creep-resistant magnesium alloys [19]. Mg-Gd and Mg-Sc alloy systems also show an improvement for the creep-resistant [15].
1.3 Grain refinements
Magnesium alloys are attractive for lightweight structural applications in the transportation industry because of their low density and high specific strength and stiffness [20]. However, the symmetry of the hexagonal close-packed (HCP) crystal structure has the
limiting number of independent slip systems, resulting in poor formability and ductility near room temperature [3]. Fortunately, this can be resolved by refining the grain size which can bring about sufficient room temperature ductility and even superplasticity at high strain rates and low temperatures [1,2].
Although the mechanical and physical properties of all crystalline materials are determined by several factors, the average grain size of the material generally plays a significant, and often a dominant, role. Thus, the strength of all polycrystalline materials is related to the grain size. The widely applied equation for the relation between grain size and strength or hardness, the Hall-Petch equation [21], as written in the form of
H = Ho + kH d-1/2, (1)
σ = σo + kσ d-1/2, (2)
where H and σ is hardness and flow-stress, respectively, d is the average grain size, and Ho, σo, kH and kσ are constants. It follows from Eq. (1) that the strength increases with a reduction in the grain size and this has led to an increasing interest in fabricating materials with extremely small grain sizes. Narutani et al. [22] has reported that the Hall-Petch relationship is also well followed for magnesium, and the k value is about 280~320 MPa‧μm1/2. Wu [23]
and Lin et al. [24] also reported the kH and kσ are about 45~65 Hv‧μm1/2 and 300~350 MPa‧
μm1/2, respectively. It is noticeable that the k slope, or called the grain size strengthening efficiency, of magnesium alloys is much higher than that of aluminum alloys (~68 MPa‧
μm1/2), which means that the grain refinement of magnesium alloys has higher strengthening efficiency than aluminum alloys [25].
or
Magnesium alloys with refined grains could raise the strength and also improve the formability. In the study of AZ31 and ZK60, Bussiba et al. [26] suggested that magnesium alloys can possess better superplasticity at low temperatures or high strain rates with smaller grain size in the submicron scale. The grain refinement also affects the hardness value of magnesium alloys. Thus, the current research also focuses on producing high hardness magnesium material through effective grain refinement.
1.3.1 Grain size refinement techniques
In order to get good mechanical properties, different processes can be applied to refine the grain size, which would improve the mechanical properties such as strength, hardness, superplastic behavior, and so forth. The grain sizes of commercial alloys are generally tailored for specific applications by pre-determined thermechanical treatments in which the alloys are subjected in specified regimes of temperature and mechanical testing. However, the refining results of these usual produces are limited to the order of a few micrometers and can’t be used to produce materials with submicrometer grain sizes. Therefore, the other specific grain refining techniques that may be used to fabricate ultrafine-grained (UFG) materials with grain sizes in the submicrometer range are developed.
Two basic and complementary approaches have been developed for grain refinement and these are known as the “bottom-up” and “top-down” approaches [27]. The “top-down”
approach is dependent upon taking a bulk solid with a relatively coarse grain size and processing the solid to produce a UFG microstructure through heavy straining or shock loading. Unlike “bottom-up” approaches, these approaches can be readily applied to a wide range of pre-selected alloys and larger product sizes. Formally, the large strain is imposed on
the metal materials by severe plastic deformation (SPD) to gain fine grains via recrystallization or sub-grains by rearrangement of dislocations.
There are many different SPD processing techniques have been proposed, developed and evaluated. These techniques include rolling, equal-channel angular pressing (ECAP) [28-30], high-pressure torsion (HPT) [31], multi-directional forging [32,33], twist extrusion [34], cyclic-extrusion-compression [35,36], reciprocating extrusion [37,38], constrained groove pressing (CGP) [39], cylinder covered compression (CCC) [40], accumulative roll-bonding (ARB) [41,42]. All of these procedures are capable of introducing large plastic straining and significant microstructural refinement in bulk crystalline solids. Some of these techniques, such as ECAP, HPT, multi-directional forging and ARB are well-established methods for producing UFG materials.
