Chun Qin

Hot deformation behavior of TC11/Ti–22Al–25Nb dual-alloy in isothermal compression

Abstract The high-temperature flow behavior of TC11/Ti–22Al–25Nb electron beam (EB) weldments was investigated by the isothermal compression tests at the temperature of 900–1060 °C and the strain rate of 0.001–10 s −1 . Based on the experimental data, the constitutive equation that describes the flow stress as a function of strain rate and deformation temperature is obtained. The apparent activation energy of deformation is calculated, which decreases with increasing the strain and the value is 334 kJ/mol at strain of 0.90. The efficiency of power dissipation η changes obviously with the variation of deformation conditions. Under the strain rates of 0.01, 0.1 and 1 s −1 , the value of η increases with increasing the true strain for different deformation temperatures. While the value of η decreases with increasing the strain under the strain rates of 0.001 and 10 s −1 . The optimum processing condition is ( t opi =1060 °C, opi =0.1 s −1 ) with the peak efficiency of 0.51. Under this deformation, dynamic recrystallization (DRX) is observed obviously in the microstructure of welding zone. Under the condition of 1060 °C and 0.001 s −1 , the deformation mechanism is dominated by dynamic recovery (DRV) and the value of η decreases sharply ( η =0.02). The flow instability is predicted to occur since the instability parameter ξ () becomes negative. The hot working process can be carried out safely in the domain with the strain rate of 0.001–0.6 s −1 and the temperature of 900–1060 °C.

Microstructure characterization and mechanical properties of TC4-DT titanium alloy after thermomechanical treatment

Influence of thermomechanical treatments (mill annealing, duplex annealing, solution treatment plus aging and triple annealing) on microstructures and mechanical properties of TC4-DT titanium alloy was investigated. Results showed that thermomechanical treatments had a significant influence on the microstructure parameters and higher annealing and aging temperature and lower cooling rate led to the decrease of the volume fraction of primary α and the size of prior-β and the increase of the width of grain boundary α and secondary α. The highest strength was obtained by solution treatment and aging due to a large amount of transformed β and finer grain boundary α and secondary α at the expense of slight decrease of elongation and the ultimate strength, yield strength, elongation, reduction of area were 1100 MPa, 1030 MPa, 13% and 53% separately. A good combination of strength and ductility has been obtained by duplex annealing with the above values 940 MPa, 887.5 MPa, 15% and 51% respectively. Analysis between microstructure parameters and tensile properties showed that with the volume fraction of transformed β phase and the prior-β grain size increasing, the ultimate strength, yield strength and reduction of area increased, but the elongation decreased. While the width of grain boundary α and secondary α showed a contrary effect on the tensile properties. Elimination of grain boundary α as well as small prior-β grain size can also improve ductility.

Effect of hot working on microstructure and mechanical properties of TC11/Ti2AlNb dual-alloy joint welded by electron beam welding process

Abstract The influence of hot working on the microstructures of TC11/Ti2AlNb dual-alloy joints welded by electron beam welding (EBW) process was investigated. The tensile tests were performed at room temperature for specimens before and after thermal exposure. The results show that the fusion zone of TC11/Ti2AlNb dual-alloy joint welded by EBW is mainly composed of β phase. After deformation and heat treatment, the grain boundaries of the as-cast alloy are broken and the fusion zone mainly consists of β, α2 and α phases. The fusion zone performs poor property in the tensile test. Specimens before and after thermal exposure all fail in this area under different deformation conditions. The ultimate tensile strength of specimens after heat treatment is up to 1190 MPa at room temperature. The joints by water quenching after deformation have better plasticity with an elongation up to 4.4%. After thermal exposure at 500 °C for 100 h, the tensile strength of the specimen slightly rises while the ductility changes a little. SEM observation shows that the fracture mechanism is predominantly transgranular under different deformation conditions.

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.

Isothermal Beta Grain Growth Kinetics of TC4-DT Titanium Alloy under Two Different Prior Processing Conditions: Deformed vs. Undeformed

Abstract β grain growth kinetics of TC4-DT titanium alloy under two different prior processing conditions (deformed vs. undeformed) was investigated during isothermal heat treatments. Deformation was carried out in α+β two-phase field with deformation degree of 60% and air cooling. Heat treatment scheme with the temperatures above β transus temperature 10, 20, 30 °C and soaking time of 2, 5, 10, 30, 60, 120 min was used. The grain size parameters have been obtained by means of the image analysis software. Growth time exponent (n) and activation energy (Q) have been established for both conditions. For the undeformed samples, n varied in the range of 0.34∼0.35 and Q in the range of 86.8∼130 kJ·mol−1, while after deformation, the values of n and Q changed in the domain of 0.36∼0.39 and 76.6∼110 kJ·mol−1, respectively. Results show that the samples subjected to deformation exhibit higher growth time exponent and lower activation energy compared to the samples without deformation under the same heat treatment conditions. The variations of the growth time exponent with temperature and the activation energy with soaking time are ascribed to the interaction between diffusion of solute atoms and migration of grain boundaries both of which can be promoted by deformation. The influences of heating temperature, time and deformation on the uniformity of grain size were also studied and the intrinsic mechanism is attributed to the combined effect of β phase nucleation rate and β phase growth velocity under the above factors.

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.

Influence of Hot Working Conditions on Mechanical Properties and Microstructures at Weld Zone of the Ti3Al /TC4 Dual Alloy

The influences of different hot working conditions on the microstructures and mechanical properties at weld zone of the Ti3Al/TC4 dual alloy have been investigated. The experimental results show that forging and heat treatment has significant influence on room temperature tensile strength (RTTS for short) of the weld zone. The influence of forging strain and temperature on its elongation at temperature 500°C is also conspicuous. The results of intuitional and variance analysis show that affecting factors of its RTTS are heat treatment, forging strain, forge temperature and condition of billet in sequence. The affecting factors of its room temperature (RT for short) ductility and the tensile strength (TS for short) at temperature 500 °C are forging strain, forge temperature, heat treatment and condition of billet. Good comprehensive properties are obtained under optimal conditions: forging at 1020°C, annealing at 700°C for 12 hours, air cooled, about 40% strain and as-rolled billet. The microstructures of the weld zone worked according as those conditions are uniform and the grain size is also fine.