Developing thixo-extrusion process for additive manufacturing of metals in semi-solid state

This paper deals with a comprehensive study of the semi-solid metal extrusion and deposition (SSMED) process and influencing parameters. This process could be considered as a non-powder based metal Additive Manufacturing (AM) method. Unlike the common methods of metal AM, it utilizes the extrusion of a metallic filament in a semi-solid state (thixo-extrusion) to build the final product from a CAD file in a layer-wise fashion. A continuous wire thixo-extruder was designed and mounted on a 3 axis table to deposit semi-solid slurry layer by layer. This work seeks to provide a better understanding of the impact of the principle process variables on the quality of this layer-wise freeform fabrication method. The process parameters related to the production and pre-treatment of the metallic filament, its thixo-extrusion, rheological and microstructural aspects are included in this study. The results show that wire pre-treatment process variables have strong effect on the globular microstructure of metallic filament and this in turn has great role in the alloy's thixo-extrudability. Proper design of a thixo-extruder and selecting accurate values for the influencing parameters is the key to a successful production. The quality of the fabricated part was influenced by several factors such as microstructural, geometrical and run time parameters. Semi-solid alloy's solid fraction was the most influential factor in the thixo-extrusion process. Alloys with solid fraction values in the range of 0.3-0.6 could be thixo-extruded. It was found that temperature distribution and feed rate had a great effect on the process performance. Also design guidelines for selecting the appropriate thixo-extrusion properties were presented for different operating conditions.