The emerging technology of wire arc additive manufacturing (WAAM) has been enthusiastically embraced in recent years mainly by the welding community to fabricate various grades of structural materials. In this study, ER70S-6 low-carbon low-alloy steel wall was manufactured by WAAM method, utilizing a gas metal arc welding (GMAW) torch translated by a six-axis robotic arm, and employing advanced surface tension transfer (STT) mode. The dominant microstructure of the fabricated part contained randomly oriented fine polygonal ferrite and a low-volume fraction of lamellar pearlite as the primary micro-constituents. Additionally, a small content of bainite and acicular ferrite were also detected along the melt-pool boundaries, where the material undergoes a faster cooling rate during solidification in comparison with the center of the melt pool. Mechanical properties of the part, studied at different orientations relative to the building direction, revealed a comparable tensile strength along the deposition (horizontal) direction and the building (vertical) direction of the fabricated part (~ 400 MPa and ~ 500 MPa for the yield and ultimate tensile strengths, respectively). However, the obtained plastic tensile strain at failure along the horizontal direction was nearly three times higher than that of the vertical direction, implying some extent of anisotropy in ductility. The reduced ductility of the part along the building direction was associated with the higher density of the interpass regions and the melt-pool boundaries in the vertical direction, containing heat-affected zones with coarser grain structure, brittle martensite–austenite constituent, and possibly a higher density of discontinuities.

Microstructural evolution and mechanical properties of a low-carbon low-alloy steel produced by wire arc additive manufacturing

The weld quality comprises bead geometry and its microstructure, which influence the mechanical properties of the weld. The coarse-grained weld microstructure, higher heat-affected zone, and lower penetration together with higher reinforcement reduce the weld service life in continuous mode gas metal arc welding (GMAW). Pulsed GMAW (P-GMAW) is an alternative method providing a better way for overcoming these afore mentioned problems. It uses a higher peak current to allow one molten droplet per pulse, and a lower background current to maintain the arc stability. Current pulsing refines the grains in weld fusion zone with increasing depth of penetration due to arc oscillations. Optimum weld joint characteristics can be achieved by controlling the pulse parameters. The process is versatile and easily automated. This brief review illustrates the effect of pulse parameters on weld quality.

Effect of Pulse Parameters on Weld Quality in Pulsed Gas Metal Arc Welding: A Review

Thin plates are extensively used in automotive, shipbuilding, and railway industries. Welding technology is the main assembling method to manufacture thin plate structures because of its high productivity and ease of use. Consequently, residual stress and distortion induced by welding are an inevitable part of the manufacturing process in welding thin plate structures. Relatively low stiffness, and a high amount of heat input are the main reasons for distortion of the welded structures. In order to decrease the heat input, laser-based welding processes that generate highly localized heat with a very high intensity can be a good choice as an alternative to traditional fusion welding. To predict residual stress and distortion in welding thin plate structures, a three dimensional, thermo-metallurgical-mechanical finite element method was developed. The results of three different laser-based welding processes including aspects of Autogenous Laser Welding (ALW), Cold Wire Assisted Laser Welding (CWLAW), and Hybrid Laser-Arc Welding were compared with traditional Submerged Arc Welding (SAW) from both predicted and experimental perspectives. SYSWELD commercial code was used for the simulations in which both large and small deformation theories were employed to predict the residual stress and the final deformation. Experiments were executed to verify the simulation results. A digital high-resolution microscope was used to visualize and measure the weld cross- sectional shape and bead geometry. To measure the residual stress, an X-ray diffractometer was employed. A digital Vernier Caliper and a 3D laser-handheld scanner were used to measure displacement in the z-direction. Moreover, the mechanical properties of welds obtained by different welding processes were also verified by tensile and micro-hardness tests. It was concluded that lower heat input can markedly influence the final distortion of the welded structure. This conclusion can strongly support the idea of replacing traditional arc welding method with a laser-based one. Simulation and experimental results were matched fairly.

