通讯机构：
[Cui, Junjia] H;Hunan Univ, State Key Lab Adv Design & Mfg Vehicle Body, Changsha 410082, Hunan, Peoples R China.
关键词：
Magnetic pulse welding;Dissimilar metals;Galvanized steel sheets;Mechanical properties;Interfacial microstructures
摘要：
In this study, the relations between interfacial microstructure and mechanical behavior of aluminum alloy sheets and zinc-coated steel sheets magnetic pulse welding (MPW) joints were characterized systematically. MPW joints of aluminum alloy and ungalvanized steel sheets were prepared for comparative analysis. Results showed that some of zinc layer on the galvanized steel was washed away to form metal jet under the high-speed collision. The change of failure mode from interfacial failure to base material fracture due to interfacial microstructure evolution was the fundamental reason for different mechanical behavior of two MPW joints. Because of the difference in element content and the severe plastic shear at the interface, the crystalline grain morphologies and sizes of aluminum and zinc in the transition zone changed. Amorphous structure was also found in the transition zone. The existence of the zinc layer would form brittle and hard phases on the interface, resulting in the generation of welding defects, thereby reducing mechanical properties of the joint.
摘要：
Amorphous structure and transition zone were observed in the magnetic pulse welded Al-Fe weld. Simulations and theoretical analysis were carried out to reveal the formation mechanism of the amorphous structure and transition zone. The wave morphology in the Al-Fe interface was well reproduced in simulations, and local melt was found in the weld interface. The cooling rate in the amorphous layer was much higher than that in the transition zone due to the smaller thickness of the melting layer. Materials in the amorphous layer experienced amorphization, while the materials in the transition zone experienced recrystallization. The current work indicated local melt played an important role in the magnetic pulse welding, which was different from the common perception that magnetic pulse welding was a totally solid-state welding process. (C) 2019 Elsevier B.V. All rights reserved.
摘要：
As an advanced interference-fit joining technology, electromagnetic riveting (EMR) has wide engineering application prospects in manufacturing and assembly fields. In this paper, a theoretical model on interference fit for EMR process is derived based on the stress wave theory and thick wall cylinder theory. EMR experiments are conducted to verify the analytical model. The compared results show that the analytical solutions agree well with experimental values. The interference-fit model synthetically considers many process parameters factors, not only the material properties and size of rivet, sheets and punch, but also the riveting force. The residual stress distribution, which is an important factor on fatigue life, can be predicted by this model. In addition, this can provide scientific guidance for the process parameter design and riveting setup optimization in engineering application.
关键词：
Magnetic pulse forming;AZ31 sheet;Driver sheet;Temperature
摘要：
In this paper, formability of AZ31 sheet in magnetic pulse forming hybrid actuating with driver sheet and temperature were investigated. The effects of discharge parameters, driver sheet and temperature were investigated on bulging by the experimental analysis. Forming height, thickness of AA1060 driver sheet, forming limit curve (FLC), fracture morphology and microstructure were performed to analyze causes of deformation behaviour for driver sheet and temperature. Analysis of driver sheet and temperature effects reveals that the optimum thickness of the driver sheet and temperature are 1 mm and 200 degrees C, respectively. The FLC results show increased by increasing temperature from 25 degrees C to 200 degrees C, and then decreased by increasing the temperature such as 250 degrees C. Compared with quasi-static forming, the temperature effect for formability is unremarkable in magnetic pulse forming with driver sheet. Demonstrating similar drawability and strechability of AZ31 sheet appeared. Microstructure reveals that high strain rate effect, yield strength decreasing (because of temperature effect) and grain refinement are main reasons to affect the formability of AZ31 sheet in magnetic pulse forming with driver sheet and temperature. However, dynamic recrystallization (DRX) is not observed which is usually accompanied at temperatures higher than 150 degrees C in quasi-static forming.
摘要：
In this paper, the mechanical properties of electromagnetic self-piercing riveted (E-SPR) joints with carbon fiber reinforced plastics (CFRP)/aluminum alloy (Al) 5052 were comprehensively investigated. Microtopography observations, hardness measurements and tensile-shear strength tests were performed by comparing with regular pressure self-piercing riveted (P-SPR) joints. Results showed that the undercut value of E-SPR joints was higher than that of P-SPR joints. The hardness values on rivet legs of E-SPR joints were larger and almost no difference on rivet heads between the E-SPR and P-SPR. In addition, it was found that mechanical properties of E-SPR joints were higher than that of P-SPR joints. The shear fracture appearance indicated that E-SPR joints with higher undercut were more difficult to rupture in the bottom of Al sheet.
