Materials Science Forum Vols. 628-629

Abstract: With the deformation temperature between 250°C and 450°C as well as the strain rate between 0.01s-1 and 5s-1, the hot compression tests of AZ80 magnesium alloy were performed on Gleeble-3800 thermal simulation testing machine, so as to seek out the responses of mechanical behavior of AZ80 magnesium alloy under different deformation conditions. The results indicated that AZ80 magnesium alloy shows dynamical recrystallization when hot compessed, the recrystallization is prone to happen and the stress peak decreases with the temperature increased, and the critical strain to produce the transformation of recrystallization augments with the strain rate increased.

Abstract: Numerical simulation and test forming of the isothermal precision forging of an impeller was carried out. The forming processes were simulated with DEFORM-3D to obtain the forming characteristics and metal flow pattern. It indicated that the impeller can be forged by the method of isothermal-forging, and the deforming process can be divided into 3 stages. The forming of blades was depended on the extrusion of materials. During the forming, uneven deformation was existed in the billet, especially in the field of the blade root. The forging load was increased rapidly during the later stage of the forming process, and the maximum forging load was about 2961kN. According to the simulations, the die structure and the billet dimension were determined, the forging die was designed and manufactured, and the precision forgings of the impeller were produced successfully. Both of the simulation and the test forming indicated that the impeller forging could be performed with the assembled structure die and the isothermal extruding forming style satisfactorily. The ideal parameters to produce the precision forgings of the impeller are: a forging temperature of 450°C and a punch speed of 1mm/s. Under these conditions, the forgings of the impeller can be produced with full blades, smooth outer surface and good flow line, which can meet the requirements of the precision forging of impellers.

Abstract: This paper obtained work piece of fine blanking with negative clearance by experiment. The microstructure and fractography photograph in regions like rollover zone, shearing band, under sheared surface and fracture band were scanned by instrument of scanning electron microscope (SEM-JSM-6360LV). The fine-blanking with negative clearance makes the metal deformation regions into a state of triaxial stress precise and it causes an intense metallic stream that brings about inter-dislocation and distortion of metallic crystal product under the action of stronger force, so the metallic crystal is reset and the isometric metallic crystal of shearing zone is staved, sloped and pulled and it appears as dense lined and strip crystal. The paper analyzes the full course of producing and development of micro crack in the ejecting stage in the fine-blanking processing of negative clearance. And the course of final rupture is also analyzed. The relationship curve of blanking stroke and micro hardness is measured by Vickers Hardness instrument (MH-6). The deforming principle of fine blanking deformation with negative clearance is analyzed by means of microstructure, metallographic photograph and micro hardness.

Abstract: Many cracks can occur in heavy forging as a result of inherent defects of heavy ingot and forging process. According to the results of current research, internal crack of metallic material can heal under high temperature. In order to get to the bottom of internal crack healing in heavy forgings, some 45 steel samples containing internal crack were normalized at higher temperature than normal. The phenomena show that new ferrite grains nucleate and grow up on the crack surfaces during the crack healing, and the ferrite grains nucleation is universal but selective on crack surfaces. The relationship of atom diffusion and ferrite grain nucleation and their actions on crack surface migration will be discussed and expounded, and that proves that ferrite grain nucleation and growth will accelerate crack healing.

Abstract: In order to reduce vehicle weight and improve crash performance, dual-phase (DP) steels are increasingly used for lightweight auto-body. the high cooling rates in Resistance spot welding (RSW) process will make the nugget very brittle and result in interfacial fracture modes. In this study, a servo resistance welding system is firstly introduced to control process parameters during the welding process. The influences of different process parameters (weld current, weld time and electrode force) on the weld strength, nugget diameter, facture modes and weld indentation are analyzed. Finally, an on-line weld failure mode inspection method is proposed based on servo gun’s feedback characteristics.

Abstract: The flanging on both sides of wheel cowling tail of front fender is stretch flanging. An asymmetric flanging model is built according to the asymmetric twist defect in flanging. The strain distribution of the flanging regions, which is analyzed through finite element method (FEM), is in reasonable agreement with related theories. The result shows that the built model is appropriate to investigate the flanging twist defect of the fender. The accuracy of the model is proved by the comparison of the simulation result with the practical production. Also it shows that the severity of the distortion mainly relies on the asymmetry of the fender and some suggestions are proposed on distortion control through the analysis of the factors influencing laws on the distortion angle, which is define to measure the twist, like radius ratio of the two sides of the fender, flanging height, the clearance between the die and punch of the flanging tools, the fillet radius of the flanging die.

Abstract: The finite element model of bulging at elevated temperature was established by using of pam-stamp 2007. The bulging tests of boron steel sheets were also performed on the self-designed and manufactured tool system. The simulated results of bulging height, limit strain and the minimum temperature of specimens are consistent with that of tests. It demonstrates the validity and accuracy of the finite element model established in pam-stamp. The influences of the friction coefficient, the temperature and the stamping velocity on the hot bulging are analyzed numerically. The friction condition should be improved and the minimum temperature gradient is recommended in practical hot stamping production.

Abstract: Transient liquid-phase (TLP) bonding of aluminium-based metal matrix composite (MMC) has been investigated. An attempt was made of using an Ag/Cu /Ag film as an interlayer for bonding to improve the joint strength. The oxide on the surface of SiCp/Al MMC was etched completely by plasma. Ag/Cu/Ag film of 8µm thickness was prepared by magnetron sputtering method on the clean bonding surface in the same vacuum chamber. Compared with the same thickness of single Cu foil and Ni foil interlayer, which are widely used for joining SiCp/Al MMC material, the shear strength of 192.8MPa was obtained, which was 86.5% that of base metal. Discussion was made on the bonding process and microstructure.

Abstract: This paper introduces a method to measure the online strain in the plate shell forming process. It solves the problem that the inside stress online is almost impossible measured in the process of the upper and lower molds forming. The key technology is the design of the moving test frame. This frame can move with the plate’s movement to protect the sensors and ensure the output of the test signal, at the same time, it does not interfere with the shell’s process conditions. It is the first time to apply this method to the large-sized high-strength steel ball shell plate, and it could accurately measure the stress distribution and changes laws within the shell plate under certain specific condition in the process of punching to online stress test and analysis during the punching forming process. The test results show that the periphery of each pressure point is residual tensile stress, and the inside is the residual compressive stress, more evenly distributed. The existence of residual compressive stress is in favor of the anti-fatigue capacity of the plate.

Abstract: Powder metallurgy process is a net-shape manufacturing technique that can eliminate or reduce machining time. It is economical and environmental friendly since no scrap is being produced and no high energy consuming process such as melting is involved. Unfortunately, conventional powder metallurgy is not capable of producing complex parts. However, a recently developed binder-treated warm compaction technique can overcome this problem by increasing the flowability of the mixed powder. In this study, by using a commercially available water-atomized iron powder, a cross-shaped part was prepared by warm compaction of a binder-treated iron-base powder at approximately 80 °C and then sintered at 1120 °C. Microstructure, mechanical property and shape consistency of the prepared part were examined. Results showed that parts with high density and high green strength can be obtained without significant shrinkage or expansion. The present paper demonstrated that the binder-treated warm compaction process can expand the capability of powder metallurgy techniques to produce complex parts.