Understanding the microforming related tribological science of engineered magnesium alloys at high temperatures

Microforming is a promising approach to micromanufacturing miniaturized components. The material flow and tribological aspects of microforming are affected by the size effect. The size effect phenomenon is influenced by parameters such as the initial microstructure, deformation temperature, lubricant type, and billet geometry. The scope of this study is to establish tribology-based scientific knowledge by considering all the mentioned parameters. As a case study to mimic the tribological interaction during microforming, a micro double cup extrusion (MDCE) test is performed on engineered Magnesium QE22 materials. The experiments were performed on various grain sizes, lubricants, and temperatures. A comprehensive investigation of all the conditions indicated that the ultrafine-grained (UFG) microstructure is the best-suited initial microstructural condition for maintaining excellent surface morphology, surface roughness, and microstructural homogeneity. The coarse grain (CG) microstructure exhibited substandard surface properties and microstructural heterogeneity. Electron backscatter diffraction (EBSD) microstructural analysis revealed tribological interactions with the activated micromechanisms under coarse-grained (CG), fine-grained (FG), and ultrafine-grained (UFG) conditions. Under CG conditions, the activation of twins induced dynamic recrystallization resulting in a greater cup height ratio and coefficient of friction (COF). This shows the incompetence of the CG microstructure in accommodating friction-induced shear. On the other hand, the UFG microstructure demonstrated a resilient microstructure that easily accommodated the induced frictional shear by activating the grain boundary sliding (GBS) mechanism. The activation of the GBS mechanism resulted in complete anhelation of the frictional subsurface layer, thereby eliminating the tribological size effect that remained unaffected even when the billets were downsized.