A new one-sided joining method, friction stirring blind riveting (FSBR) was successfully implemented to form lap-shear joints for dissimilar metals from pairs of 3.05 mm thick cast Mg AM60, rolled 1.5 mm thick Al AA6022, and extruded 3.15 mm thick Al AA6082 specimens. The concept of this process is riveting the two workpieces with reduced force under frictional heat and fastening the workpieces through blind riveting once the rivet is fully inserted. In this research, the process was experimentally analyzed and optimized for four joint combinations. It was demonstrated that switching the positions of Mg and Al alloy specimens has a significant effect on the process window and maximum tensile load of the joints. Three quality issues of the FSBR joints were observed and discussed. During tensile testing, the sheet closer to the rivet tail work-hardens due to tail forming process but has worse loading condition than the sheet closer to the rivet head. For AA6xxx sheets, precipitate hardening due to frictional heat is another strengthening mechanism in FSBR compared to the conventional riveting process, which leads to higher tensile loads in FSBR joints.
Application of hot stamping technology of advanced high strength steel is one of the effective approaches for automotive light-weighting. It is important to investigate the effects of hot stamping process parameters on forming quality for the application of hot stamping technology. A kind of hot stamping tool system for process tests of U-shape sample is introduced. It includes cooling system and temperature measuring system and it employs springs to provide blank holder forces. The effects of process parameters, blank holder force, cooling rate, initial temperature of blank, stamping velocity and die unloading temperature of blank, on the dimensional accuracy and microstructure of hot stamping samples can be investigated on the hot stamping tool system. Satisfied hot stamping U-shape samples were obtained by optimizing the process parameters.
This study investigates the effect of thermal exposure (i.e., 80°C for 14 days) on the static and fatigue characteristics of adhesive-bonded aluminum joints. Results showed that thermal exposure reduced the quasi-static strength of the adhesive-bonded joints by up to ~16%. The thermal exposure slightly decreased the fatigue resistance of the adhesive-bonded aluminum joints at high cycle regime (>~106 cycles) but significantly degraded the fatigue resistance at low cycle regime (~103–104 cycles). The effects of thermal exposure on the properties of adhesive and interfacial bond adhesion between the adhesive and aluminum were analyzed. It was found that the thermal exposure degraded the properties of adhesive due to that adhesive was oxidized, which led to the decreases of the static strength and fatigue resistance at low cycle regime for the adhesive-bonded aluminum joints. The oxidation of adhesive decreased the content of O-H group in the adhesive, which likely reduced the hydrogen bond at the adhesive/aluminum interface. The decrease in the content of hydrogen bond weakened the bond adhesion at the adhesive/aluminum interface, and consequently slightly reduced the fatigue resistance at high cycle regime for the adhesive-bonded aluminum joints.
The cruciform biaxial tensile test can be used to map the hardening evolution of the yield surface over a wide range of loading paths, which is useful for calibrating and validating the advanced material models. However, when cruciform specimens following ISO Standard 16842 are used, equivalent plastic strain in the gauge region is limited to only ~0.03 for DP590. In this study, a new method was developed to strengthen the arms of ISO Standard cruciform specimen in order to achieve greater plastic deformation in the gauge region. Arm strength of cruciform specimens was enhanced by laser deposition of thickening layers using materials compatible with the base metal. Furthermore, the slit geometry in the arms was adjusted to improve strain distribution and delay fracture. To verify the proposed method, cruciform specimens of different base materials (Cr4, DP590, DP780 and DP980) were tested in a biaxial tensile testing system with the aid of digital image correlation (DIC) techniques to characterize the strain fields within the gauge region. The laser-deposition-affected zones were negligible for the base materials according to optical microscopy. For DP590, the laser deposition method provided an increase of equivalent plastic strain in the gauge region from ~0.03 to ~0.11 for various loading paths. Consequently, evolution of an experimental yield locus was obtained at equivalent plastic strains up to ~0.11 for DP590. Continuing work with Cr4, DP780 and DP980 materials increased equivalent plastic strains to different degrees under nearly plane strain conditions (biaxial force ratio of 1:2).
Abstract Laser treatment is used to improve the surface adhesion performance of metal materials, including aluminum alloy. However, the adhesion performance of laser-treated aluminum alloy is reduced after the hygrothermal exposure during storage before bonding, obtaining the reliable surface adhesion is a challenge. In this paper, the nanosecond laser-treated aluminum alloy was exposed in a hygrothermal environment with 80°C 95%R.H. for 48h. Scanning electron microscopy, transmission electron microscopy and adhesion strength testing were used to characterize the physical/chemical properties and adhesion reduction of the aluminum surface. Subsequently, a simple and effective method of heat treatment was proposed to recover the adhesion strength. Finally, molecular dynamics (MD) simulation was utilized to explore the underlying recover mechanism. The experimental results revealed a AlOOH layer with ~410 nm thickness was generated on the laser-treated aluminum surface after hygrothermal exposure, which reduced the adhesion strength by 38% (from 30.9MPa to 18.9MPa). After heating at 120 °C for 24 hours, the adhesion strength (30.4MPa) of aluminum surface recovered to the level before hygrothermal exposure. MD results suggested that there are two mechanisms for the adhesion recovery by heat treatment: 1) the atomic kinetic energy of AlOOH increased and the structural order decreased, which strengthened its electrostatic and hydrogen bond interactions with adhesive molecules; 2) the mechanical property of AlOOH were enhanced.
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