Yumei Yue

Growth of metal and metal oxide nanowires driven by the stress-induced migration

High quality Al and CuO nanowries are fabricated by simply heating the Al and Cu samples in air. Although the experimental operations and the stress-induced migration processes are quite similar, the causes of the driving forces and the growth mechanism are completely different. For the growth of Al nanowires, the driving force is determined to be the compressive stresses caused by the thermal expansion mismatch between Al film and Si substrate, and the growth mechanism is proposed to be the extrusion of atoms from the bases of nanowires (EAFB). For the growth of CuO nanowires, the driving force is determined to be the compressive stresses caused by the formation of Cu oxide layers, and the growth mechanism is proposed to be the formation of oxide molecules on surfaces of the nanowires (FOOS). The direct experimental observations of both EAFB and FOOS are presented. It is also demonstrated that stress distribution on the macroscopic level, which is caused by thermal or mechanical manipulation, can also in...

Design of Friction Stir Welding Tool for Avoiding Root Flaws

In order to improve material flow behavior during friction stir welding and avoid root flaws of weld, a tool with a half-screw pin and a tool with a tapered-flute pin are suggested. The effect of flute geometry in tool pins on material flow velocity is investigated by the software ANSYS FLUENT. Numerical simulation results show that high material flow velocity appears near the rotational tool and material flow velocity rapidly decreases with the increase of distance away from the axis of the tool. Maximum material flow velocity by the tool with the tapered-flute pin appears at the beginning position of flute and the velocity decreases with the increase of flow length in flute. From the view of increasing the flow velocity of material near the bottom of the workpiece or in the middle of workpiece, the tool with the half-screw pin and the tool with the tapered-flute pin are both better than the conventional tool.

Investigation of Ultrasonic Assisted Friction Stir Spot Welding of Magnesium Alloy to Aluminum Alloy

A new welding method: ultrasonic assisted friction stir spot welding (UAFSSW) was put forward in the present study. UAFSSW was successfully applied to weld dissimilar AZ31 Mg alloy and 6061 Al alloy. Results show that for either conventional FSSW or UAFSSW, sound joints are obtained with the upper Mg alloy and lower Al alloy configurations. Ultrasonic vibration is beneficial for the upward flow of lower aluminum alloy, the increase of the stir zone (SZ) width and the refinement of the grains in the SZ. All cross sections of the Al–Mg joints exhibit the formation of intermetallic compounds (IMC) in the SZ. The crack of the conventional FSSW joint propagates exactly along the interface between the dissimilar materials and exhibits an inverted V-shaped morphology. After reaching the highest point of the hook defect, crack of the UAFSSW joint extends to the keyhole, leaving a portion of Mg alloy on the lower sheet. Conventional FSSW and UAFSSW joints show different IMC compositions at the faying interface.

Effect of Rotational Speed on Microstructure and Mechanical Properties of Refill Friction Stir Spot Welded 2024 Al Alloy

Refill friction stir spot welding (RFSSW) was successfully used to weld alclad 2024 aluminum alloy with different thicknesses. Effects of tool rotational speed on the weld formation, microstructure, and mechanical properties of the RFSSW welds were mainly discussed. Results show that keyhole is successfully refilled and welding defects such as flash, annular groove, and material adhesion can be observed. A bright contrast bonding ligament is found embedded in the weld and it is thicker in the center. Defects of hook, void, lack of mixing, and incomplete refilling can be found at the thermo-mechanically affected zone/stir zone (TMAZ/SZ) interface, which can be attributed to weak metallurgical bonding effect. With increasing the tool rotational speed, thickness of the bonding ligament decreases, grains in the SZ coarsen, hardness of the SZ decreases, and lap shear load of the welds decreases. When changing the rotating speed, impact strength shows rather complicated variation trend.