Numerical simulation and experimental validation of residual stress and welding distortion induced by laser-based welding processes of thin structural steel plates in butt joint configuration

The assessment of carbon equivalent formulas in predicting the properties of steel weld metals

Effect of welding process on the ballistic performance of high-strength low-alloy steel weldments

Heat-affected zone softening in high-strength low-alloy steels

Thermal Stability and Residual Stresses in Additively Manufactured Single and Multi-material Systems


 The assessment of welding induced residual stress is of significant interest due to its adverse effect on the structural stability and mechanical performance. Incorporation of intricate phenomena thermal gradient, phase development, volumetric dilation, and phase transformation strain during FE modelling, not only act as a reliable method for residual stress calculation but also serves as a directive to reduce tensile residual stress in a weldment which is sensitive to the selection of materials. In order to investigate the same for continuous and pulse laser welded Ti6Al4V alloy, the sequentially coupled thermal-metallurgical-mechanical models are established. The internal state-dependent variables are implemented to capture the growth of phase evolved during diffusionless transformation using Koistinen-Marburger theory in the cooling cycle. The role played by a phase transformation induced strain on the generation of residual stress is systematically investigated. The volumetric dilation and associated phase fraction form the basis for the estimation of phase transformation strain in the present study. The accomplishment of highest martensitic fraction (~ 95 %) produced a phase transformation strain of 7.95 &#215; 10-3 in pulse mode of operation. As a result, reversal of residual stress from tensile to compressive is perceived for pulse laser welded specimen. Similarly, a sign of enriched martensitic transformation is noticed that puts the weld surface into a compression state and mitigates the overall tensile residual stress. The assumption of diffusional phase transformation during heating cycle and non-diffusional transformation during cooling phase is appropriate to predict the residual stress.

Evaluation of Phase Transformation Strain and its Influence on Residual Stress Generation in Laser Welded Ti6Al4V Alloy


 Co based superalloy Mar M 509 having excellent high temperature oxidation and hot corrosion resistance is studied via the laser powder bed fusion (LPBF) process. The microstructure and mechanical properties of Mar M 509 in the as-printed (AsP) and heat-treated (HT) condition are compared, as a function of two build orientations (longitudinal (L) and transverse (T)), to establish a working range for application of the alloy. The AsP condition has a distinct cellular microstructure (500–600 nm) with 50–60 nm carbide particles decorating the cell boundaries. The L build orientation displays a strong <001> texture, has columnar grains with a grain size of 8–35 μm (along major axis) and a grain aspect ratio of 4, while the T orientation displays a more equiaxed, but bi-modal microstructure with a grain size of 5–28 μm. The room temperature mechanical properties show variability between L and T with T having 15% higher hardness and 34% higher 0.2% yield strength (YS), 30% lower elongation than L. After a short cycle heat treatment at 1250°C, the weld bead structure and cellular boundaries are broken down and there is substantial grain growth in both L (25–33 μm along major axis) and T orientations (5–42 μm), along with coarsening of carbides (250–350 nm). The dislocation density reduces substantially, indicating recrystallisation, and the lattice parameter of the matrix drops significantly, suggesting solute depletion that contributes to precipitate growth and enrichment of the carbides. There is a drop in the yield strength from 860 MPa to 740 MPa in L and from 1150 MPa to 840MPa in T and an increase in ductility from 14% to 23% in L.

B Satish Kumar

Papers

Heat-affected zone softening in high-strength low-alloy steels

Thermal Stability and Residual Stresses in Additively Manufactured Single and Multi-material Systems

Evaluation of Phase Transformation Strain and its Influence on Residual Stress Generation in Laser Welded Ti6Al4V Alloy

Thermal Stability of Additively Manufactured Mar M 509

Influence of Heat Input on Solidification Cracking in Additively Manufactured CM247LC Ni-based Superalloy