Ze Kun Yao

Effect of post-weld heat treatment on microstructure and properties of electron beam welded joint of Ti2AlNb/TC11

Abstract The present paper reports the influence of post-weld heat treatment (PWHT) on microstructure and properties of electron beam welded dissimilar joint. Ti2AlNb and TC11 alloys were used to fabricate the joints. Three PWHTs were applied to the welded joints. The structures were analysed using optical microscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy techniques. The results show that weld metal of the fusion zone is mainly composed of α2 and β phases. As the energy input increases under different PWHTs, the decomposition degree of metastable phases (α′/β) rises, but the tensile strength and impact toughness of the joint reduce. Under each condition, the tensile strength of the joint is higher than that of the TC11 base metal.

Effect of isothermal deformation on microstructure and properties of electron beam welded joint of Ti2AlNb/TC11

Abstract The present paper reports the influence of hot work (isothermal deformation accompanied with heat treatment) on microstructure and properties of electron beam welded dissimilar joint. Ti2AlNb alloy and TC11 alloy were used to fabricate the joints. Isothermal deformation and heat treatment were performed under certain conditions. The structures were analysed using optical microscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results show that the as welded metal of Ti2AlNb/TC11 joint is mainly composed of α2 and β phases. The metastable β phase transformed into α+β phases during deformation and heat treatment processes. There are no big differences in tensile strength of joints under different conditions. However, the impact toughness of the weld has improved 72% after hot work.

Microstructure evolution of friction welded dissimilar joint during subsequent hot work

Abstract The present paper reports the influence of hot working conditions on the microstructure of Ti2AlNb/TC11 dissimilar joint. Linear friction welding technique was used to fabricate the joints. The microhardness and tensile properties of the joints have been tested. It was found that the fine structure of linear friction welds underwent abnormal grain growth and abnormal grain boundary phase growth in the post-weld solution heat treated condition. This phenomenon significantly deteriorated the ductility of the joint. After appropriate hot work, abnormal big grains/phases disappeared, and the joint exhibited good tensile properties due to its fine structures.

Microstructural Evolutions of a High Temperature Titanium Alloy Processed by Thermal Mechanical Treatments

TG6 alloy is a new titanium alloy which has been designed to reach the service temperature of 600°C. The microstructural evolutions of TG6 alloy under different thermal mechanical treatments were studied. It was found that lamellar microstructures with basketweave α lamella are obtained for TG6 alloy forged in β field. The evolution mechanism is transformed from dynamic recrystallization to dynamic recovery and the thickness of α lamella increases with increasing forging temperature. The aspect ratio of α lamella decreases firstly and then increases with increasing deformation degree. Grain boundary α lath appears when the deformation degree is less than 40%. The thermal mechanical treatments, including deformation in β field firstly and then deformation in α+β field result in the transformations of the microstructures drastically. The deformation degree in α+β field decides the final microstructure. The deformation degree of 20% in α+β field results in partial globular α phase.

Linear Friction and Electron Beam Welded Joints of Ti2AlNb/TC11

Dissimilar joining of Ti-22Al-25Nb alloy and an α+β titanium alloy TC11 were carried out using electron beam process and linear friction process, respectively. The microstructure and tensile properties of the joints were investigated. The results show that both EBW and LFW could be adopted to the dissimilar joining of Ti-22Al-25Nb/TC11. The EB welded joint exhibits higher strength than that of the LF welded joint, due to the precipitation of O phase in the fusion zone of the former one. These results can be expected to be of great technical interest as basic data for the use of EBW or LFW in aerospace industry.

Research of Superplasticity Deformation Behavior about Coarse-Grain Ti-22Al-25Nb Alloy

This study investigates the superplastic deformation mechanism of coarse-grain materials .Superplasticity deformation behavior of Ti-22Al-25Nb alloy with 200~410μm coarse grain has been investigated through tensile test at 940~990°C and 3.3× (10-2~10-4) s-1 strain rate. Results prove coarse-grain Ti-22Al-25Nb alloy primary depends on slipping of crystal planes between two phases to achieve superplasticity. Transformation from lath O phase to ring equiaxed O phase occurs as temperature rises and its volume percent falls, but percentage of recrystallized B2 phase increases. Dislocation can move round ring equiaxed O phase. Good plasticity can be preserved because O phase can flow as solid particle in semisolid slurry in B2 grain.

