Advanced Materials Research Vols. 291-294

Abstract: An experimental study of the pulsed laser milling process for a sintered polycrystalline diamond is presented. The characteristics and quality of the cavities machined with a Yd laser under different pulse energies, pulse overlaps, scan overlaps and numbers of passes are discussed, together with the effects of these parameters on the cavity profile, depth of cut and surface roughness. A statistical analysis is also presented to study the relationship between the process parameters and surface roughness. It shows that the optimum pulse overlap and pulse energy may be used to achieve good surface finish, whereas scan overlap and number of passes can be selected to improve the depth of cut without much effect on the surface finish.

Abstract: Single sided resistance spot welding (SSRSW) developed from resistance spot welding (RSW) is a feasible plan to join vehicle structure to closed-form tube and is increasingly used in automobile manufacturing. A ring spot weld can be formed between the two workpieces during this process. The weld strength is critical to the performance of vehicle. The mechanical property of the ring nugget, however, is lack of attention. In this study, a simulation model of workpieces with ring nugget for tensile-shear test was established and the stress and strain distributions around the weld were researched in detail. It was found that the maximum stress is generally concentrated at the out edge around the ring nugget. The failure region, however, may be in the heat affected zone (HAZ) instead of in the place of maximum stress, which should be judged by stress failure criteria (SFC) in the model. In addition, the tensile stress and shear stress on the surface of the lower sheet and upper tube were illustrated.

Abstract: Modeling of resistance spot welding process on galvanized steel sheet was investigated. Mathematical models developed by nonlinear multiple regression analysis and artificial neural network approach were employed in the prediction of welding quality factors, namely nugget diameter, penetration rate and tensile shear strength, under some welding conditions. According to the prediction models on quality, the prediction systems of welding process parameters were formulated respectively on the basis of Newton-Raphson iterative algorithm and cascade forward back propagation algorithm in order to obtain the desired welding quality. The results showed that the prediction precision of cascade forward back propagation algorithm was higher than Newton-Raphson iterative algorithm. The current duration had the largest prediction error, followed by electrode force and welding current. Therefore, it was concluded that the current duration was the most difficult parameter in prediction system of welding process in order to obtain the desired welding quality.

Abstract: The dissimilar welds between magnesium alloy and aluminum alloy formed by friction stir welding has been investigated. The results show that the cavity and tunnel defect often appear in the welds, and the formation of fragile intermetallic compounds which exhibit virtually no ductility and lower strength cause the poor property of the welds during FSW.

Abstract: Microstructures, tensile properties and work hardening behavior of friction stir welded (FSWed) AA2219-T62 aluminum alloy (in its one-third bottom slice of a 20 mm thick plate) were evaluated at different strain rates. While the yield strength was lower in the FSWed joint than in the base metal, the ultimate tensile strength of the FSWed joint approached that of the base metal. In particular the FSW resulted in a significant improvement in the ductility of the alloy due to the prevention of premature failure caused by intergranular cracking along the second-phase boundary related to the presence of the network-like grain boundary phase in the base metal. While stage III and IV hardening occurred after yielding in both base metal and FSWed samples, the FSW led to stronger hardening capacity and higher strain hardening exponent and rate due to the enhanced dislocation storage capacity associated with the microstructural change after FSW. The fracture surface of the FSWed joint was mainly characterized by dimples and tearing ridges along with micropores.

Abstract: Against the T-joint connection problem, the CO2 laser and TIG arc hybrid welding Experiment Platform is set up. The implemented T-joint titanium alloy connection test adopts a novel fashion that while welding the upside of the sheet the bilateral back formed well. The preferred solution is presented based on studies such as arrangement of heat source, wire feeding, shielding gas selection and gap adaptability. The results reveal that the technology provides a new efficient method for the T-joint titanium alloy connection.

Abstract: An investigation was performed to study the response of laminated composites with bolt joints. The configuration was single-lap single countersunk head bolt. The major focus was to characterize bearing behavior of laminates and the effect of some parameters such as lap types, fastener types, hole diameter, and stiffness of lapped plate on the bearing strength and stiffness. For this purpose, hundreds of specimens were tested to obtain ultimate loads and stiffness of joints. According to different configurations of joints, three contrast tests were carried out, and some useful conclusions were drawn. Firstly, comparing the stiffness, the joints with protruding head bolts are slightly stiffer than those with countersunk head bolts. And bearing strength of the joints with protruding head bolts are about 10% higher than those with countersunk head bolts. Secondly, ultimate load and stiffness of single-lap double bolts are similar to double-lap single bolt, which are twice bigger than single-lap single bolt. Thirdly, the stiffness of joints can be improved by using bigger hole diameter or stiffer lapped plates. When the hole diameter and stiffer lapped plates are in a reasonable range, bearing strength increase. However, larger stiffness of joint structure may cause transient breaking, which is dangerous for the structure. So in engineering, it is needed to balance the bearing strength and stiffness of joints.

Abstract: The extruded AZ31B Mg alloy sheet was welded with friction stir welding. The microstructures and mechanical properties of the welded joint were investigated. The results show that the grains in the weld nugget zone were small, uniform and equiaxed. The grains in thermo-mechanical affected zone were stretched and relatively small, but were not as small and uniform as those in the weld nugget zone. The grains in the heat-affected zone were relatively coarse. The fracture of the welded joint occurred mainly in the heat affected zone. The tensile strength of the welded joints was up to 257.4 MPa and was 87.9% of the base material strength. The microhardness in the weld nugget zone was higher. The microhardness in the thermo-mechanical affected zone and heat affected zone were lower than that in the weld nugget zone. The microhardness in the weld nugget zone increased from the upper surface to the bottom.

Abstract: The salt-bath annealing was used to study the recrystallization of structural steels as well as the effect of chemical compositions on the microstructure and properties of structural hot dip galvanized steel sheets was investigated in term with the production condition of the No.2 hot dip galvanizing line in Pangang. The optimum annealing temperature range was determined. Experimental results show that the recrystallization temperature of Grade40 and Grade33 is approximately 720°C and 680°C, respectively. Considering the stability of mechanical properties, their optimum annealing temperature are determined to be approximately 720°C. The recrystallization temperature of Grade50 is above 700°C. In order to ensure the margin of strength and the stability of properties of Grade50, the optimum annealing temperature is approximately 780°C.

Abstract: In this paper, the effect of welding current of the tungsten inert gas arc (TIG) welding process on weld quality of TZM alloy was investigated and the fracture microstructure and mechanical properties of the welding joints of TZM alloy was analyzed by x-ray flaw detection, metallographic microscopic analysis, scanning electron microscopy and mechanical properties test. The results indicate that the weld bead with excellent mechanical properties can be obtained using the welding process parameter with the welding current of 210A, welding speed of 4 mm/s and the argon gas flow rate of 8~12 L/min. The microstructure of weld presents the large columnar grains in the center of the weld bead and the equiaxed grains in heat-affected zone instead of the lamellar fiber texture of the TZM alloy matrix, which resulted in the weakened strength of the matrix and the improved plasticity of it.