Using lower heat generation for obtaining finer grain size and higher hardness

The basic principle for friction stir processing is the repeating stirring action through the rotation tool during the entire process. The heat is mainly generated from the friction between shoulder/pin and the plastic deformed materials in every stirring action. Hence, the rotating tool is the main source of friction heat. Besides the decreasing rotation rate, using smaller tool is another effective way to lower the input heat energy.

In addition, to lower the input heat energy, accelerating the heat conduction rate or the cooling rate are both feasible for refining the resulting microstructures. Fig. 3-43 shows the variation of recrystallized grains in the nugget zone of the plates with different thicknesses.

Under the same conditions and tool size, the thinner plate can possess finer resulting grain size after FSP. Shortening the distance between the rotation tool and the bottom back-up plate in the thinner specimen could increase the thermal conduction rate.

Other than the process parameters, cooling facility is also the method used for controlling the working temperature during FSP. The function of cooling facility is to accelerate the cooling rate, to repel the unnecessary heat immediately and to maintain the working temperature as low as possible during FSP. The applied cooling facility in this study is the copper made back plate containing three cooling channels with cooling water passing through them. Fig. 3-44 shows the microstructure of AZ31 alloy nugget zone after FSP with different back plates or cooling facilities. With a proper or more effective cooling facility, the finer resulting recrystallized grain size in the stirred zone would be yielded due to the less experienced heat history.

Furthermore, adding liquid nitrogen and keeping specimen contacting with the liquid nitrogen is a more effective way to cool the specimen and to bring out the friction heat. 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. Figure 3-45 shows the variation of recrystallized grains in the nugget subject to the different cooling conditions. The finest recrystallized grains could be

~0.55 μm in the bottom of the nugget zone of the liquid nitrogen cooled FSPed AZ31 alloy specimen. A summary of the recrystallized grain size for the different FSP parameters is presented in Table 3-7. The hardness profile of FSPed AZ31 alloy with liquid nitrogen cooling is shown in Fig. 3-46. Through liquid nitrogen cooling system, the resulting microstructure of nugget zone can possess finer grain size and higher hardness as compared with other approaches.

Other results by using the combination of smaller tool size, thinner plate, and liquid nitrogen cooling facility for the FSPed AZ31 Mg alloys are shown in Fig. 3-47. The finest grain size obtained at this stage is around 450 nm, as shown in Fig. 3-47, the SEM images of the specimen using 800 rpm, 90 mm/min, and a tool with shoulder diameter, pin tool diameter and length of 10, 3 and 3 mm, respectively.

3.3.2 The combination of composite and liquid nitrogen cooling methods

Since the cooling facility plays an important role for accelerating cooling/conduction rate which can result in finer grains, the combination of Mg/ZrO2 composites and liquid nitrogen cooling facility are performed. The 2G4P Mg/monoclinic phase ZrO2 composite is chosen because of finer grain sizes than other Mg-based composites. After the fabrication of 2G4P Mg/ZrO2 composite, the subsequent FSP pass with liquid nitrogen cooling facility is performed. A smaller tool with shoulder diameter, pin tool diameter and length of 10, 3, and 3 mm, respectively, was used to perform the subsequent liquid nitrogen cooling pass. The pin rotation rate and advancing speed for the sequent pass was 800 rpm and 90 mm/min,

respectively.

Fig. 3-48 shows the resulting microstructure in the nugget zone for the Mg/ZrO2

composite with subsequent cooling pass. The grain sizes can be further refined into submicro-scale and the average grain size is about 350~450 nm which is less than the finest grain size of all previous alloys and composites. The measured hardness value also increases to about 135 Hv which is also higher than all other composites in the present study, as shown in Fig. 3-49. Therefore, the improvement of cooling rate sure plays an important for producing finer grains during FSP.

Fig. 3-50 shows the TEM observations of the microstructure for the Mg/ZrO2 composite with subsequent cooling pass. It can be seen that grain boundaries were pinned by the reinforced ZrO2 particles in some small grains. There are also some nano-grains in the specimen. In addition, the corresponding selected area diffraction (SAD) pattern also exhibits diffraction rings indicating many small grains having random misorientations.

3.3.3 Brief conclusions

Based on above experiments, it can be concluded that using a smaller tool and a thinner plate as well as applying an efficient cooling system can produce finer FSPed microstructure.

So far the recrystallized grain size of AZ31 Mg alloy after FSP is just around ~400 nm. The average hardness value can reach to around ~100 Hv for pure AZ31 alloy and ~135 Hv for Mg/ZrO2 composite with the second cooling pass. However, the desire for much finer Mg grains by using the present cooling approach is difficult to achieve. Therefore, the need for another cooling design is emerged. The new designed cooling system has been applied as described below.