| Aluminum alloy materials have been widely used in high-speed trains, withexcellent comprehensive mechanical properties,6N01used in car body and7N01used in the vehicle chassis of high speed trains are two kinds of aluminum alloysthat can be reinforced by heat treatment.But the welding deformation can be asevere problem, which requires straightening after welding. Heat-straightening, amethod of flame heating combined mechanical pressurized, can effectively controlthe welding distortion, but may deteriorate the performance of frame membersunder inappropriate process. At the same time the theory of heat-straighteningapplied in aluminum alloy materials is still not thoroughly understood at themoment. So with regard to the aluminum alloys treated by heat-straightening atdifferent temperatures, the research on the changing mechanism of microstructureand mechanical properties in the base metal and welding joints is urgently in need,which provides the basis for reasonable heat-straightening in high-speed trains.With the construction of the hardwares and software programming completed,a set of temperature detecting system based on accurate thermocouple was firstlydeveloped, which can measure multi-channel temperatures precisely, and thetemperature detecting system provided a quick response and the sampling frequencycan come up to20times per minute. Technology parameters of the temperaturepattern in heat-straightening were tested with the temperature detecting system.Considering a local and transient temperature pattern produced in heat-straightening,flame heating was not good for accurate control on the straightening temperature, sothermal simulation experiments were carried out to analyze the heat-straightening.The differences on the properties of the6N01-T5aluminum alloy heatstraightened in different temperatures were investigated. Results showed that theinfluences of heat-straightening on the base metal and welding joints were different.Static mechanical properties were not reduced after heat-straightening in lowtemperature, while the tensile strength, bending strength and hardness went downand the softening zone widened when it was over250℃. Loading frequency had noobvious effect on the fatigue life of welding joints, while the fatigue strength ofheat-straightening joints was improved under low loading stress, this case alsoapplied to heat-straightening within250℃under high load stress, butheat-straightening at high temperature made properties worse. In addition, with thehelp of TEM microstructures in various zones of heat-straightening joints wereanalyzed to explain the mechanism, which helped to reach a conclusion that the thermal cycle of heat-straightening affected the precipitation, growing anddissolution of the second phase of GP, β’’, β’ and β, at the same time the grains ofalloys recrystallized and grew, resulting in changes in properties. The most suitabletemperature to heat-straightening for6N01was200~250℃.The differences on the properties of the6N01-T5aluminum alloy heatstraightened in different temperatures were also investigated. Results showed that250~350℃was the most sensitive temperature for strength properties, but it kept ahigher value at high temperature. The hardness of welding zone was less affected bythe heating temperature, while hardness of the base metal and HAZ decreased below300℃and rose within300~350℃, and softening of materials heated at hightemperature was not so obvious due to natural aging. Under low loading stressheat-straightening generally improved the fatigue strength of joints, however, whenstress was higher the effect was unconspicuous, and welding defects seemed to bemain factors influencing the fatigue performance of welding joints. Changingmechanism of properties was analyzed in this paper. Heat-straightening at differenttemperatures put alloys into different aging states, so were grain morphology andstrengthening phases. Generally330~400℃was thought to be suitable temperatureto heat-straightening for7N01. |
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