The interface of Fe₃Al/Q235 dissimilar materials joint, which was made by vacuum diffusion welding, combines excellently. There are Fe₃Al, FeAl phases and 𝛼-Fe (Al) solid solution at the interface of Fe₃Al/Q235. Aluminum content decreases from 28% to 1.5% and corresponding phase changes from Fe₃Al with DO₃ type body centred cubic (bcc) structure to 𝛼-Fe (Al) solid solution with B2 type bcc structure. All phases are present in sub-grain structure level and there is no obvious brittle phases or micro-defects such as pores and cracks at the interface of Fe₃Al/Q235 diffusion joint.

The chemical composition of the second phase precipitation in the vacuum diffusion-bonded zone of Fe₃Al intermetallic compound and Q235 carbon steel was analysed by means of electron probe microanalyser (EPMA). The relative content of the second phase precipitation and grain size was evaluated through a micro-image analyser. The percentage of Fe and Al content in the diffusion zone was measured by EPMA. The results indicated that the relative content of the second phase precipitation rich in carbon and chromium at the Fe₃Al/Q235 interface was much higher. With the transition from Fe₃Al intermetallic compound to Q235 carbon steel across Fe₃Al/Q235 interface, the grain diameter decreased from 250 𝜇m to 112 𝜇m, Al atom content decreased from 27% to 15%, while Fe atom content increased from 76% to 96%.

The effect of microstructural characteristics on fracture behaviour mechanism for electron beam welding of Ti–6Al–4V was investigated. The results indicated that the welded microstructure composed of coarse needle 𝛼 + 𝛽 phases presenting disordered and multidirectional short needle morphology to make fracture mechanism complex. The coarse grains in weld seam with microhardness 536 HV were easy to be fractured in the region where welding heat input was ≥ 68.8 kJ/m. There exists flat curves of Ti, Al and V, Fe concentration distribution fluctuation to cause microstructural amplitude-modulated decomposition to increase the joint ductility and cleavage strength. The uneven distribution of the partial micropores located at the interior of the specimen acting as crack initiation sites lead to non-linear branch propagating path. The 𝛼 + 𝛽 interlaced structure results in the fracture location near 𝛼/𝛽 interface. The existence of stacking fault structure caused pile-up of dislocation to produce micropores to be new fracture initiation sites.