摘要：
The durability of hybrid carbon fiber reinforced plastics (CFRP)/aluminum alloy (Al) structures is receiving increasing attentions in engineering applications. In this paper, the mechanical performance of CFRP/Al electromagnetically riveted lap joints after exposure in neutral salt spray environment for various ageing time was studied. The shear and fatigue tests were conducted. The mechanical property degradation laws and failure mode evolution were obtained. The results showed that the shear and fatigue properties of the CFRP/Al electromagnetically riveted lap joints linearly decreased with the increase of ageing time. In addition, the fatigue test results showed that three weeks was the critical ageing time for failure mode changing. Fatigue crack initiated from the riveted hole (below 3 weeks) and the edge of the overlapping region (over 3 weeks) of Al sheet, respectively. Meanwhile, the fracture analysis showed that the cracks of specimens after long-time ageing (over 3 weeks) were initiated from the several corrosion pits simultaneously. (c) 2018 Elsevier Ltd. All rights reserved.
摘要：
In this paper, the corrosion mechanism and tensile properties of basalt fibers in sodium hydroxide (NaOH) solution with various concentrations and temperatures were studied. The hydroxyl ions disrupt the (-)Si(-)O(-)Si(-) and (-)Si(-)O(-)Al(-) bonds leading to the formation of insoluble hydroxides. With the continuation of the hydration reaction, a hydration layer (corrosion shell) with high content of calcium, iron, manganese and titanium ions was formed on the fiber surface. The corrosion shell enabled an increase in the strength and elongation at break of basalt fibers, significantly. Results showed that the tensile strength of fibers was strongly dependent on temperature and concentration. After the basalt fibers were immersed in 1 mol/L NaOH solution at 50 degrees C for 1 h, 3 h, 6 h, 1 day and 3 days, their retention ratios of strength were 67.6%, 57.8%, 52.5%, 49.0%, 58.2%, respectively. Higher temperature accelerated the corrosion rate of basalt fibers, shortened the formation time of the corrosion shell and increased mass loss. From 25 to 70 degrees C, the mass loss of fibers increased from 2.4% to 33.8% for fibers immersed in 1 mol/L NaOH for 3 days. The experimental results from quantitative x-ray fluorescence (XRF) showed that the mass loss of basalt fibers was mainly due to the leaching of silicon, aluminum and potassium ions.
摘要：
A novel design and manufacturing method, dubbed “precast,” of the cooling system and tools for a hot forming process was proposed in this paper. The integrated structures of the punch and blank holder were determined by analyzing the bending and reverse-bending deformation of the forming parts. The desired crashworthiness performance of an automotive front bumper constructed with this process was obtained by a tailored phase transformation, which generated martensite-bainite in the middle and full martensite transformation in the corner areas. Varying cooling effects in the formed parts caused the highest temperature to be located in the bottom and the lowest on the end of the formed parts. Moreover, the microstructural distributions demonstrated that the bottom possessed a relatively lower content of martensite, while, conversely, the end possessed a higher content. This was precisely the most desired phase distributions for the hot formed parts. For the six-process cycle stamping, the temperatures reached a stable status after an initial rapid increase in the first three process cycles. The microstructural results verified the feasibility of the hot forming tools under multiprocess cycles.
摘要：
In this work, the numerical simulations and electromagnetic riveting (EMR) experiments were conducted to investigate microstructure evolution and the forming mechanism of adiabatic shear bands (ASBs). And the effects of rivet dies on microstructure distributions in formed heads and mechanical properties of riveted structures were systematically explored. The impact velocity and deformation distribution results demonstrated that the proposed numerical method was accurate and reliable. The simulation results showed the slope angle of rivet dies notably affected the plastic flow of materials, and then determined the microstructure distribution in formed heads. The combined effects of inhomogeneous plastic flow and thermal softening were accounted for the forming of ASBs. The formed heads had two obvious ASBs (upper and lower ASB) for the 40° rivet die and flat rivet die. The formed heads only had the lower ASB and no clear upper for the 60° rivet die and 80° rivet die. The pull-out test results showed that the specific rivet die could improve the mechanical properties of the EMR joints, which contribute to the engineering applications of EMR riveted structures.
