Welding Metallurgy of

Wire Arc Additive Manufacturing (WAAM) is attracting significant attention in industry and academia due to its ability to capture the benefits of additive manufacturing for production of large components of medium geometric complexity. Uniquely, WAAM combines the use of wire and electric arc as a fusion source to build components in a layer-by-layer approach, both of which can offer significant cost savings compared to powder and alternative fusion sources, such as laser and electron beam, respectively. Meanwhile, a high deposition rate, key for producing such components, is provided, whilst also allowing significant material savings compared to conventional manufacturing processes. However, high quality production in a wide range of materials is limited by the elevated levels of heat input which causes a number of materials processing challenges in WAAM. The materials processing challenges are fully identified in this paper to include the development of high residual stresses, undesirable microstructures, and solute segregation and phase transformations at solidification. The thermal profile during the build poses another challenge leading to heterogeneous and anisotropic material properties. This paper outlines how the materials processing challenges may be addressed in WAAM by implementation of quality improving ancillary processes. The primary WAAM process selections and ancillary processes are classified by the authors and a comprehensive review of their application conducted. Strategies by which the ancillary processes can enhance the quality of WAAM parts are presented. The efficacy and suitability of these strategies for versatile and cost effective WAAM production are discussed and a future vision of WAAM process developments provided.

https://purehost.bath.ac.uk/ws/files/183236429/StrategiesAndProcessesForHighQualityWireArcAdditiveManufacturing_Manuscript.pdf

Invited review article: Strategies and processes for high quality wire arc additive manufacturing

Revisiting fundamental welding concepts to improve additive manufacturing: From theory to practice

Additive manufacturing (AM) of metals, also known as metal 3D printing, typically leads to the formation of columnar grain structures along the build direction in most as-built metals and alloys. These long columnar grains can cause property anisotropy, which is usually detrimental to component qualification or targeted applications. Here, without changing alloy chemistry, we demonstrate an AM solidification-control solution to printing metallic alloys with an equiaxed grain structure and improved mechanical properties. Using the titanium alloy Ti-6Al-4V as a model alloy, we employ high-intensity ultrasound to achieve full transition from columnar grains to fine (~100 µm) equiaxed grains in AM Ti-6Al-4V samples by laser powder deposition. This results in a 12% improvement in both the yield stress and tensile strength compared with the conventional AM columnar Ti-6Al-4V. We further demonstrate the generality of our technique by achieving similar grain structure control results in the nickel-based superalloy Inconel 625, and expect that this method may be applicable to other metallic materials that exhibit columnar grain structures during AM. 3D printing of metals produces elongated columnar grains which are usually detrimental to component performance. Here, the authors combine ultrasound and 3D printing to promote equiaxed and refined microstructures in a titanium alloy and a nickel-based superalloy resulting in improved mechanical properties.

/pdf/grain-structure-control-during-metal-3d-printing-by-high-23r8jaxm4i.pdf

Grain structure control during metal 3D printing by high-intensity ultrasound.

Application of ultrasonic vibrations in welding and metal processing: A status review

Grain refining by ultrasonic stirring of the weld pool

Grain refining of magnesium welds by arc oscillation

Predicting susceptibility of magnesium alloys to weld-edge cracking

Grain refining can improve the solidification-cracking resistance and mechanical properties of welds. Mg alloys are increasingly used for vehicle weight reduction, AZ91 Mg being the most widely used Mg casting alloy. The present study was conducted to study grain refining in gas–tungsten arc welding of AZ91 Mg, focusing on identifying the mechanism of grain refining. Significant grain refining was obtained by transverse arc oscillation. Dendrite fragmentation was identified as the grain refining mechanism, instead of heterogeneous nucleation or grain detachment. This was done by using an overlap welding procedure in which the weld made with transverse arc oscillation (test weld) overlapped with another weld made previously without arc oscillation (preweld). It was found that repeated prewelding followed by welding with transverse arc oscillation resulted in very fine grains.

Tao Yuan

Papers

Grain refining by ultrasonic stirring of the weld pool

Grain refining of magnesium welds by arc oscillation

Predicting susceptibility of magnesium alloys to weld-edge cracking

Mechanism of grain refining in AZ91 Mg welds by arc oscillation

Microstructure and properties research of Al-Zn-Mg-Cu alloy with high strength and high elongation fabricated by wire arc additive manufacturing