The primary purpose of this paper is to study the size effect of surface roughness and realize the quantitative description of the surface roughness in micro-forming process. This work is a continuation of the previous work by the authors. The effects of the initial surface roughness of the specimen, the grain size, and grain orientations on the surface roughness of micro-upsetting products were investigated. The ratio of the number of grains of the surface layer to the total number of grains was adopted to characterize the size effect. The variation of the size effect on the contact normal pressure during the compression process was also analyzed. And the quantitative description of the evolution law of surface roughness for micro-formed parts was realized. The corresponding micro compression experiment was done in order to testify the prediction model.

The microstructure evolution of a Mg–Gd–Y alloy was studied using uniaxial tension combined with an electron backscatter diffraction technique. The results show that large amounts of slip transfer phenomena can be observed around the grain–boundary area after tension, and the activation of these slips depends largely on the misorientation of grain boundaries. The Mg–Gd–Y alloy shows almost randomized grain–boundary misorientation, but transferred slip traces were preferred at boundaries with misorientation around the [0001] axis between 0–30°. Theoretically, materials with a higher fraction of slip transfer at the grain–boundary area would improve the ductility. Upon comparing the two groups of magnesium alloy with different grain–boundary misorientation distributions, the one with more grain boundaries favored for slip transfer achieved higher elongation during a tension test. Therefore, in addition to weakening the texture, adjusting the misorientation of the grain boundaries appears to be a new method to improve the ductility of magnesium alloys.

Adding rare earth elements to magnesium alloys is an effective way to improve their wear resistance. However, the effect achieved is closely related to the friction condition. In this paper, two different types of welding wires, AZ91 magnesium alloy and AZ91 + gadolinium (Gd), were used for surface welding. Dry sliding friction and wear experiments were performed on the surfacing alloys using the pin-on-disc test. The effects of Gd addition on the wear resistance and wear mechanism of the alloy were systematically studied under low to high loads. The results show that as the load increases, the friction coefficient of the surfacing AZ91 alloy gradually decreases as the wear rate increases. A mild–severe wear transition occurred at 100 N. The addition of Gd only slightly increased the wear rate under a load of 15 N. The wear rate was significantly decreased with loads in the range of 30 to 100 N and mild–severe wear transition was avoided. The influence of both Gd addition and load on the wear mechanism were considered. The overall wear resistance of the surfacing magnesium alloy was determined.

Extruded Mg-Gd-Y alloy tubes were obtained by using cast ingot and extruded bar billets. Microstructure and mechanical properties were also studied with two different cooling methods: air cooling and water cooling. The result shows that by using an extruded bar as billet extruded tubes achieves higher elongation comparing to using cast ingots due to favored texture for the activation of basal slip. Using the water-cooling method, extruded tubes achieve a higher yield strength compared to the air cooling method due to their fine grain size. Using cast ingot billets and the water-cooling method, the elongation is only 6% due to large unrecrystallized grains caused by inhomogeneous deformation and unfavored texture for the activation of basal slip. Using the extruded bar billet and the water-cooling method, the tube has uniformed small grains and much more randomized texture caused by the inhibition of preferred grain growth process. The highest texture intensity is only 1.852 in this kind of tube. Both high yield strength (195.3 MPa) and high elongation (23.9%) are achieved in this tube.

The intergranular cracking in the Mg-Gd-Y alloy was investigated by an in situ tension test combined with an electron backscattered diffraction (EBSD) measurement and digital image correlation (DIC). During the tension test, the crack was found at the triangle point of the grain boundary area with profuse slip traces. DIC results show that inhomogeneous strain distribution can be found at the triangle point area, leads to serious deformation incompatibility at this point. This also leads to a weak point for crack initiation, and as tension strain increases, the crack would extend from the weak point along the grain boundary.

The rolling correction process can eliminate machining distortions of aluminum alloy 7075-T651 structural parts. The thermal stability of the corrected structural parts under the action of temperature loading, especially the macroscopic shape stability, is key to ensure the safe service of mechanical equipment. In this study, different thermal loads were used to postprocess aluminum alloy 7075-T651 structural parts after rolling correction. The thermal stability of the rolled samples was analyzed by characterizing the microstructure and physical and mechanical properties. The results show no obvious change compared to rolling in the distortion of the parts after temperature treatments at 120 °C, 160 °C, 230 °C and 300 °C; the distortion changes were only 10.48%, 2.74%, 8.13% and 8.70%, respectively. The residual stresses in the rolling areas of the samples decreased by 35.58%, 26.08%, 75.97% and 83.13%, respectively. The microhardness also showed a decreasing trend. There was no obvious change after treatment at 120 °C, but the hardness decreased by approximately 5%, 23% and 56%, respectively, after treatments at other temperatures. However, the rolling stress relaxed under thermal stress. The microstructure change analysis shows that the material microstructure is mainly dominated by static reversion at lower thermal loads. With increasing thermal load, the samples are mainly affected by the static recrystallization effect, the microstructure is gradually blurred, and the hardness decreases significantly. In conclusion, although the residual stresses introduced by rolling would occur in different degrees of stress relaxation under the thermal load, the microstructure changes caused by thermal load did not significantly affect the macroscopic distortion of the samples, and the macroscopic shape of the structural parts after rolling correction had good thermal stability.