Extrusion is the process by which a block of metal is reduced in cross section by forcing it to flow through a die orifice under a high pressure [43]. Semi-finished products of cylindrical bars or hollow tubes are most generally produced by extrusion. Lin and Huang [24,44] have extruded the AZ31 and AZ91 magnesium alloys, which are usually manufactured by thixomolding and casting, to refine the grain size from the initial average 75 μm to 1~5 μm via the occurrence of dynamic recrystallization during extrusion. These refined magnesium alloys can perform good superplasticity and higher room temperature tensile strength.
Rolling is the most widely used metalworking process because it lends itself to high production and close control of the final product. Rolling can be carried out at elevated temperatures (hot rolling), wherein the coarse-grained, brittle, and porous structure of the ingot or continuously cast metal is transformed into a wrought structure, with much finer
grain sizes. The rolling temperature plays an important role in rolling process. Rapid grain growth may be bought up by high rolling temperature that grains may not be effectively refined, while surface or edge cracking may take place at low rolling temperatures. Chang et al. [45] reported that grain sizes of AZ31 magnesium alloys could be refined from 13 μm to below 10 μm by rolling 2 mm thick plate to 0.5 mm for AZ31, and its thin sheet exhibited 300~325 MPa tensile strength. In addition, Kim et al. [46] also rolled AZ61 sheets with a thickness of 2.15 mm to 0.5 mm after nine passes at 648 K, and effectively refined grains from 16 μm to 8.7 μm.
In ARB process, the rolled material is cut, stacked and rolled again. In order to obtain the bulk material, the stacked sheets are bonded during rolling simultaneously (roll-bonding).
Therefore, the achieved strain is unlimited in this process because repetition times are endless in principle. Fig. 1-1 illustrates the principle of ARB process. Perez-Prado et al. [47] reported that grain sizes of AZ31 and AZ91 alloys can be refined from 38 μm to 3 μm and 23 μm to less than 1 μm, respectively. Therefore, ARB process is also an efficient way for grain refinement and the homogeneity of the microstructure can be improved by increasing the number of rolling process.
The ECAP process, performed by pressing samples into equal-diameter channel with a different exit direction, is a shear deformation maintaining the same input dimension of the pressed (or extruded) materials. The detailed illustration is shown in Fig. 1-2 [48]. This process can impose severe plastic deformation on materials after several passes to induce micro-scaled grains [48] or even to submicro or nano-scaled extrafine grains [49]. Therefore, this process appears to be a powerful technique. Mabuchi et al. [50] refined the AZ91 alloy to 1 μm by ECAP after eight passes, and the resulting alloys also showed a high elongation of 661% at a low temperature of 473 K. Recently, Matsubara et al. [51] and Lin et al. [52] have
developed a two-stage extrusion plus equal channel angular pressing (ECAP) to fabricate the UFG Mg alloys. The original coarse grain size can be reduced to less than 10 μm after extrusion at 300oC and it is further reduced to around 0.7 μm after subsequent 8-pass ECAP at 200oC.
As for the “bottom-up” approach, UFG materials are fabricated by assembling individual atoms or by consolidating nanoparticulate solids. Examples of these techniques include inert gas condensation [53], electrodeposition [54], spray forming [55,56], ball milling with subsequent consolidation [57] and cryomilling with hot isostatic pressing [58], where cryomilling essentially denotes mechanical milling in a liquid nitrogen environment. In practice, these techniques are often limited to the production of fairly small samples that may be useful for applications in fields such as electronic devices but are generally not appropriate for large-scale structural applications.
The spray forming process is an inert gas atomization of a liquid stream into variously sized droplets which then are then propelled away from the region of atomization by fast flowing atomizing gas, as shown in Fig. 1-3 [55]. Droplets are subsequently deposited and collected by a substrate on which solidification takes place. Finally a coherent and near fully dense perform is produced. This process can produce bulk scale nano-grained materials via rapid solidification at a cooling rate about 102 ~103 K/s. Therefore, this process is very powerful tool, but it has an expensive issue related to patent. However, Chen and Tsao [56]
reported that spray-formed AZ91-3.34 wt% Si alloys can possess finer structures and better workability than the as-cast counterpart.
