Volume 37, Issue 5 -

Trailing intensive cooling with liquid nitrogen has successfully applied to friction stir welding of 2 mm thick 2060 Al-Li alloy. Welding temperature, plastic strain, residual stress and distortion of 2060 Al-Li alloy butt-joint are compared and discussed between conventional cooling and trailing intensive cooling using experimental and numerical simulation methods. The results reveal that trailing intensive cooling is beneficial to shrink high temperature area, reduce peak temperature and decrease plastic strain during friction stir welding process. In addition, the reduction degree of plastic strain outside weld is smaller than that inside weld. Welding distortion presents an anti-saddle shape. Compared with conventional cooling, the reductions of welding distortion and longitudinal residual stresses of welding joint under intense cooling reach 47.7 % and 23.8 %, respectively.

Potassium hexatitanate (K 2 Ti 4 O 9 ) whiskers were prepared by the kneading–drying–calcination method. After the preparation of products under different calcination temperatures and holding times, their morphology and structure were characterized by thermogravimetric and differential thermal, X-ray diffraction (XRD), scanning electron microscopy and transmission electron microscopy. The XRD analysis showed that the reaction mixture was completely converted to K 2 Ti 4 O 9 crystals at 800 °C when the T/K ratio was 3. Based on the analysis of LS (liquid–solid) growth mechanism, the corresponding transformation reaction mechanism during the roasting was elucidated. K 2 Ti 4 O 9 whiskers grow mainly through the parallel action at a low temperature. With the increase in temperature, the series effect is obvious.

The application of accurate constitutive relationship in finite element simulation would significantly contribute to accurate simulation results, which plays a critical role in process design and optimization. In this investigation, the true stress–strain data of 3Cr20Ni10W2 heat-resisting alloy were obtained from a series of isothermal compression tests conducted in a wide temperature range of 1203–1403 K and strain rate range of 0.01–10 s −1 on a Gleeble 1500 testing machine. Then the constitutive relationship was modeled by an optimally constructed and well-trained back-propagation artificial neural network (BP-ANN). The evaluation of the BP-ANN model revealed that it has admirable performance in characterizing and predicting the flow behaviors of 3Cr20Ni10W2 heat-resisting alloy. Meanwhile, a comparison between improved Arrhenius-type constitutive equation and BP-ANN model shows that the latter has higher accuracy. Consequently, the developed BP-ANN model was used to predict abundant stress–strain data beyond the limited experimental conditions and construct the three-dimensional continuous response relationship for temperature, strain rate, strain, and stress. Finally, the three-dimensional continuous response relationship was applied to the numerical simulation of isothermal compression tests. The results show that such constitutive relationship can significantly promote the accuracy improvement of numerical simulation for hot forming processes.

Microstructure and tensile properties of P92 steel in the normalized and tempered, and thermal aged at 923 K for 5000 h and 10,000 h conditions have been investigated. Laves phase precipitate was observed in the thermal-aged steels. The size of Laves phase precipitate increased with increase in thermal exposure. This was also confirmed from the observation that the area fraction of Laves phase precipitate was higher in the 5000 h aged condition which decreased with further increase in thermal exposure. On the other hand, the size and area fraction of M 23 C 6 precipitate were found increased in the 5000 h aged steel, further continued to enhanced precipitation of fine M 23 C 6 in the 10,000 h aged steel. This resulted in significant increase in area fraction and comparable size with the steel aged for 5000 h. Hardness of the steel was decreased with increase in the duration of ageing. Thermal-aged steels exhibited lower yield stress, ultimate tensile strength and relatively higher ductility in comparison with steel in the normalized and tempered condition. The increase in lath width and recovery of dislocation structure under thermal-aged condition resulted in lower tensile strength and hardness. An extensive Laves phase formation and coarsening by loss of tungsten in the matrix led to decrease in the tensile strength predominantly in the 5000 h aged steel. The tensile strength of 10,000 h aged steel was comparable with that of 5000 h aged steel due to enhanced precipitation of fine M 23 C 6 in the steel due to enhanced mobility of carbon in the absence of tungsten in the matrix.

The major challenges for conventional diffusion bonding of joining Ti alloy and Al alloy are the undesirable interfacial reaction, low matrixes and joint strength. To avoid the problem in diffusion bonding, a novel two-stage sintering powder metallurgy process is developed. In the present work, the interface characterization and joint performance of the bonds obtained by powder metallurgy bonding are investigated and are compared with the diffusion bonded Ti/Al joints obtained with the same and the optimized process parameters. The results show that no intermetallic compound is visible in the Ti/Al joint obtained by powder metallurgy bonding, while a new layer formed at the joint diffusion bonded with the same parameters. The maximum tensile strength of joint obtained by diffusion bonding is 58 MPa, while a higher tensile strength reaching 111 MPa for a bond made by powder metallurgy bonding. Brittle fractures occur at all the bonds. It is shown that the powder metallurgy bonding of Ti/Al is better than diffusion bonding. The results of this study should benefit the bonding quality.

