Study On Structures And Properties Of Twin Wire MIG Welded Joint Of 6082-T6 Aluminium Alloy

Aluminium alloy is considered as one of the dominant materials to realize lightweight owing to its unique properties such as low density,good thermal conductivity,high specific strength,excellent corrosion resistance,favorable processability and so on.The extensive application of any new material depends not only on its own properties but also on the progress of welding and other secondary processing techniques.Metal inert gas welding is one of the most widely used welding methods for aluminium alloy,with the rapid development of manufacturing,the deposition rate and speed of the traditional single wire welding is close to the limits which this technique can achieve,so it is difficult to satisfy the demand of growing industry only relying on the improvement of welding power source,welding materials and shielding gas.As an efficient welding method,twin wire high-speed welding is developing faster and faster in modern international welding field and it will play a inestimable role to the whole manufacturing industry especially combined with high automation robots.From the development at home and abroad,twin wire pulsed welding technique which combines twin wire welding with pulsed welding obviously increases welding speed and improves the welding quality and its technical advantages are becoming increasingly prominent.Twin wire pulsed MIG welding has the following features:wide range of current regulation,welding parameters of each wire whose material and diameter can be various are able to be set individually;high wire feed rate and welding speed and greatly increased deposition rate;it can still keep low heat input while the deposition rate increases,stable arc and controlled droplet transfer;low spatter and small deformation,good weld shaping and high production efficiency.Therefore,it is of great theoretical significance and practical value to study the twin wire MIG welding of aluminium alloy.6082 aluminium alloy whose strengthening phase is Mg2Si belongs to Al-Mg-Si series alloy and can be strengthened by heat treatment.It is widely used in aerospace,construction, transportation and other industries because of its moderate strength,good weldability and corrosion resistance.In this paper,comparative tests between single wire and twin wire MIG welding were conducted and the structures and mechanical properties of single wire and twin wire welded joints were analyzed.Effect of welding parameters on the structures,mechanical properties and corrosion resistance of twin wire pulsed MIG welded joints were studied.The main conclusions are given as follows:(1)6082-T6 aluminum alloy MIG welded joint can be approximately divided into weld zone,heat affected zone and base metal. Weld zone is mainly composed ofα-Al solid solution andα-Al+β-Al8Mg5 eutectic phase;Heat affected zone is coarsening Mg2Si particle phase;Base metal mainly consists ofα-Al solid solution andβ-Mg2Si and simple substance of Si.(2)The hardness distribution of 6082-T6 MIG welded joint is:the hardness of weld zone is minimum, the hardness of HAZ increases with approaching to base metal and that of base metal reaches the maximum.Compared with single wire MIG welding,the hardness and tensile strength of twin wire MIG welded joint which has smaller grains and narrower HAZ increase slightly owing to relatively the low heat input of twin wire welding.Gas porosity of twin wire welded joint leads to severe stress concentration and low elongation.(3)With the increase of master current of twin wire welding, the weld penetration increases and the hardness of welded joint decreases slightly because the grains of weld zone and Mg2Si precipitation particles of HAZ coarsen and the grain boundary shape changes from discontinuous distribution of the short rod into a continuous network distribution.When the current is small,the tensile strength and elongation of welded joint are low due to the appearance of incomplete penetration.The increased current which causes the increase ofβ-Al8Mg5 precipitation phase in the grain boundary leads to decreased corrosion resistance.(4)When the welding speed increases from 100cm/min to 125cm/min,the hardness of welded joint increases because the grains of weld zone are refined and the Mg2Si particles of HAZ become smaller.The increase of welding speed makes theβ-Al8Mg5 precipitation phase in the grain boundary decreased which results in the increase of weld corrosion resistance.At the welding speed of 100cm/min,the gas porosity of weld is less and the tensile strength and elongation of welded joint are the maximum.(5)The structure and hardness of welded joint are not susceptible to current modes.When the current pattern is direct current plus pulse current,the arcs between master wire and slave wire interfere each other intensively which makes the welding progress unstable and increases the probability of external gas dissolving into the weld,so the tensile strength and elongation of welded joint decrease severely due to much gas porosity in the weld.The corrosion resistance of weld is the best when the master wire passes direct current and the slave wire passes pulse current.(6)Compared with the welding wire ER5356,ER5087 contains Zr and more Mn.these two elements can play a role of refining grain so higher hardness and tensile strength and elongation can be acquired by using welding wire ER5087,but the corrosion resistance of weld reduces because the element Mn makes the Mg solubility in the matrix decreased which leads to increasingβ-Al8Mg5 precipitation phase along the grain boundary.(7)The MIG welded joint of aluminium alloy has a high susceptibility to hydrogen porosity.Gas porosity can reduce the compactness of weld and bearing area of welded joint which makes the strength and ductility of welded joint decreased severely,so reasonable welding technological measures and materials must be taken to reduce the source of hydrogen.