Pulse Metal Inert Gas based Wire Arc Additive Manufacturing of an Austenitic Stainless Steel

Abstract

Metal additive manufacturing (AM) is having a major impact on the future of production processes providing new alternatives to conventional forging, casting, and machining processing routes. To aid in industrial adoption, qualification and certification processes, an understanding of the effects of the process parameters on the material properties of AM material to obtain a balance between repeatable satisfactory material properties and productivity is required. This thesis reports on Wire Arc Additive Manufacturing, a Directed Energy Deposition process for austenitic Type 316L stainless steel, one of the most commonly used stainless steels due to its combination of strength and corrosion resistance.

This research investigates the effects of deposition rate (0.75 kg/hr and 3.00 kg/hr), heat input ( and cooling strategy (60°C and 150°C interpass temperature control, and in-process LN2 cryogenic cooling localised to the melt-pool) on the materials characterisation and mechanical performance of Type 316L stainless steel produced via WAAM. The addition of silicon to the wire composition is also explored. WAAM machine is developed by the author to study these effects encompassing a Pulse Metal Inert Gas based setup, advantageously feeding the wire coaxial to the welding torch.

The results showed that the process parameters influenced the solidification mode, morphology, δ-ferrite content, texture, grain size, oxide inclusion size and distribution, relative density, and substrate distortion. The results from the mechanical analysis showed that whilst the yield strength and ultimate tensile strength readily exceed the ASTM A240/A240M (2004) specification the process parameters that induce higher cooling rates result in a statistically significant increase in tensile strength. Higher deposition rates however, reduced ductility, which was below that required by ASTM A240/A240M (2004). Relations between oxide inclusion frequency and FeritscopeTM measurements prior to deformation with uniform elongation are established, whereas the relative density was found to have a negligible effect on uniform elongation. The wire with additional silicon composition reduced the oxide number frequency and provided higher tensile strength and Young’s Modulus than the Type 316L wire composition at 0.75 kg/hr.

In-process cryogenic cooling localised to the melt-pool was demonstrated to improve the overall performance of the WAAM material whilst eliminating interpass dwells periods. The grain size and texture of the low linear heat input, 0.75 kg/hr 316L samples were significantly changed in comparison to the large directional columnar grain growth observed for other processing conditions. This provided major benefits to Young’s Modulus for which for the majority of the experiments was significantly lower than the specification for wrought material

Date of Award	24 Jun 2020
Original language	English
Awarding Institution	University of Bath
Sponsors	Renishaw plc
Supervisor	Stephen Newman (Supervisor), Vimal Dhokia (Supervisor) & Alborz Shokrani Chaharsooghi (Supervisor)