# Grain Size in Aluminum Alloy 6061 under Hot Ring Compression Test and after T6 Temper
## 1 Introduction  
Formerly, materials used in structures were mostly steel based, because steel is easy to acquire, high in strength, and low in cost. However, aluminum alloy has a density about one third as much as steel. In addition, aluminum alloy has characteristics better than steel, such as high electrical and thermal conductivity, light weight, and simple to process. All these qualities have already made aluminum alloy a popular material for many applications, like transportation, the defense industry, automation and general household industries. In recent years, countries around the world are increasingly concerned about the issue of environmental protection. Because of excellent recycling characteristics, and under the trends of green manufacturing in less carbon and reducing waste, aluminum alloy is favored by industries to develop more diverse applications.  
Although there are two categories of cast and wrought aluminum alloys, in most applications the parts are made from wrought materials. To manufacture such parts, raw aluminum alloys are heated in furnace and then placed on the die, and the punch moves toward the die to squeeze the alloys by carefully controlling the metal flow. This not only can easily form the alloys into the desired shape by plastic deformation, but can also enhance the mechanical properties of the alloys to achieve better strength. However, an incorrect process setup or improper temperature control can often result in grain coarsening of aluminum alloys, which can reduce the strength of the alloys or more likely cause them to break. Usually, the grain coarsening occurs on the surface of parts [1; 2], which bringsthe surface poor appearances and makes it difficult to process further. If the situation of peripheral coarse grain can be improved, not only can the alloys be strengthened but the surface treatment costs can also thus be reduced, which would help industries upgrade their market competitiveness.  
The peripheral coarse grains are mostly found in aluminum extrusions. [PERSON] et al. [1] had found that they could be made by some extrusion speed, amount or rate, and temperatures just above recrystallization temperature, at irregularities such as corners or junctions in thickness. They also showed that there were no peripheral coarse grains found on non-recrystallization or full recrystallization. Peripheral coarse grain can also be enlarged by subsequent solid solution [2]. The higher the temperature of solid solution, the more the peripheral coarse grain. Not only does extrusion have peripheral coarse grain, but forging also has it after heat treatment, despite the extrusion billet showing no peripheral coarse grains [3,4].  
Whether peripheral coarse grain could be solely caused by hot forming is still unclear. In view of this question, this study is designed to investigate by removing the billet surface layers to eliminate the effect from the previous processes preparing the specimen. Experimental study is then executed in different working temperatures and surface conditions, specifically the frictional boundary conditions. In order to master the frictional boundary conditions, this study attempted to use a ring compression test [5,6] with different lubricants to determine the friction coefficient. At the same time, the ring compression test can provide diverse non-uniform strain distributions, which may allow a glimpse into grain size change after hot forging processes and after subsequent heat treatments of aluminum alloys.