Effect Of Defects On Fatigueperformance Of Laser Welded Ti–6Al–4V Alloy

The welding of Ti-6Al-4V titanium alloy was carried out using YLS-5000 ytterbium doped multimode fiber laser manufactured by Apache Corp X-ray non-destructive testing was used to investigate the welding joints after welding and analyze defects in the welding.Optical microscopy,scanning electronic microscopy,tensile testing machine,fatigue testing machine were used to investigate the welding microstructure,tensile properties and fatigue properties.The defects such as porosity and crack in the welded joint were counted and measured.The influence of welding defects on the microstructure and fatigue property of the joints is analyzed.The results indicate that:all welding joints have different degrees of oxidation and cold cracking.And the number of cracks in the weld is the most,when the welding speed is0.015m/s.Fusion zone(FZ)consists of single acicular martensit?'phase.The size of acicular martensite were different and the configuration is perplexing.At the same time,there were some pores in FZ.These pores resulted the deformation of the original?grain boundaries.And the dislocation density increases rapidly in the grain boundaries.Heat Affected Zone(HAZ)consists of acicular?'martensite and primary?martensite.The hardness of joints increases with the increase of the content of acicular?'phase.The research of mechanical property results that:the tensile strength of welding joints is only 1/3 of the base metal.The tensile fracture is brittle fracture.Some morphology characteristics like cleavage facet,cleavage step,river pattern,tear edge and so force can be observed on fracture surface.The acicular martensite structure can increase the hardness of grains and destroy the uniformity of microstructure.The porosity and crack defects in the weld increase the brittle tendency of the joints,and the crack is more harmful than the pore.When welding speeds were 0.01m/s,0.012m/s and 0.015m/s,the stress-life relationship were respectively (?).This shows that at the same stress level,the welding joint can obtain a higher fatigue life when the welding speed is 0.015m/s.Under the condition of external cyclic loading,fatigue crack easily was initiated near the weld surface defects.The fatigue life of welded joints depends on the location of fatigue crack initiation.The dense pores accumulation of close to the weld surface is the most harmful on the fatigue performance of welded joints.The harm of welding crack is bigger than that of the single porosity.