Besides the refinement technologies mentioned above, there are still other methods to refine the grain size. Andrade et al. [59] used shock-wave deformation to refine Cu alloys to
0.1 μm at a high strain rate (~104 s-1). Garces and Perez [60] conducted the physical vapor deposition (PVD) technique to obtain columnar grain structure of the diameter of 0.2 μm of the Mg-14wt%Ti–1wt%Al–0.9wt%Mn alloys. Although, there are numerous developed methods for grain refinement, most of them are limited to be applied. Some techniques are still in laboratory research trial, such as ARB process. Some are difficult to produce bulk materials, such as PVD method and most of the “bottom-up” approaches. Some are because of high production cost, such as spray forming. Thus, the traditional extrusion and rolling are still among the first choices to be adopted in industry because of the lower cost and higher feasibility.
1.4 Metal matrix composites
A composite is a combined material created by the synthetic assembly of two or more components - a selected filler or reinforcing agent and a compatible matrix binder – in order to obtain specific characteristics and properties. The criteria that a composite can be used is judged by the following points [61].
z It must be man-made.
z It must be a combination of at least two chemically distinct materials with a distinct interface separating the constituents.
z The separate materials forming the composite must be combined three dimensionally.
z It should be created to obtain properties which would not otherwise be achieved by any of the individual constituents.
Composite materials have long been designed to couple the weaker but more ductile
phase (e.g. polymers or metals) with the stronger but more brittle one (e.g. ceramics), so as to tailor the needed properties of the resulting mixed materials. Among them, metal matrix composites (MMCs) have attracted attention since 1970’s. Metal matrix composites, in general, consist of at least two composites: one obviously is the metal matrix (in most case, an alloy is the metal matrix), and the second component is a reinforcement (in general, an intermetallic compound, an oxide, a carbide or a nitride). MMCs have several advantages that are very important for their use as structural materials. These advantages include a combination of the following properties [62]:
z High strength
z High elastic modulus
z High toughness and impact properties
z Low sensitivity to temperature changes or thermal shock z High surface durability and low sensitivity to surface flaws z High electrical and thermal conductivity
z High vacuum environment resistance
In addition to conductivity of MMCs, the most obvious advantages of MMCs are their resistance to severe environments, toughness, and retention of strength at high temperatures.
The reinforcements of the MMCs can be divided into two major groups, discontinuous and continuous. The continuous reinforcements, such as carbon fibers, glass fibers and silicon fibers, are in the form of continuous long fibers or short fibers. As for discontinuous reinforcements, the most prominent discontinuous reinforcements have been SiC, Al2O3 and TiB2 in both whisker and particulate form.
Continuous fibers could improve the elastic modulus and ultimate tensile strength of composites significantly. Because continuous fibers have a large aspect ratio of axial length
to diametric width, such continuous fibers could effectively carry most of load so as to enhance the elastic modulus and strength of entire composites. From these viewpoints, continuous fibers seem to be perfect reinforcements, but they also suffer some disadvantages.
The strength along the radial direction is greatly lower than that along axial direction, leading to an anisotropy problem [63]. This character will be unfavorable to some engineering applications. In order to improve this issue, most fibers are weaved into different directions to decrease anisotropy.
The composites reinforced by particulates or whiskers usually do not have a strong anisotropic character, sometimes can be almost isotropy. But discontinuous reinforcements would do not effectively share the load. Although particulates or whiskers reinforced MMCs would not have compatible elastic modulus and strength as the continuous fibers reinforced ones, the former can be processed by conventional methods such as extrusion, rolling and forging to meet the desired shapes.
