Reduction of Defects by Friction Stir Processing for Additively Manufactured Cast Aluminum Alloys (AlSiMg)

Abstract

In the field of metal 3D printing, in which metal powder is repeatedly layered by melting, decrease in strength and durability due to defects (pores, etc.) occurring in the layered metal is a major obstacle to commercialization. In this study, friction stir processing (FSP) was applied as a means to remove defects such as pores generated inside cast aluminum alloys fabricated using direct energy deposition, and to improve the microstructure. The AlSiMg alloy used in this study is commonly utilized for general casting parts in industry and is widely employed in the aviation and automobile industries where weight reduction is desired. FSP was performed using two types of tools with different shoulder shapes and their effect on the defects, microstructure, and hardness of the FSPed area was evaluated. Further, the effect of FSP on defect removal was studied. Prior to FSP treatment, many spherical pores (defects) with a diameter of 500 μm or less were formed in the deposited material. A larger stir zone area was formed on the cross section of the FSPed specimen treated with Tool 2 (open grooves on the shoulder) compared to that treated with Tool 1 (closed grooves on the shoulder). In addition, the maximum depth of the thermo-mechanically affected zone was greater in the former, but the decrease rate with increasing feed rate was smaller. For each tool, the change in the microstructure of the material for each feed rate was observed, and the stirred part, the part subjected to heat and mechanical change, and the part only subjected to heat were classified by examining the alloyed Si content through FE-EPMA. In addition, the removal or deformation of defects under the influence of FSP was observed microscopically, and the results were shown. Changes in hardness at each location were also measured and displayed. The results shows that FSP of DEDed cast aluminum is effective for removing defects, such as pores, and improving the microstructure. And Tool 2 with open grooves on the shoulder exhibits a relatively better stirring performance and is more stable.