Evolution for the dynamic recrystallization (DRX) volume fraction of Ti-5Al-5Mo-5V-3Cr-1Zr near β titanium alloy during hot deformation was characterized by using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. To determine the equation parameters, a series of thermal simulation experiments at the temperature of 1023–1098 K and strain rate of 0.001–1 s ‒1 to the true strain of 0.7 were conducted to obtain the essential data about stress σ and strain ε . By further transforming the relationship of σ versus ε into the relationship of strain hardening rate d σ /d ε versus σ , two characteristic strains at the beginning of DRX (critical strain ε c ) and at the peak stress (peak strain ε p ) were identified from the d σ /d ε-σ curves. Sequentially, the parameters in the JMAK equation were determined from the linear fitting of the different relationships among critical strain ε c , peak strain ε p and deformation conditions (including temperature T , strain rate ε˙ $\dot \varepsilon $ and strain ε ). The as-obtained JMAK equation was expressed as X DRX =1-exp[-0.0053(( ε-ε c )/ ε c ) 2.1 ], where ε c =0.6053 ε p and ε p =0.0031ε˙ $\dot \varepsilon $ 0.0081 exp(28,781/ RT ). Finally, the JMAK equation was implanted into finite element program to simulate the hot compression of thermal simulation experiments. The simulation predictions and experimental results about the DRX volume fraction distribution showed a good consistency.

The weld pool contains abundant information about the welding process. In particular, the type of the weld pool surface shape, i. e., convex or concave, is determined by the weld penetration. To detect it, an innovative laser-vision-based sensing method is employed to observe the weld pool surface of the gas tungsten arc welding (GTAW). A low-power laser dots pattern is projected onto the entire weld pool surface. Its reflection is intercepted by a screen and captured by a camera. Then the dynamic development process of the weld pool surface can be detected. By observing and analyzing, the change of the reflected laser dots reflection pattern, for shape of the weld pool surface shape, was found to closely correlate to the penetration of weld pool in the welding process. A mathematical model was proposed to correlate the incident ray, reflected ray, screen and surface of weld pool based on structured laser specular reflection. The dynamic variation of the weld pool surface and its corresponding dots laser pattern were simulated and analyzed. By combining the experimental data and the mathematical analysis, the results show that the pattern of the reflected laser dots pattern is closely correlated to the development of weld pool, such as the weld penetration. The concavity of the pool surface was found to increase rapidly after the surface shape was changed from convex to concave during the stationary GTAW process.

The chemical composition and fracture toughness of thermal barrier coatings (TBCs) before and after heat treatment were characterized, and the cracks around the interface between the coating and the substrate could be successfully eliminated and meanwhile the porosity of the coatings tended to reduce. The XRD analysis revealed the coatings were composed of non-transformable tetragonal t’ phase of ZrO 2 and γ $\gamma $-(Ni, Cr) with minor Ni3Al (γ′ $\gamma ^' $) precipitates. Additionally, the relationship between the heat treatment and wear resistance was systematically studied. The results indicated that both the hardness and fracture toughness increased after quenching process. The oxidation wear became more prominent after heat treatment, which probably resulted from the better bonding strength of coatings. Dense and homogeneous microstructure introduced by vacuum oil-quenching improved stabilization of the weight gain during thermal cycle test.

The iso-thermal crystallization behavior of phosphate-enriched phase has been experimentally investigated in the rapidly quenched CaO–SiO 2 –FeO–Fe 2 O 3 –P 2 O 5 steelmaking slags under different cooling schedules. The experimental results indicate that increasing endpoint temperature from 1453 to 1533 K and prolonging holding time from 2 to 60 min can result in an increasing tendency of the size of phosphate-enriched phase in the shape of one-dimensional rod. The crystallization kinetics of phosphate-enriched phase in steelmaking slags has been described by Avrami equation. The Avrami constant n$$n$$ was obtained to be 0.472, while the crystallization rate constant k$$k$$ was recommended as lnk=57.40+12,273.96/T−8.25lnT−5.5×10−3T$$\ln k{\rm{= 57}}{\rm{. 40 + 12,273}}{\rm{. 96}}/T - {\rm{8}}{\rm{. 25}}\,\ln T - {\rm{5}}{\rm{. 5}\times{\rm 10}^{- 3}}T$$. Thus, the apparent activation energy E$$E$$ of crystallization is recommended as E=537.60−206.015T$$E{\rm{= 537}}{\rm{. 60}} - {\rm{206}}{\rm{. 015}}T$$ kJ/mol.

As a special hardenable α titanium alloy, Ti-2.5 Cu alloy was a candidate material for high temperature components requiring high strength and plasticity. The effect of prestrain on the precipitation behaviors was investigated in the present study. Tensile tests show that elongation up to 22 % can be obtained after solid solution (SS) treatment. Thereafter, prestrain in tension with 5 %, 10 %, 15 % and 20 % was carried out for the SS samples and then duplex aging was applied. Transmitting electron microscopy (TEM) investigations show that larger Ti 2 Cu particles were observed in the prestrained condition than free aging one, as prestrain significantly speeds up the precipitation kinetics. The strength firstly increases and then decreases for the prestrained samples after duplex aging, where the competition between precipitation hardening and recovery softening should be responsible. With the consideration of SS, precipitation and recovery, a strength model for duplex aging combined with prestrain was established, which is in well agreement with experiments. Present study may provide a promising way to obtain the strength of deformed hcp materials in industry application.

Equilibrium phases in alloy A286 and the influence of element contents on gamma prime (γ′) and carbides were studied by Thermo-Calc calculation. The phase equilibria, solidification behavior, and precipitation behavior of equilibrium phases were investigated. In the results, Al is considered as the critical element for γ′ phase control, and C is the vital element for the regulation of MC and M 23 C 6 phases. The influence extents of different elements on the transition temperature and the maximum content of γ′ phase, MC phases, and M 23 C 6 phases are determined in order as Al>Ti>Cr>Mo, C>Ti>Ni>Cr>Mo, and C>Ni>Cr>Mo, respectively.