It has long been known that the second phase inclusions or particles can inhibit grain growth in metallic materials. Therefore, one of the critical microstructure parameter is the particle interspacing Ls, which can be roughly estimated from [64]
,
2 / 1
3 2 ⎟⎟
⎠
⎞
⎜⎜
⎝
= ⎛
f
s r V
L π (3)
where <r> is the average particle radius and Vf is the particle volume fraction. Previous reinforcing ceramic particles in the 1980’s usually measure around 10-50 μm in diameter, later improvements have lead to the smaller particles with uniform size distribution in the range of 0.5-5 μm. With the typical reinforcing particles of Vf=20% and <r>=10 μm in typical aluminum base composites, Ls will be around 30 μm. The resulting grain size after
casting would also generally be in this range. Further hot extrusion may refine the grain size down to around 5~10 μm, and toughening effects will only be moderately enhanced. With the abundant resources lately of the nano particles or nano carbon tubes (or wires) fabricated by various physical or chemical means, the reinforcements can be substantially smaller. If the reinforcement size is lowered to be submicron size or nano size, Ls can be reduced to submirco or nano range. It means that the grain size can be refined to submicron or nano grains. For example, Wang and Huang [65] ever added 1 vol% silica with 50 nm to 6061 aluminum alloy and extruded this composite to obtain 0.7 μm grain size.
1.4.1 Processing of metal matrix composites and magnesium matrix composites
There are various processes to produce particulates reinforced MMCs, but they could approximately be divided into solid-state and liquid-state methods.
1.4.1.1 Liquid-state methods
In principle, the liquid state route represents a very simple processing concept whereby particulates or whiskers are mixed into a light alloy melt, subsequently cast and then fabricated in a manner analogous to conventional unreinforced alloys. Sometimes, these particulates or whiskers may be manufactured into performs, and the melted alloy is subsequently introduced into performs. The related processes include the stir casting [66-68], squeeze casting [69-71], molten metal infiltration technique [72], semi-solid slurry stirring technique [73] and plasma spray [74]. Moreover, in order to uniform disperse these particles, Lan et al. [75] reported the use of ultrasonic non-linear effects to disperse nano-sized ceramic particles in molten metal to fabricated the nano-sized SiC particle reinforced AZ91D magnesium composites. This way could effectively disperse SiC particle and reduce severe
clustering occurrence. Detailed properties of composites by various processing routes are listed in Table 1-4.
1.4.1.2 Solid-state methods
The solid state method applies higher diffusion ability at elevated temperatures to sinter the entire composites and the metal matrix is remained to be the solid state during processing.
The most typical solid state methods are the powder metallurgy (PM) [76-79] and diffusion bonding [80]. In addition, the PM process would generally be followed by a secondary processing such as extrusion, rolling, forging and superplastic forming to form final shapes of productions as well as to reduce the porosity. Detailed properties of composites made by the PM method are included in Table1-4.
It is known that many microstructure factors would influence the final performance of MMCs. Table 1-5 [81] presents a list of microstructural factors that influence mechanical properties and fracture in discontinuously reinforced MMCs.
The above mentioned metal based composites, either with the conventional reinforcements ~20 μm or the more advanced ones ~0.5 μm in dimension, have micro-range grain size. According to Eq. (3), when the reinforced particles are smaller, the resultant composite will have finer grain size. The success in fabrication of various nano-sized powders, wires or tubes has offered the new possibility in modifying existing commercial materials in terms of their functional or structural characteristics. At present, nano-sized composites were focused on the polymer matrix modified by ceramic nano particles so as to significantly improve its mechanical or physical properties [82-86]. It is less addressed about nano-sized particulates reinforced metal matrix composites except for few papers [75,79].
The nano-sized particulates reinforced metal matrix composites might be able to stabilize grain size to less than 200 nm and enhance the ductility at elevated temperatures.
Although the nano-sized particulates reinforced metal matrix composites could have a better properties, uniform dispersion of these nano-sized particles would be an extremely difficult task. Because of the high surface area ratio, nano-sized powders tend to cluster together, sometimes forming micro-sized aggregates. After secondary treatments, these aggregates will act as defect to form the initiation of crack to degrade the final performance.