Structure Evolving at Bonding Interface of Dual-Alloys Jointed with Different Method under Coupling Action of Heat and Force

During near isothermal forging and heat treatment structure change of bonding interface in Ti3Al/TC11 and Ti2AlNb/TC11 dual alloys jointed with different method has been investigated. The results show that the solidification structure at dual-alloy joint welded by electron beam in vacuum has evolved into forging structure, columnar grains have been changed into equiaxed grains through breaking, crystal lattatice rebuild and re-crystallizing, and the mechanical bonding plus metallurgical bonding structure at joint welded by linear friction weld has transformed into metallurgy structure altogether, because constant high temperature during near isothermal forging can cause the diffusion of alloy elements and reconstruction of lattice structure.

Research on the Mechanism of Interface Strengthening for Ti3Al/TC11 Dual Alloy

The Ti3Al/TC11 dual alloy bar joined by electron beam welding was deformed by near isothermal forging and then processed by gradient heat treatment. Afterwards, the mechanism of interface strengthening has been investigated according to the microstructure evolution and properties. The results show the brittle phases formed in the re-solidification can be broken by near isothermal forging; the lattice distortion energy is increased obviously due to deformation, so the nucleation ratio of recrystallization is improved, which method is fine-crystal strengthening. After the dual alloy deformed by near isothermal forging is further processed by gradient heat treatment, the room temperature tensile strength of the joint is higher than that of the Ti3Al alloy; the high temperature tensile strength of the joint is equal to or higher than that of the TC11 alloy. The reason is that the finer and phases are precipitated from the phase transus microstructure in the welding interface, which produces the effects of fine-crystal strengthening and dispersion strengthening.

Temperature Effects of ECAP and Annealing after ECAP on Microstructure of TA15 Alloy

A study was conducted by optical microscope (OM) and transmission electron microscope (TEM) on the microstructure evolution of TA15 alloy by severe plastic deformation (SPD) and annealing after SPD. In this study, equal channel angular pressing (ECAP) was taken as the method of SPD. The chief aim of the present work is to investigate the temperature effects of ECAP and annealing after ECAP on microstructure of TA15 alloy. The results indicate that equiaxed microstructure has been obtained by ECAP at the temperatures of α+β phase region, and that with the increase in pressing temperature, equiaxed grains have become coarser and the content of α phase has reduced. β grains have been coarsened severely since the pressing temperature was above the α-β transformation temperature (Tβ). Annealed at proper temperature after ECAP, the α phase of TA15 alloy has been more homogeneous, prior α phase has been well globularized, and grains have not grown obviously. According to the testing of TA15 alloy, the optimized temperature parameters of ECAP and annealing after ECAP are 900°C and 700°C. Observation and Analysis of the TEM morphological images illustrate that a quantity of twinning deformations have been produced by ECAP at the temperatures below Tβ, which leads to the continued plastic deformation through the restarting of many slip bands.

High strain rate superplasticity of ultrafined Ti–10V–2Fe–3Al compact prepared by sintering and isothermal forging

Abstract High strain rate superplasticity was obtained for powder Ti–10V–2Fe–3Al (Ti-1023) alloy prepared by powder sintering and isothermal forging technology. The selected powder was cold isostatic pressed, sintered and isothermal forged to prepare this powder alloy. Tensile testing was conducted at optimum superplastic temperaure of 1023 K with different initial strain rate, and the elongation to failure, the flow stress and the microstructure were analysed. The experiment results exhibited that the microstructure of this powder alloy is extraordinary uniform and fine, resulted in considerable enhancement of optimum initial strain rate increased from 3·3×10−4 s−1 of conventional cast and wrought Ti-1023 alloy to 3·3×10−3 s−1 of this powder alloy. The elongation to failure increased first and then decreased with initial strain rate from 3·3×10−4 to 3·3×10−2 s−1. The strain rate sensitivity m is about 0·46 near initial strain rate of 3·3×10−3 s−1, larger than conventional cast and wrought Ti-1023 alloy. Microstructure observations showed that dynamic recrystallisation and grain growth were present during superplastic deforming.