Huijie Liu

Mechanical properties of underwater friction stir welded 2219 aluminum alloy

Underwater friction stir welding of 2219 aluminum alloy was carried out in order to further improve the joint performances by varying welding temperature history. The results indicated that the tensile strength of the joint can be improved from 324 MPa by external water cooling action in normal to 341 MPa. However, the plasticity of the joint is deteriorated. The underwater joint tends to fracture at the interface between the weld nugget zone and the thermal mechanically affected zone on the advancing side during tensile test, which is significantly different from the normal joint.

Thermal modeling of underwater friction stir welding of high strength aluminum alloy

The thermal modeling of underwater friction stir welding (FSW) was conducted with a three-dimensional heat transfer model. The vaporizing characteristics of water were analyzed to illuminate the boundary conditions of underwater FSW. Temperature dependent properties of the material were considered for the modeling. FSW experiments were carried out to validate the calculated results, and the calculated results showed good agreement with the experimental results. The results indicate that the maximum peak temperature of underwater joint is significantly lower than that of normal joint, although the surface heat flux of shoulder during the underwater FSW is higher than that during normal FSW. For underwater joint, the high-temperature distributing area is dramatically narrowed and the welding thermal cycles in different zones are effectively controlled in contrast to the normal joint.

Repair welding process of friction stir welding groove defect

Abstract The groove defect formed in the friction stir welding dramatically deteriorates weld appearances and mechanical properties of the joints owing to its larger size and penetration. Therefore, the friction stir repair welding was utilized to remove such a groove defect, and the focus was placed on the mechanical properties and microstructural characteristics of the repair joints so as to obtain an optimum repair welding process. The experimental results indicate that the groove defect can be removed by friction stir repair welding, and the offset repair welding process is superior to the symmetrical repair welding process. In the symmetrical repair welding process, a large number of fine cavity defects and an obvious aggregation of hard-brittle phase Al 2 Cu occur, accordingly the mechanical properties of the repair joint are weakened, and the fracture feature of repair joint is partially brittle and partially plastic. A good-quality repair joint can be obtained by the offset repair welding process, and the repair joint is fractured near the interface between the weld nugget zone and thermal-mechanically affected zone.

Microstructures and interfacial quality of diffusion bonded TC21 titanium alloy joints

Abstract Diffusion bonding of TC21 titanium alloy was carried out at temperature ranging from 780 °C to 980 °C for 5-90 min. The interfacial bonding ratio, deformation ratio, microstructures and microhardness of the diffusion bonded joints were investigated. Results show that joints with high bonding quality can be obtained when bonded at 880 °C for 15-30 min. The microhardness increases with increasing the bonding temperature, while it has a peak value (HV367) when bonding time is prolonged up to 90 min. Fully equiaxed microstructures, bi-modal microstructures and fully lamellar microstructures were observed when bonded in temperature range of 780-880 °C, at 930 °C or 980 °C, respectively. The volume fraction of α phase first increases and achieves the maximum when bonded at 880 °C for 60 min, and then descended.

Microstructural zones and tensile characteristics of friction stir welded joint of TC4 titanium alloy

TC4 titanium alloy was friction stir welded using a W-Re pin tool, and the defect-free weld was produced with proper welding parameters. The joint consists of stir zone, heat affected zone and base material. The stir zone is characterized by equiaxed dynamically recrystallized a phases and transformed β phases with fine a+β lamellar microstructure. The microstructure of the heat-affected zone is similar to that of the base material, but there is an increase in the volume fraction of β. Transverse tensile strength of the joint is 92% that of the base material, and the joint is fractured in the stir zone and the fracture surface possesses typical plastic fracture characteristics. The stir zone is the weakest part of the joint, through which the tensile characteristics of the TC4 joint can be explained.

Characteristics and Formation Mechanisms of Welding Defects in Underwater Friction Stir Welded Aluminum Alloy

Underwater friction stir welding (FSW) has been demonstrated to be a promising method for strength improvement of heat-treatable aluminum alloy joints. However, when improper welding parameters are utilized, welding defects, such as voids can be produced in the joints, leading to dramatically deteriorated mechanical properties. Thus to obtain high-quality underwater joints, it is necessary to understand the variables that promote the formation of these defects. In this study, the characteristics of welding defects in underwater joints were examined, and the formation mechanisms of the defects were investigated by analyzing the material flow patterns during underwater FSW. The results indicated that welding defects can occur at both low- and high-rotation speeds (HRS). The defects formed at HRS can be divided into two types according to the welding speed. When a HRS and a low welding speed are chosen, the material beneath the tool shoulder tends to be extruded into the pin stirred zone (PSZ) after flowing back to the advancing side. This results in a turbulent flow condition, creating void defects in the PSZ. When a high welding speed is coupled with the HRS, a large amount of material from the thermo-mechanically affected zone is dragged into the pin hole, which causes the material of the shoulder stirred zone to fill the pin hole in a downward flow direction. This leads to turbulent flow in PSZ, and creates voids or even groove defects in the as-welded joints.

Texture of friction stir welded Ti–6Al–4V alloy

The α+β titanium alloy, Ti–6Al–4V, was welded by friction stir welding using a W–Re pin tool, and the defect-free weld was produced with proper welding parameters. Texture of the Ti–6Al–4V friction stir weld was studied by orientation imaging microscopy. The as-received Ti–6Al–4V sheet mill annealed was composed of elongated primary α and transformed β. A typical rolling texture was observed in the base material. The microstructure of the stir zone was significantly different from that of the base material. The stir zone was characterized by the presence of considerable amount of equiaxed dynamically recrystallized grains and a texture around {φ1=30°, ϕ=62°, φ2=30°} was developed during the friction stir welding.

Particle-in-cell simulations for the effect of magnetic field strength on a cusped field thruster

The particle-in-cell plus Monte Carlo method is used to simulate a cusped field thruster. Three different values of magnetic field strength are simulated and convergent results are obtained successfully. The simulation results indicate that with the increase of the magnetic field strength, electron density peak tends to move in the radial direction towards the axis of the discharge channel. The central gap area of electron density is decreased due to the stronger electron confinement, which correspondingly reduces the inevitable central electron leak in the cusped field thrusters. However, it is found that the stronger confinement of electrons reduces electron density near the wall, which increases the neutral gas leak there. It is also validated that the main potential drop in the discharge channel appears near the channel exit. With the enhancement of magnetic field strength, potential drops at separatrices increase correspondingly. At the same time, several potential wells and barriers can be found near the wall, which accords well with recent theoretical and experimental results.

Improving the particle distribution and mechanical properties of friction-stir-welded composites by using a smooth pin tool

Friction stir welding (FSW) is a very promising technique for joining particle-reinforced aluminum-matrix composites (PRAMCs), but with increase in the volume fraction of reinforcing particles, their distribution in welds becomes inhomogeneous. This leads to an inconsistent deformation of welds and their destruction at low stresses. In order to improve the weld microstructure, a smooth pin tool was used for the friction stir welding of AC4A + 30 vol.% SiC particle-reinforced aluminum-matrix composites. The present work describes the effect of welding parameters on the characteristics of particle distribution and the mechanical properties of welds. The ultimate strength of weld reached, 309 MPa, was almost 190% of that of the basic material. The mechanism of SiC particle conglomeration is clearly illustrated by means of schematic illustrations.