Study On Microstructures And Mechanical Properties Of MIG Welded6082Aluminum Alloy

6082aluminum alloy belongs to Al-Mg-Si alloy, which contains Mg and Si elementsand can be strengthened by heat treatment. Due to its excellent mechanical properties, highcorrosion resistance and good weldability,it is widely used in the fields of high-speed trains,subway-rail transportation and automotive manufacturing. Aluminum alloy welding is oneof the key techniques in manufacturing high speed train cars of aluminum alloys, which themost widely used and very mature one is metal inert-gas welding (MIG welding).In view of the practical problems encountered in the high-speed train production in theChangchun Railway Vehicles Co., Ltd (CNR), studies on the microstructure andperformances of6082-T6aluminum alloy MIG welded joints are crucial. The studiesmainly concentrate on the thermal cycle of aluminum alloy welding, the impact of weldingrepair and air humidity on the mechanical properties and porosity tendency. The weldingtest was done in the CNR. After that, the samples are brought back to university. The jointmicrostructures were observed using metallographic and scanning electron microscope inthe Jilin University. Meanwhile, tests of the micro-hardness, tensile properties, flexuralproperties and impact properties of joints were done..The main results of the study are as follows:1The welding thermal cycle and its parameters are described to be one of the mainrepresentation of the welding thermal process and characteristics of welding metal. Andthermal cycle parameters are the main factors for influencing the following: porositytendency on ageing strengthening aluminum alloy, the second phase dissolution andprecipitation process and properties of welding joints. For welding pool, with a smallerwelding thermal cycle parameters VH, higher TM and higher TH (T1), hydrogen has a greatertendency to be absolved into the pool; a smaller TH (T2) and higher VC can inhibit the risingof the bubbles, and promote the formation of weld porosity. For heat-affected zone, withsmaller welding thermal cycle parameters VH, higher TM,TH and VC, the formation of solidsolution region are accelerated; with a high TM and high TH, the β phase (Mg2Si)aggregation, growth are promoted, and form over ageing zone.26082-T6aluminum alloy welding has a high resistance to crack with ER5087aluminum alloy welding wire. Under the circumstance of this experiment, there is nosolidification crack in the weld zone and no liquation cracking in heat-affected zone. MIGwelded joint is composed of weld zone, heat-affected zone and base metal zone. The weldof the6082-T6aluminum alloy is composed of fine equaled grains and coarse columnar grains. The former is in the weld center zone, the latter is growing vertically to the fusionline. There are two low hardness zones in the welded joint. One is the coarse grain zonewhere the heat affect zone is adjunct to the fusion line. Another is the over aging zone in theheat affect zone which is about9-11mm from the weld center. The coarse grain zone iscaused by the free precipitation because of the welding heat like the solid solution treatment.The over aging zone is given rise to by the accumulation and growth of the precipitatedMg2Si enhancement.3The moisture of air has a significant impact on pores in the aluminum alloy weld.With the increase of the moisture, there is an obvious increase of pores. Before the welding,the situation of base material and welding wire is especially vital to the welding quality.When the welding wire is in the high moisture environment for a long period, there is anobvious increase of pores in weld. The tensile samples all breaks in the heat-affected zoneand the tensile strength is dispersed, which is a proof that moisture has little impact on thetensile strength of the welded joint. Moisture has a great influence on the bending propertiesand impact properties of the welded joint. Bending fractures and impact fractures mostlyappear in the zone of intensive pores.4The times of the welding rework have a significant effect on the jointmicrostructures and mechanical properties. With the increase of rework times, the width ofsolid solution zone and over aging zone increases. At the same time, the从-Al phase grainsin the solid solution zone coarsens and the β phase in over aging zone accumulates andgrows. The low hardness zones all appears in the over aging zones and their widths increasewith more times of the rework. For ensuring the mechanical properties of the welded6082-T6aluminum alloy joints, it’s not recommended to use the original welding process torepair the large defects in the thin plates. While we can use the original welding process torepair the defects in medium or thick plates.