Solidification behaviour controls the size and shape of grains, the microstructure, the extent of segregation, the distribution of inclusions, the extent of defects such as porosity and hot cracks, and ultimately the properties of weld metal. In the past, theories related to single crystal growth technology and the solidification of castings have been extrapolated to describe weld metal solidification. However, within the past two decades there have been several new developments in solidification theory. Furthermore, welding involving high energy beam processes and significantly greater cooling rates have received much emphasis. Traditional solidification theories as they apply to castings are not appropriate in describing solidification under these latter conditions. This paper provides a comprehensive review of the solidification behaviour of a weld pool incorporating recent advances in solidification theories. Basic principles from ingot, single crystal, and rapid solidification processes are u...

Correlation between solidification parameters and weld microstructures

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

Solidification cracking can occur in the fusion zone during the solidification of the weld metal Liquation cracking, on the other hand, can occur in the partially melted zone during the solidification of the liquated material These two types of cracking are reviewed in this article, including the factors that affect cracking and the remedies

Solidification and liquation cracking issues in welding

The generally reported observations pertinent to any proposed interpretation of the columnar to equiaxed transition (CET) in as solidified alloys are initially considered. The review then proceeds to consider the proposed mechanisms of equiaxed grain formation, the influence of alloy and processing conditions on the CET, criteria for the termination of columnar growth and, finally, deterministic/stochastic models for predicting the CET. In conclusion, the present level of understanding of the CET and current modelling capabilities are summarised and assessed.

Columnar to equiaxed grain transition in as solidified alloys

The presence of Nb rich Laves phase in Inconel 718 weld fusion zones is known to be detrimental to weld mechanical properties. In the present study, an attempt was made to control the formation of Laves phase in alloy 718 gas tungsten arc welds using pulsed current. Welds were produced in 2 mm thick sheets of the alloy with constant current and pulsed current and were subjected to post-weld solution treatment at 980°C followed by aging. Detailed microstructural studies and tensile tests at 650°C were conducted. The results show that the use of current pulsing (i) refines the fusion zone microstructure, (ii) reduces the amount of Laves phase and exerts a favourable influence on its morphology and (iii) improves the response of the fusion zone to post-weld heat treatment and weld tensile properties.

Control of Laves phase in Inconel 718 GTA welds with current pulsing

The mechanisms of grain refinement have been examined for magnetically stirred gas tungsten arc (GTA) welds completely penetrating thin sheets of several aluminum alloys. Grain refinement in unstirred welds may be brought about by adding sufficient titanium to produce heterogeneous nucleation by Ti-rich particles. In some alloys magnetic stirring is shown to extend the range of welding conditions which produce a partially equiaxed structure, and to widen the equiaxed fraction of partially equiaxed welds. This is attributed to magnetic stirring lowering the temperature gradient, allowing nucleation and growth of Al-rich grains further ahead of the columnar interface growing in from the fusion boundaries. In alloys with low Ti levels, magnetic stirring may cause refinement by sweeping grains from the partially molten zone ahead of the advancing solidification interface. This mechanism requires that the partially molten zone be sufficiently wide, and that the grain size in this zone remain small.

B. P. Pearce

Papers

Grain refinement in magnetically stirred GTA welds of aluminum alloys