Hongze Fang

Ultrasonic stress evaluation on welded plates with Lcr wave

Abstract The acoustical theory is introduced. Ultrasonic stress measurement experimental installation is established, consisted of a variable angle transducer and an oscillograph with special higher digitiser board. Acoustoelastic coefficient of longitudinal critically refracted waves propagating in Q-235 steel is calibrated by the tensile testing machine. In the present paper are discussed the longitudinal stress and transverse stress on twin wire welded plate, and a comparison is made with the result of finite element method obtained by Marc Software. The present paper also discusses the longitudinal stress distributions of multilayer welded plate and longitudinal residual stress gradient of 2219 Al alloy is obtained in the ultrasonic travel time change expression formal. Instability acoustoelasticity effect in elastic region of welding residual stress field is discovered. The sources of measurement error are pointed out. In conclusion the potential of the ultrasonic method in different industrial cases is particularised.

New technique to control welding buckling distortion and residual stress with non-contact electromagnetic impact

Abstract A new technique using non-contact electromagnetic forces has been proposed for controlling welding buckling distortion and residual stresses in welded thin plates. The experimental results show that the method can successfully eliminate the buckling distortion and reduce the residual stresses. Three-dimensional finite element modelling has been developed to study the evolution of the stress and strain throughout the welding and electromagnetic impacts. The predicted welding distortion and residual stresses are in good agreement with the experimental results. The numerical analyses show that the reduction in distortion and stress is a result of the change of the plastic strain field in the weld region: electromagnetic impacts reduce longitudinal compressive plastic strain in the local region near the weld, and even produce the tensile plastic strain. Moreover, it is found that the residual stress can promote the changes of the longitudinal plastic strain state under electromagnetic impact.

Effect of cyclic heat treatment on microstructures and mechanical properties of directionally solidified Ti–46Al–6Nb alloy

The Ti–46Al–6Nb (mole fraction, %) ingots that were directionally solidified by cold crucible were cyclic heat treated at 1330 °C in the α phase region. The microstructures and mechanical properties of the ingots before and after heat treatment were investigated. The results show that the large columnar grains are changed into equiaxed grains after heat treatment. The grain size decreases with increasing the cyclic times, which is caused by the recrystallization and the transition from the large grain of small lamellae to the small grain of large lamellae. Four times of cyclic heat treatment refines the grain size from 1.33 mm to 0.59 mm, nevertheless the lamellar spacing increases from 0.71 μm to 1.38 μm. Extending the holding time and increasing the cyclic times of heat treatment eliminate the β-segregation at the grain boundary and the interlamellar. The compression testing shows that the compressive strength of the directionally solidified ingot in the parallel and perpendicular directions are 1385.09 MPa and 1267.79 MPa, respectively, which are improved to 1449.75 MPa and 1527.76 MPa after two and four times of cyclic heat treatment, respectively, while that is 1180.64 MPa for the as-cast sample. The fracture mode of the sample after cyclic heat treatment is quasi-cleavage fracture.

Controlling welding hot cracking based on electromagnetic force

Abstract Controlling welding hot cracking based on electromagnetic force was carried out. The results show that the welding cracking can be inhibited with this method, which lies on the voltage of capacitor and the frequency of applying force. The sheet bears the downward axial force and the less radial force of flat spiral coil. The radial force points at the centre of the sheet, so it is able to compress the weld metal. Moreover, the axial force also has the ability to compress the weld metal by means of the backing plate under weldment. Controlling welding hot cracking is mainly due to the improving of mechanical condition of weld metal within brittle temperature range.

Microstructure and mechanical properties of nanostructured bainite weld with regeneration

Because of the brittle martensite crystalline structure, nanostructured bainitic steel is very difficult to be welded and easily form cracks in the welded joint, which limits the scope of their application. Regeneration treatment can lead to nanostructured bainite formation in the welded joints, preventing further degradation of the welded joint. Detailed changes of microstructures and mechanical properties of the weld are characterized here. Coarse inter-dendrite structures appear in the weld due to welding segregation, and are confirmed to be retained austenite by TEM. Moreover, an extraordinary combination of strength and ductility of the weld is achieved. The ultimate tensile strengths are 1913MPa and 2115MPa when regeneration temperatures are 250 °C and 230 °C. The corresponding elongations are 5.14% and 2.3%. In addition, the tension fracture behaviour and crack propagation mode of the weld are investigated.

Solution of stress intensity factor for mode I centre crack in undermatched butt joint with certain reinforcement

Abstract In order to improve the fracture resistance of undermatched butt joint, the influence of joint shape parameters on the stress intensity factor (SIF) of undermatched butt joint with mode I centre crack was studied, and the solution of SIF was obtained by combining the analytical method with the finite element method as well. The results are useful to guide the shape design of undermatched butt joint with mode I centre crack under both static load and fatigue load conditions.

Study on tempered zone in nanostructured bainitic steel welded joints with regeneration

Abstract Microstructure evolution of nanostructured bainite in tempered zone during welding and regeneration has been analysed. The effect of welding heat input on microstructure and mechanical properties is investigated. Results show that cementite precipitation and nanostructured bainite regeneration occur in tempered zone. When heat input is reduced, the size and amount of cementite decrease, while the volume per cent of retained austenite increases. At the same time, the tensile strength in tempered zone increases with welding heat input decreasing. A welded joint of nanostructured bainitic steel with tensile performance almost the same as base metal is obtained by laser beam welding with regeneration.