•Dissimilar electron beam welding of high-Nb containing TiAl/Ti600 alloys was investigated for the first time.•The effect of thermal compensation on microstructure and mechanical properties of welded joints was studied.•Thermal compensation could obtain free-crack joint and improve the strength and plasticity of the joint.•An intrinsic change of microstructure was presented to interpret the improvement of strength and plasticity of the joint.High-Nb containing TiAl (TNB) and Ti600 alloys are the alternative materials for the tail-nozzle structure of rocket engines because of their low density, high specific strength and high temperature performance. This paper aims to investigate the microstructure evolution and the change in mechanical properties of the TNB/Ti600 electron-beam welded joints after thermal compensation treatment by electron beam defocusing mode. Compared with the electron beam welding of no-thermal compensation treatment, results showed that the cold cracks of the joint could be eliminated effectively by thermal compensation treatment. The microstructure in the fusion zone was composed of fine rod-shaped α-Ti + α2-Ti3Al phase. Moreover, the precipitated phase in heat affected zone on the TNB alloy side increased noticeably. The hardness of fusion zone considerably diminished to 400 HV, at the same time both of the tensile strength of the joints at room temperature and 600 °C increased substantially, reached 601 and 613 MPa respectively, consisting with the evolution of the microstructure of the joint.Download high-res image (309KB)Download full-size image

High Nb containing TiAl alloys showed susceptibility to cold cracking when welding was performed without preheating and post-heating; however, crack-free welds could be made with pre- and post-heating. The room-temperature strength of the crack-free welds was 480 MPa, and failure occurred in the base metal. The fusion zone obtained at a beam current of 7 mA mainly comprised coarse β/B2-phase grains, with acicular α2 and γ phases distributed at the grain boundaries. The fusion zone obtained at 13 mA mainly contained fine γ- and α2-phase grains. The fusion zone obtained with pre- and post-heating was dominated by the acicular α phase, reticular β/B2 phase, and blocky γ phase. The heat-affected zone could be divided into two characteristic regions according to the morphologies of the α2 and γ phases. Changes in microhardness were analyzed with respect to the phase composition and microstructural morphology.

A commercial Ti-Zr-Ni-Cu amorphous filler foil was applied to braze the high-temperature Ti600 and Ni-25at%Si, resulting in the good joint between both of alloys. The interfacial microstructure of Ti600/Ti-Zr-Ni-Cu/Ni-25at%Si brazed joint at 1213 K for 10 min is mainly comprised of the continuous Ti2Ni phase. Most of Ti-rich phases such as α-Ti, β-Ti, Ti3Al, Ti2Ni, (Ti,Zr)2(Ni,Cu) and (Ti,Zr)2Si formed in the vicinity of the Ti600 substrate except that Ti5Si3 precipitates near the Ni-25at%Si region, resulting in the formation of six distinct layers. The corresponding formation mechanism were clarified. The Ti5Si3 and Ni31Si12 were firstly formed during the initial stage followed by diffusion reactions at Ni-25at%Si side. The continuous Ti2Ni phase formed through the reaction of Ti(L) + Ni(L) → Ti2Ni. α-Ti, Ti3Al, (Ti,Zr)2(Ni,Cu) and (Ti,Zr)2Si in the brazing joint precipitated through a solid-solid phase transformation.

•Direct solidification of Ni3Si from the liquid is suppressed.•Full lamellar structures was obtained from Ni-25 at%Si melts.•β1-Ni3Si lamellas exist between Ni-rich lamellas and Ni31Si12.•Star-like β1-Ni3Si appears in Ni-rich lamellas.Directional solidification experiments have been performed on Ni-25 at% Si alloy using electron beam floating zone method. The Ni-25 at % Si hypereutectic alloy can form a fully regular eutectic microstructures consisting of γ-Ni31Si12 and Ni, with thin β1-Ni3Si layers and star-like β1-Ni3Si existing at the boundary and in the Ni-rich lamellas, respectively. The results indicate that there is full lamellar αNi-γ (Ni31Si12) eutectic structures at the nominal composition of Ni3Si which can be develop as in situ composite with improved toughness at the nominal composition of Ni3Si.

Electron beam welding experiments on 2.5-mm-thick as-forged Ti–43Al–9V–0.3Y intermetallic plates were carried out. Cold cracking occurred in the direct electron beam welding joints because of the brittleness of the microstructure and the residual stress. A crack-free electron beam welding joint was obtained by a new composite control method. This process can prolong the high temperature stage to promote the phase transformation and to give the expected microstructure of the joint. The microstructures and tensile strengths of the joints were examined by TEM, X-ray diffraction, electron back scattering diffraction and universal test machine. The weld obtained by the composite control method was characterized by an analysis of the combined phase structure of the γm grain and the γ/α2 colony microstructure. The tensile strength of the joint that was obtained by the composite control method was found to be 411.3 MPa, which is 20% higher than the direct welding joint.Highlights► Crack free electron beam welded TiAl alloy was obtained by composite control method. ► The weld obtained by composite control method was characterized by γmγm grain and γ/α2γ/α2 colony. ► Grains in the weld was refined, which is help to reduce the local thermal stress. ► Increase of the deformed structure in the weld can release thermal stress.

•Cracks easily occurred in electron beam welded hard alloy to steel joints without intermediate.•The ductility of the weld without intermediate was poor and large residual stress would occur in the hard alloy base metal.•Effective bonding between hard alloy and steel can be obtained with Ni-based intermediate by electron beam welding.•The weld microstructure was mainly composed of (Fe,Ni) solid solution and dispersedly distributed carbides.The direct welding of hard alloy to steel by electron beam was carried out. The weld was mainly composed of molten steel. The microstructure of joint mainly consisted of the cellular dendrites and the eutectic phases. A transition layer enriched with large amounts of carbides formed at the interface of hard alloy. Finite element analysis of the welding sequence indicated that the maximum residual stress distributed at the weld or the hard alloy base metal where the macroscopic cracks tended to generate. According to the metallurgical compatibility principle, the cracks can be avoided by a powder metallurgy Ni–Fe intermediate during electron beam welding–brazing of hard alloy to steel. The microstructure of the joint was composed of (Fe,Ni) solid solution and carbides. The transition layer enriched with brittle phases did not form at the interface in the hard alloy side. Therefore, the microhardness of the weld was reduced and the compatible deformation ability of the weld was improved, and thus macroscopic cracks could be avoided.