Study On Electron Beam Welding And Heat Treatment Of Titanium Alloys

Electron beam welding (EBW) has drawn particular attention due to its highenergy density, a deep and narrow joint, a minimal heat-affected zone (HAZ), lowresidual stress and small distortion of welded materials. Due to their low specificgravity, high strength-to-weight ratio, excellent fatigue and corrosion resistance andsuperior mechanical properties at high temperature, titanium alloy has been widelyused as structural materials in the aero gas turbine industry, automotive and chemicalindustries, as well as biomedical engineering and nuclear power plants. While limitedstudies on the microstructure and mechanical properties of electron beam welded(EBWed) dissimilar titanium alloy joints were reported, especially the effect of heattreatment on the dissimilar titanium alloy joints. It is necessary to study themicrostructure and mechanical properties for the application of electron beamwelding process in the aerospace industry. The purpose of the present study was,therefore, to evaluate the microstructural characteristics, tensile and fatigue behaviorof dissimilar titanium alloy joints welded by electron beam, along with the effect ofheat treatment on the joint, the main results are as follows:10mm thick Ti-6Al-4V/Ti17and Ti-6Al-4V/BT9joints were obtained usingelectron beam welding at the same welding parameters in this study. The weldingresulted in a significant microstructural change across the Ti-6Al-4V/Ti17joint, withhexagonal close-packed (hcp) martensite α′and orthorhombic martensite α′′in thefusion zone (FZ), α′in the HAZ of Ti-6Al-4V side, and coarse β in the HAZ of Ti17side. Also, using the equation the Mo equivalent was calculated, and it showed that itis reasonable that martensite α′′and β existed in the Ti-6Al-4V/Ti17joint. A markedmicrostructural change also occurred after welding in the dissimilar Ti-6Al-4V/BT9joint, with martensite in the FZ and in the HAZ of Ti-6Al-4V side, and martensiteand some retained α phase in the HAZ of BT9side.A characteristic asymmetrical hardness profile across the dissimilar joint wasobserved. The hardness value in the FZ of Ti-6Al-4V/Ti17joint was significantlyhigher than the base metals (BMs) due to the formation of martensite. It is of specialinterest to observe that, while the hardness value in the HAZ on the Ti-6Al-4V sidewas higher than that of the Ti-6Al-4V BM and decrease gradually as the distanceincreased from the FZ border line, the hardness value in the HAZ on the Ti17side was visibly lower than that of the Ti17BM and increase gradually as the distanceincreased from the FZ border line, meaning that a soft zone appeared. The hardnessvalue in the FZ of Ti-6Al-4V/BT9joint was also higher than the BMs, and thehardness value in the HAZ on the Ti-6Al-4V and BT9sides decreased from the FZborder to the BM. No obvious soft zone in the HAZ could be seen fromTi-6Al-4V/BT9dissimilar welded joints, unlike the case of Ti17alloy welded withTi-6Al-4V alloy.Tensile properties of EBWed dissimilar titanium alloy joint under optimizedwelding parameters were investigated. At the same strain rate, the yield strength (YS)and ultimate tensile strength (UTS) of the dissimilar Ti-6Al-4V/Ti17andTi-6Al-4V/BT9joints lay in-between the two BMs, and slightly higher thanTi-6Al-4V BM, whose abnormal behavior was explained using a schematic diagramand the limiting strength ratio (LSR). The YS and UTS of the Ti-6Al-4V/Ti17andTi-6Al-4V/BT9joints increased, both hardening capacity and strain hardeningexponent decreased with increasing strain rate.Fatigue behavior of dissimilar joint was studied. At the same strain ratio (-1)and strain rate (1×10-2s-1), Ti-6Al-4V/Ti17and Ti-6Al-4V/BT9joints showedessentially symmetrical hysteresis loops and equivalent fatigue life, and exhibitedcyclic stabilization at lower strain amplitudes up to0.6%, while cyclic softeningoccurred after initial cyclic stabilization at higher strain amplitudes. Strain ratio alsohas a marked effect on the fatigue properties. In the constant strain ratio (-1) andstrain rate (1×10-2s-1), with the average total strain amplitude increasing from0.2%to1.2%, the cyclic stress amplitude increased from about200MPa to1000MPa.Fatigue life decreased from about107to200. The fatigue life of Ti-6Al-4V/Ti17andTi-6Al-4V/BT9joints was almost the same as their BMs, which suggested that theselected welding process was quite reasonable. At the same strain amplitude (0.8%)and strain rate (1×10-2s-1), Ti-6Al-4V/Ti17joints basically exhibited asymmetricalhysteresis loops in tension and compression at all strain ratio except R=-1. At astrain ratio of R=0and0.5, a large amount of plastic deformation occurred in theascending phase of the first cycle of hysteresis loops of Ti-6Al-4V/Ti17joint due tothe high positive mean strain values. Fatigue life of the Ti-6Al-4V/Ti17joint wasobserved to be the longest at R=-1, and it decreased as the strain ratio deviated fromR=-1. Fatigue crack initiated on the specimen surface or near-surface defect andfatigue crack propagation zone was predominantly characterized by typical fatigue striations along with secondary cracks which were perpendicular to the crackpropagation direction.The effect of heat treatment on the microstructure and mechanical properties ofthe dissimilar joints was discussed. The annealing with solution of Ti-6Al-4V/Ti17joints and Ti-6Al-4V/BT9joints (STA) had a more significant effect on themicrostructure and mechanical properties than annealing at630oC and550oC.Though the annealing with solution resulted in a significant change in themicrostructure, hardness and strength of both joints, the fatigue life of both joints arealmost the same as the as-welded joint. However the microstructure, hardness, tensileproperties and fatigue properties of both joints annealing at630oC and550oC arebasically the same as the as-welded joints. Therefore, it is more reasonable forTi-6Al-4V/Ti17joints and Ti-6Al-4V/BT9joints annealing at630oC and550oC,respectively.