Methods in dispersing the nano powders have been limitedly disclosed, mostly still protected by patents.
1.5 Friction stir welding and friction stir processing
1.5.1 Introduction of friction stir welding (FSW)
Traditional welding methods can be divided into two categories. One is fusion welding, which fuses the edge of metals to the liquid state, and then solidifies to integrate these metals.
There are many kinds of fusion welding technologies such as shield metal arc-welding (SMAW), tungsten inert gas arc-welding (TIG), electron beam welding (EBW) or laser beam welding (LBW), etc. Another is solid-state welding, which heats the metals to a softening state (not above melting point), and then applies a pressure to join two components by rapid diffusion at elevated temperatures. The methods of solid-state welding include the friction welding and superplastic welding [87]. The fusion welding always contains solidification structures, such as the inter-dendritic, eutectic phases, to influence the final properties after welding, and is sometimes subject to the phenomenon of pores. In addition, the fusion
welding also dissolves the precipitates to matrix to lower the original strength for some precipitation strengthening alloys, especially for highly alloyed 2000 and 7000 series aluminum alloys. The phenomena mentioned above are the disadvantage of fusion welding.
A novel and revolutionary welding technology, friction stir welding (FSW), was invented at The Welding Institute (TWI) of UK in 1991 as a solid-state joining technique, and it was initially applied to aluminum alloys [4]. The friction stir welding technique was also patented by TWI worldwide.
The basic concept of FSW is simple. A non-consumable rotating tool with a specially designed pin and shoulder is inserted into the abutting edges of sheets or plates to be joined and traversed along the line of joint. A schematic diagram of friction stir welding is shown in Fig. 1-4 [88]. In general, the pin is typically slightly shorter than the thickness of the workpiece. The pin is mounted in a shoulder that may be three times the diameter of the pin.
The pin and shoulder are pressed against the workpiece, rotated at several hundred revolutions per minute, and advanced along the joining surface. The tool serves two primary functions: (a) heating of workpiece, and (b) movement of material to produce the joint. The heating is accomplished by friction between the tool and the workpiece and plastic deformation of workpiece. The localized heating softens the material around the pin and combination of tool rotation and translation leads to movement of material from the front of the pin to the back of the pin. As a result of this process a joint is produced in “solid state”.
An entire process of friction stir welding is shown in Fig. 1-5.
1.5.2 Characteristics of friction stir welding
1.5.2.1 Microstructure of welding zone in friction stir welding
Based on the microstructural characterization of grains and precipitates in the FSW joint region, three distinct zones, the nugget zone (NZ) or termed as the stir zone (SZ) or dynamically recrystallized weld zone (DXZ), the thermo-mechanically affected zone (TMAZ), and the heat-affected zone (HAZ), have been identified as shown in Fig. 1-6. The part of base metal (BM) is the original metal materials, not undergoing any influence of welding process. The region between BM and TMAZ is HAZ. This zone experiences a thermal cycle with the occurrence of grain growth, but does not undergo any plastic deformation. The HAZ retains the same grain structure as the base metal. Beyond the nugget zone there is TMAZ which experiences both temperature and deformation during FSW. A typical micrograph of TMAZ is shown in Fig. 1-7 [89]. The TMAZ is characterized by a highly deformed structure. The base metal elongated grains were deformed in an upward flowing pattern around the nugget zone. Although the TMAZ underwent plastic deformation, recrystallization did not occur in this zone due to insufficient deformation strain. The part of DXZ is in the center location of welding zone. This DXZ has an apparent characteristic of fully recrystallized and equiaxed grains due to intense plastic deformation and frictional heating during FSW.
Generally, from the relation of the rotational direction of the pin tool and the direction of moving forward, the welding zone can be classified into the advancing side and retreating side, as shown in Fig. 1-8. The advancing side is on the position of the same direction of the rotational direction of the pin tool and forward direction. On the contrary, the retreating side is on the other position of the opposite direction of the rotation direction and forward direction.
In general, the difference of the transition region between DXZ and TMAZ is apparent
