The set-up of friction stir processing

The simplified FSP machine is a modified form of a horizontal-type miller, with a 5 HP capability, as shown in Fig. 2-5.

2.2.1 The design of tool and fixture

There are several kinds of fixed pin tools used in this research. The largest one is 6 mm in diameter, 6 mm in length and 18 mm in shoulder diameter. The smallest tool has shoulder

diameter, pin tool diameter and length of 10, 3 and 3 mm, respectively. Smaller tool is conducted in order to reduce the heat generation caused by the shoulder and stirring pin during the process. All the tools have one fixed pin which has screw with a pitch distance of 1 or 0.8 mm. The material of the tool is SKH9, one kind of high speed steel. The details of the pin tools are listed in Table 2-2.

The back plate made by steel was used firstly in the experiments. In order to increase cooling effects, the copper made black plate was used in most experiments. The back plate is designed to contain three cooling channels with cooling liquid passing through them. The entire fixture design is shown in Fig. 2-6.

2.2.2 The special cooling condition during friction stir processing

In order to achieve ultimate fine grain size, optimal parameters and specific cooling system, such as liquid nitrogen, were applied. Firstly, the liquid nitrogen was added directly to chill the FSP specimen. Then, the black plate fixture, as shown in Fig. 2-6, was encircled to be a tank. The tank was filled with liquid nitrogen and the specimens are constantly contacted by liquid nitrogen. Therefore, with the accompanying of the liquid nitrogen cooling, the undesired heat causing grain growth could be eliminated more quickly. The finer recrystallized grain could be obtained without the long grain growth stage. However, the back plate fixture part is still not direct contacted and cooled by the liquid nitrogen. Hence, this cooling type is not efficient enough until the new effective cooling system is designed.

2.2.2.1 Newly designed effective cooling system

Based on extensive experimental experience, it is concluded that the primary heat

release during FSP is made from the bottom of the pin to the back plate beneath the sample. A newly designed cooling system is used, as shown in Figure 2-7. In order to transfer the heat generated between the tool and sample during FSP as quickly as possible, a thin copper mould and liquid nitrogen are used. Two tunnels are machined beneath the surface of the copper mould, and the liquid nitrogen can be immerged and flow through. The specimen was set and fixed direct on the thin copper mould and the whole system was also encircled and placed in the tank. Therefore, the liquid nitrogen can submerge the thin copper mould and flow through the tunnels.

2.2.3 The methods of adding nano-sized powders into AZ31 alloys

One or two deep shallow grooves ~6 mm depth, 1.25 mm wide and 100 mm length are cut on the rectangular samples of the AZ31 Mg alloy. The rectangular specimens of the AZ31 Mg alloy were sampled from the billet and were 55 mm in width, 130 mm in length and 10 mm in thickness. The appearance of the samples with one (1G) or two (2G) grooves is shown in Fig. 2-8. Then, these grooves were inserted with nano-sized ZrO2 powders, in average, one groove is filled with 500 mg ZrO2 powders. The AZ31 sample with grooves and inserted ZrO2 powders was conducted multiple FSP to fabricate composites, as shown in Fig. 2-8(b) and (c). In order to prevent the nano-particles from being displaced out of the groove(s), surface “repair” was accomplished before FSP with a modified FSP tool that only has a shoulder without pin. The detailed procedure is shown in Fig. 2-9. In addition, the samples have the grooves inserted with nano-sized SiO2 powders, with one groove filled with 100 mg SiO2 powders, were prepared and the results are compared with those of the ZrO2 containing composites.

2.2.4 The parameters of friction stir processing

2.2.4.1 FSP parameters for modified AZ31 alloys

The advancing speed of the rotating pin during FSP varies between 45 mm/min and 800 mm/min, with a variable pin rotation rate of 600 rpm (rotation per min) to 1800 rpm for pure AZ31 Mg alloys. The fixed pin tool with 6 mm in diameter and 6 mm in length was used. The shoulder diameter was 18 mm, and the applied tilt angle of the fixed pin tool was 3°. The AZ31 plate was 10 mm in thickness. In addition, a K-type thermocouple was inserted into the sample, as shown in Fig. 2-10, to measure the temperature variation during the entire FSP pass.

2.2.4.2 FSP parameters for fabricating intrinsic reinforced Mg-Al-Zn alloys

The fixed pin tool was 6 mm in diameter and 5 mm in length, with a pitch distance of 1 mm. The shoulder diameter was 16 mm, and the tilt angle of the fixed pin tool was 2^o. The pin rotation was set to be 1500 rpm, and the pin advancing speed was 20 mm/min. Most FSP experiments were conducted with air cooling, selected FSP runs were performed using copper clamping kits with water. Some selected FSP runs were even conducted using liquid N2 to chill the specimens and the rotating pin. A special container was placed around the specimens so that the specimen and pin could be submerged in the liquid N2, as shown in Fig. 2-3(c).

For comparison, the Mg70Al5Zn25 alloy system was also prepared by rapid quenching melt spinning with a cooling rate around 10⁵ K/s. Melt spinning is a technique used for rapid cooling of liquids. A wheel is cooled internally, usually by water or liquid nitrogen, and rotated. A thin stream of liquid is then dripped onto the wheel and cooled, causing rapid solidification. This technique is used to develop materials that require extremely high cooling rates in order to form, such as metallic glasses. The schematic drawing of the melt spinning is

shown in Fig. 2-11 (a). The melt spinning process was conducted in our group and the picture of the facility is shown in Fig. 2-11 (b).

2.2.4.3 FSP parameters for fabricating extrinsic reinforced Mg-AZ31 based composites

The fixed pin tool with 6 mm in diameter and 6 mm in length was used. The shoulder diameter was 18 mm, and the applied tilt angle of the fixed pin tool was 2°. The plate was 10 mm in thickness. The FSP parameters of advancing speed of 800 rpm and pin rotation of 45 min/min were applied in the present experiment. The FSP was conducted for one to four passes (denoted as 1P to 4P), the advancing direction for the subsequent pass was in opposite direction to the previous pass (i.e. proceeding in a forward and then backward way).

2.2.4.4 FSP parameters for producing the ultrafine grained AZ31 alloys

A tool with shoulder diameter, pin tool diameter and length of 10 mm, 3 mm and 3 mm, respectively, was used to perform the FSP. The tool rotation rate of 800~1200 rpm was adopted and the advancing speed was 28~37 mm/min. A tool tilt angle 1.5^o was used. In order to obtain rapid heat sink during FSP, the newly efficient cooling system with liquid N2

was conducted. The thin plate used in present study is 4 mm in thickness. Finally, Fig. 2-12 shows the experimental procedures for this research.

2.3 Microhardness measurements

A Shimadzu HVM-2000 microhardness machine with standard Vicker’s diamond