Effect of Tool Geometry on Material Flow Behavior of Refill Friction Stir Spot Welding

Using LY12 aluminum alloy as the research object, material flow behavior of refill friction stir spot welding process is simulated. The simulation results are investigated by studying the macrostructure and microstructure of the refill friction stir spot welding joints. Effects of tool geometries on material flow are mainly discussed. Results show that bonding ligament and grains in different regions of the stir zone are affected by the material flow behavior. Rational groove on the sleeve can increase the material flow velocity. From the viewpoint of reducing the unfavorable bonding ligament, the groove on the sleeve inner wall is better than the smaller width groove on the sleeve outer wall. Compared with the concentric circle grooves on the sleeve bottom, the scrolled groove is more beneficial to decrease the bonding ligament thickness and increase the welding spot area. Disregarding the higher heat input, sleeve with bigger outer diameter greatly increases the flow velocity and the welding spot area compared to changing the groove geometry.

Effect of Temperature Field on Formation of Friction Stir Welding Joints of Ti–6Al–4V Titanium Alloy

Abstract In order to investigate the formation mechanism of tunnel defect produced near the bottom of stir zone (SZ) in friction stir welding joint of Ti–6Al–4V titanium alloy, the temperature distribution during welding process was analyzed by numerical simulation and experiment. Results show that macrostructure morphology of SZ in cross section presents “bowl” shape owing to the characteristic of temperature distribution. Obvious temperature gradient appears along the thickness direction of joint. Decreasing rotational velocity reduces peak temperature and temperature gradient, which is beneficial to eliminate tunnel defect.

Effect of Trailing Intensive Cooling on Residual Stress and Welding Distortion of Friction Stir Welded 2060 Al-Li Alloy

Abstract 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.

Effect of Welding Speeds on Mechanical Properties of Level Compensation Friction Stir Welded 6061-T6 Aluminum Alloy

Abstract In order to eliminate the flash, arc corrugation and concave in weld zone, level compensation friction stir welding (LCFSW) was put forward and successfully applied to weld 6061-T6 aluminum alloy with varied welding speed at a constant tool rotational speed of 1,800 rpm in the present study. The glossy joint with equal thickness of base material can be attained, and the shoulder affected zone (SAZ) was obviously reduced. The results of transverse tensile test indicate that the tensile strength and elongation reach the maximum values of 248 MPa and 7.1% when the welding speed is 600 mm/min. The microhardness of weld nugget (WN) is lower than that of base material. The tensile fracture position locates at the heat affected zone (HAZ) of the advancing side (AS), where the microhardness is the minimum. The fracture surface morphology represents the typical ductile fracture.

Effect of Plate Thickness on Tensile Property of Ti–6Al–4V Alloy Joint Friction Stir Welded Below β-Transus Temperature

Abstract Defect-free joint of Ti–6Al–4V alloy was successfully friction stir welded below β-transus temperature and then tensile tests were performed. Microstructure, macrostructure, tensile properties and fracture position are mainly discussed in order to investigate how surface indentation and plate thickness influence the tensile property. Weld zone (WZ) attained below β-transus temperature that owns better tensile strength limit than base metal (BM). During the tensile test, the elongation is decided by whether BM yields. Compared with friction stir welding joint using 2.5 mm thick plate, it is very difficult for joint using 2 mm thick plates to get bigger elongation due to surface indentation. Due to the higher tensile property of the WZ, the joint without surface indentation fractures at BM, reaching the 58.46 % elongation of BM.

Effect of Mold Fixture on Stress and Deformation of Composite Structures

During the process of autoclave formation of thermosetting composite structures, the temperature distribution is strongly influenced by the forming fixture, which then contributes to forming error of the composite structure. On the base of 3D thermomechanical model by the software ABAQUS effects of mold fixture on temperature, stress and deformation are investigated. Numerical simulation results of the composite structure show that the deformation in the two sides and stress in the middle are both maximum. Comparison of composite structures shows that the column mold fixture, in contrast to oblique support mold and vertical plate mold ones, manifests uniform temperature and deformation distributions during the process of autoclave formation. Therefore, from the viewpoint of controlling the deformation of the composite structure, the column mold fixture is superior to the oblique support or vertical plate mold fixtures.