Ti6Al4V (Ti64) alloys manufactured by selective laser melting (SLM) are well known for their susceptibility to failure at a low ductility of less than 10% due to the formation of an (α′) martensitic structure. Annealing and solution treatments as post-heat treatments of α′ are considered a good way to improve the mechanical performance of SLM-manufactured Ti64 parts. In this research, the effect of heat treatment parameters such as temperature (850 °C and 1020 °C) and cooling rate (furnace and water cooling) on the microstructure and mechanical properties of the SLM Ti64 structure was investigated. It was shown that the tensile strength/ductility of the Ti64 alloy produced by SLM was determined by the post-heat treatment. The experimental results revealed that heat treatment at 850 °C followed by furnace cooling resulted in the best possible combination of ductility (13%) and tensile strength (σ_y = 932, σ_u = 986 MPa) with a microstructure consisting mainly of 78.71% α and 21.29% β. Heat treatment at 850 °C followed by water cooling was characterized by a reduction in hardness and the formation of predominantly α plus α′′ and a small amount of β. HT850WC exhibited yield and tensile strengths of about 870 and 930 MPa, respectively, and an elongation at fracture of 10.4%. Heat treatment at 1020 °C and subsequent cooling in the furnace was characterized by the formation of an α + β lamellar microstructure. In contrast, heat treatment at 1020 °C and subsequent water cooling formed semi-equiaxial β grains of about 170 µm in diameter with longer elongated α grains and basket-weave α′. Post-treatment at 1020 °C followed by furnace cooling showed high ductility with an elongation of 14.5% but low tensile strength (σ_y = 748, σ_u = 833 MPa). In contrast, post-treatment at 1020 °C followed by water cooling showed poor ductility with elongation of 8.6% but high tensile strength (σ_y = 878, σu = 990 MPa). The effect of aging at 550 °C for 3 h and cooling in a furnace on the microstructure and mechanical properties of the specimens cooled with water was also studied. It was found that aging influenced the microstructure of the Ti6Al4V parts, including β, α, and α″ precipitation and fragmentation or globularization of elongated α grains. The aging process at 550 °C leads to an increase in tensile strength and a decrease in ductility.

The paper aims at investigating the microstructure, failure mode transition, peak load and energy absorption of DP600 dual phase steel during the tensile-shear test. It was found that the welding current has profound effect on the load–displacement characteristics. In the low welding current, welds failed in interfacial failure mode. Increasing welding current resulted in sufficient weld nugget growth to promote double-sided pullout failure mode with improved mechanical properties. Further increase in the welding current caused expulsion and failure mode was changed to single-sided pullout with reduced energy absorption capability. It was found that the fusion zone size is the key parameter controlling the mechanical properties of DP600 resistance spot welds in terms of peak load, maximum displacement and failure energy.

Phase transformations in dissimilar resistance spot welds of dual phase steel and ferritic stainless steel are analysed. In contrast to a full martensitic microstructure predicted by the Schaeffler and Balmforth diagrams, a ferrite–martensite microstructure was observed in the fusion zone. The formation of ferrite phase in the fusion zone can be attributed to the rapid cooling rate of resistance spot welding, which suppresses the post-solidification ferrite–austenite transformation. The grain growth and martensite formation were main metallurgical features of the heat affected zone of ferritic stainless steel side. Microstructure gradient of heat affected zone in dual phase steel side was dictated by martensitic transformation. The effect of weld thermal cycle on the mechanical performance of the joint is discussed.

A hybrid powder of Ti6Al4V, which consists of 50 wt% plasma atomised (PA) spherical and 50 wt% hydride-dehydride (HDH) irregular-shaped powder with a flowability of 36.5 s, is projected as the raw materials in this research. In addition, Ti6Al4V (Gr.23) plasma atomised 100 wt% spherical powder was used as the reference powder with a flowability of 28 s to facilitate the tensile property discussion. It was found that the powder flowability is the crucial parameter governing the tensile properties of fabricated specimens. It was shown that the decrease in powder flowability (36.5 s) resulted in increases in susceptibility to the formation of a lack of fusion defects with dropped tensile properties. The microstructure of the fabricated Ti6Al4V alloy features the formation of prior β grain boundaries and an alpha prime (α′) martensite. Furthermore, no intermetallic aluminide phase (Ti₃Al) precipitations in grain boundaries were detected through energy-dispersive X-ray spectroscopy analysis.