Cold metal transfer (CMT)-based wire and arc additive manufacturing (WAAM) of Ti-6Al-4V alloy has been investigated to manufacture walls with two different building strategies. This study focuses on the influence of the application of thermal treatments on the resulting microstructure and mechanical properties. Deep microstructural analysis revealed different grades of growth of lamellae α phase after several thermal treatments at different temperatures, which lead to different tensile mechanical properties and better strength and ductility balance compared to the as-built condition. Results are compared with equivalent forged and casting standards and the state of the art for WAAM of Ti-6Al-4V alloy. At temperatures of 920 °C, anisotropy was maintained and elongation increased by 70% while yield strength and UTS was slightly decreased by 8%.

The CMT-Twin-based wire and arc additive manufacturing (WAAM) process for 5356 aluminium alloy has been investigated focusing on the optimisation of welding parameters to maximise the deposition rate while avoiding segregation-related problems during solidification. For that, different conditions have been studied regarding interpass dwell time and the use of forced cooling. The larger heat input produced by the double-wire CMT-Twin process, compared to the single-wire CMT, creates vast segregations for less intensive cooling conditions and short dwell times that can induce cracks and reduce ductility. Thermography has been applied to set a maximum local temperature between consecutive layers avoiding those segregations and pores, and to optimise the total manufacturing time by varying the interpass dwell time along the height of the wall. Only a constant interpass long dwell time of 240 s and the new optimised strategy were effective in avoiding merged segregations, reducing the latest total manufacturing time by 36%. Obtained tensile properties are comparable to other works using WAAM for this alloy, showing lower properties in the vertical orientation. The use of CMT-Twin-based welding technology together with variable interpass dwell time controlled by thermography is an interesting alternative to build up parts with wall thicknesses around of 10 mm in a reduced time.

Reduced hot cracking susceptibility is essential to ensure the flawless manufacturing of nickel superalloys typically employed in welded aircraft engine structures. The hot cracking of precipitation strengthened alloy 718 mainly depends on chemical composition and microstructure resulting from the thermal story. Alloy 718 is usually welded in a solution annealed state. However, even with this thermal treatment, cracks can be induced during standard industrial manufacturing conditions, leading to costly and time-consuming reworking. In this work, the cracking susceptibility of wrought and investment casting alloy 718 is studied by the Varestraint test. The test is performed while applying different welding conditions, i.e., continuous tungsten inert gas (TIG), low frequency pulsed TIG, continuous laser beam welding (LBW) and pulsed LBW. Welding parameters are selected for each welding technology in order to meet the welding quality criteria requested for targeted aeronautical applications, that is, full penetration, minimum cross-sectional welding width and reduced overhang and underfill. Results show that the hot cracking susceptibility of LBW samples determined by the Varestraint test is enhanced due to extended center line hot cracking, resulting in a fish-bone like cracking pattern. On the contrary, the minor effect of material source (wrought or casting), grain size and pulsation is observed. In fact, casting samples with a 30 times coarser grain size have shown better performance than wrought material.

In this work, weldability and hot cracking susceptibility of five alloy 718 investment castings in laser beam welding (LBW) were investigated. Influence of chemical composition, with varying Si contents from 0.05 to 0.17 wt %, solidification rate, and pre-weld heat treatment were studied by carrying out three different weldability tests, i.e., hot ductility, Varestraint, and bead-on-plate tests, after hot isostatic pressing (HIP) and solution annealing treatment. Onset of hot ductility drop was directly related to the presence of residual Laves phase, whereas the hot ductility recovery behaviour was connected to the Si content and γ grain size. LBW Varestraint tests gave rise to enhanced fusion zone (FZ) cracking with much more reduced heat-affected zone (HAZ) cracking that was mostly independent of Si content and residual Laves phase. Microstructural characterisation of bead-on-plate welding samples showed that HAZ cracking susceptibility was closely related to welding morphology. Multiple HAZ cracks were detected in nail or mushroom welding shapes, typical in keyhole mode LBW, irrespective of the chemical composition and thermal story of castings. In all LBW welds, Laves phase with a composition similar to the eutectic of the pseudo-binary equilibrium diagram of alloy 718 was formed in the FZ. The composition of this regenerated Laves phase matched with the continuous Laves phase film observed along HAZ cracks. This was strong evidence of backfilling mechanism, which is described as wetting and infiltration of terminal liquid along γ grain boundaries of parent material. The current results suggest that this cracking mechanism was activated in three-point intersections resulting from perpendicular crossing of columnar grain boundaries with fusion line and was enhanced by nail or mushroom weld shapes and narrow and columnar γ grain characteristics of castings. Neither Varestraint nor hot ductility weldability tests can reproduce this particular cracking mechanism.