Investigating plasma keyhole welding with multiple wires for fusion welding with chemically graded weld seams

Abstract

Fusion welding processes are characterised by high temperature gradients, caused by the local heat input, which in turn leads to the formation of thermal and transformation induced (tensile-) stress. If the resulting stress exceeds the yield point, distortion occurs, which has a strong influence on the geometrical precision of the component. To counter these effects, in the past few years Low Transformation Temperature (LTT) materials were successfully used as filler wire. Here, the martensite start temperature is reduced and the increased thermal expansion during the martensitic phase transformation is used to induce compressive stress. Thus the formation of tensile stress through thermal shrinkage can be actively counteracted by the formation of compressive stress at reduced temperature. This counteracts the thermal distortion with the use of phase transformation. However, a solid technical solution for locally inducing LTT injections needs to be found. The following article presents a new approach of generating chemically graded weld seams in order to change the microstructure from ferritic to martensitic LTT alloy within the welding process. Therefore, a multi-wire plasma keyhole process is used to create a new alloy in-situ with varying nickel and chromium contents. To obtain a chemically graded structure the wire feed speeds are changed continuously within the welding process. For the multi-wire plasma keyhole process, a conventional plasma welding torch is extended by additional wire feeders, developed by the RWTH Aachen University Welding and Joining Institute. The graded structure is achieved by combining a high chromium-nickel filler wire with a conventional low alloy filler wire. The length of the graded area is varied so that the transition from high to low alloy area is either long or short. The results show, that it is possible to establish a stable welding process and change the chemical composition within the weld seam in-situ. Cross sections show a changing microstructure from low alloy ferritic microstructure to a martensitic microstructure. The influence of the transition area on the resulting microstructure and mechanical properties, such as hardness, is investigated. The experiments were performed in order to develop a basic knowledge about the mixing of dissimilar materials in plasma keyhole welding and the effects on the microstructure.