Abstract
This study aimed to conduct a comparative study on the microstructure and mechanical performance of 5083, 6005A and 7N01 Al joints used in China Railway High-speed (CRH) trains. We connected 10 mm-thick plates by three-layer and three-pass gas metal arc welding (GMAW). The results indicated that 6005A and 7N01 Al joints were more sensitive to grain boundary liquation in the partially melted zone (PMZ) than 5083 Al joins. Besides, recrystallization was obtained in heat-affected zones (HAZ). The 5083 Al joints experienced the most severe recrystallization and the grain size changed from 6.32 (BM) to 32.44 (HAZ) μm duo to intracrystalline strain induced by cold-rolled processes. The 7N01 Al alloys experienced the lowest extent of recrystallization and the grain size increased from 5.32 (BM) to 22.31 (HAZ) μm. Furthermore, significant precipitate evolution in the HAZ was observed. Original thin β” precipitates dissolved in HAZ of 6005A Al joints and transformed to the softer β phase. However, less precipitation transition was examined in 5083 and 7N01 Al joints. The precipitates’ evolution produced a softening region in HAZ of 6005A joints where the hardness was only 55 HV. The microhardness profile of the other two Al joints was less affected. The tensile strength of 5083, 6005A, and 7N01 Al alloy joints reached 323, 206 and 361 MPa, respectively. The 5083 Al and 6005A Al joints failed at HAZ near the fusion line while 7N01 Al joints failed at the fusion zone owing to the high strength of the base metal. The liquation, coarse grains by recrystallization, and precipitate evolution all decreased local strength, resulting in the fracture at HAZ.
Highlights
Aluminum alloys have been widely used as significant lightweight materials in aerospace, automotive and transportation industries due to their low density, high specific strength and excellent corrosion resistance [1,2,3]
5083, 6005A and 7N01 aluminum alloys were adopted in China Railway High-speed (CRH) trains to meet the increasing lightweight demand and further reduce the carbon consumption
Ma et al [7] demonstrated that the grain size in the fusion zone reached 200 μm while that in the base metal was less than 50 μm, leading to a 15% decrease in tensile strength
Summary
Aluminum alloys have been widely used as significant lightweight materials in aerospace, automotive and transportation industries due to their low density, high specific strength and excellent corrosion resistance [1,2,3]. Successful joining of 10 mm-thick sheets can be obtained by multi-layer and multi-pass GMAW, the multiple thermal-mechanical processing steps usually result in a coarse grain structure and softening zone in weld joints. These defects will deteriorate mechanical properties of welds in aluminum alloy applications. The tensile properties of 6005A or 7N01 Al alloys are much better while the weldability is not as good Both of them suffered from softening and hot-cracking. Previous studies usually concerned the microstructure and mechanical properties of thin joints under single-pass welding. This study aimed to provide experimental data and a theoretical basis for high-quality and efficient welding of aluminum alloy for high-speed trains
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