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

Microforming is a promising approach to micro-manufacture 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 downsizing. The scope of this article is to establish the tribology based scientific knowhow 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. The comprehensive investigation of all the conditions indicated that the UFG microstructure is the best-suited initial microstructural condition for maintaining excellent surface morphology, surface roughness, and microstructural homogeneity. The CG microstructure exhibited substandard surface properties and microstructural heterogeneity. EBSD microstructural analysis establishes tribological interactions with the activated micro mechanisms in all the CG, FG, and UFG conditions. In the CG condition, the activation of twin induced dynamic recrystallisation resulted in a greater cup height ratio and coefficient of friction.  This shows the incompetence of the CG microstructure in accommodating the friction-induced shear. On the other hand, the UFG microstructure condition demonstrated a resilient microstructure that accommodated the induced frictional shear with ease by activation of 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.