MIG welding also known by the names MAG (metal active gas) or GMAW (gas metal arc), is a welding process that is now widely used for welding a variety of materials. The essential feature of the process is the small diameter electrode wire which is fed continuously into the arc from a coil. As a result the process can produce quick and neat welds over a wide range of joints. In this paper microstructure micro hardness and tensile strength analysis of dissimilar metals (aluminium and copper) weld by metal inert gas welding is studied. Since aluminium and copper has almost same electrical properties welding of both will replace copper with aluminium thus attaining lower density and lower price.
Keywords: microstructure, micro-hardness, tensile strength, cold metal transfer
INTRODUCTION:
Copper and aluminium alloys are widely used in marine engineering in areas such as pipelines, storage tanks, ships' hulls and cladding for offshore structures. Copper has a good corrosion resistance, excellent ductility, and good electrical and thermal properties and used in making electrical components and radiator. Copper can be replaced with aluminium since both has similar electrical properties and can be attained at a lower price and at a lower density. This key problem can be solved by the welding of these two metals. Due to the dissimilarities in the chemical and physical properties welding of dissimilar metals is difficult compared to homogeneous metals. This challenge is being solved here. Aluminium is an attractive material in engineering as it has a low density, high specific strength, and good corrosion resistance, favourable thermal and chemical conductivity. Aluminium alloys will replace steel and have found applications in aerospace, aircraft, automotive, electronics and other fields.
OBJECTIVE:
To determine the microstructure, micro hardness, and tensile strength of aluminium fused with copper using metal inert gas welding and to study the behaviour of these two materials.
LITERATURE REVIEW:
I have read science direct paper on Microstructure characteristics and mechanical properties of cold metal transfer welding Mg/Al dissimilar metals by Jing Shang, Kehong Wang, Qi Zhou, Deku Zhang, Jun Huang, Guangle Li in which they studied the microstructure of Mg/Al cold metal transfer (CMT) weld joint was studied by means of optical microscopy, scanning electron microscope and x-ray diffraction and a good bond strength was got.
I have read journal on meeting challenges in welding aluminium alloys through pulse gas metal arc welding by P. Praveen , P.K.D.V. Yarlagadda which says about solution to the challenges in welding aluminium like joining of different grades of aluminium alloys, joining of reinforced and unreinforced aluminium alloys, reduction of cracks in weld and weld repairs.
ALUMINIUM WELDING:
Aluminium has an excellent strength to weight ratio, intrinsic recyclability; corrosion resistance makes it good for manufacturing material. It has a wide range of aerospace application includes rocket casings to space station modules. Without compromising safety and performance they are used in automobile industry to build body frames that make it light weight.
Chemical properties of aluminium are that they are highly susceptible to hydrogen porosity during fusion welding. Hydrogen readily dissolves into the molten weld pool and produces gas pores on solidification.
Physical properties of aluminium includes that its density is low and about one-third of that of steel. It has a melting point of 660 degree Celsius. Thermal conductivity is 6 times more than steel. Thermal expansion is about twice that for steel. Aluminium does not changes its colour when heated.
Mechanical properties include that even though pure aluminium is weak they are highly ductile. The modulus elasticity is about 70,000MPa which is one-third that for steel, which means it deflects three times more than steel under the same applied force and hence it can absorb more energy under the impact. When decreased to lower temperature aluminium's toughness and ductility increases.
COPPER WELDING:
Copper has excellent resistance to corrosion in the atmosphere and in fresh water. Its chemical properties include that it has two oxidation stages cuprous and cupric. It is not reactant to water but reacts with atmospheric oxygen forming a brown black layer of copper oxide.
Physical properties of copper includes its non reactive that it won't even become brittle in sunlight. The thermal conductivity of copper is about 394W/mk, which is twice that for aluminium and thirty times that for stainless steel. So it can be used where rapid heat transfer is required and the application of this includes heat sink, radiators in automobiles etc. Toughness won't be affected at even at sub zero temperature. It is completely recyclable without loss in properties. It is anti microbial in nature. It can be got in wide range of colours.
EXPERIMENTAL PROCEDURES:
Samples in the dimension 100 x 50 x 3mm of aluminium and copper are welded in this experiment. The oxide films on substrates surface were removed by stainless steel wire brush. Since copper has a melting point of 1084 degree Celsius and aluminium has a melting point of 660 degree Celsius so aluminium has to be preheated to a temperature of about 500 degree Celsius before welding. In this experiment we are doing the butt joint of these two metals. The welding parameters such as welding current, welding voltage, wire feed speed and welding speed are adjusted. Microstructure was determined using the optical microscope (attached with an image analyzer) after the welded specimen was polished by using a chemical etching agent. Vickers micro hardness measurements were carried out under some load in the transverse cross section of the joints by using Vickers micro hardness tester. Tensile strength of the welded specimen was also carried out. Microstructure of the welded region is analysed and fatigue and failure analysis is studied. Thus the defects like hot cracking, crater cracking, cold cracking etc can be analysed. Micro structural studies helps in understanding the micro structural changes which occur during the welding processes and how different the welded region behave from the parent material due to the effect of high temperature and strain.
RESULTS AND DISCUSSION:
By analysing the microstructure of the welded region of Al and copper give information about the formation of inter-metallic compounds, heat affected zone, the grain growth or the grain size variation, weld defects and many more. It also helps in improving the strength and the properties of the weld, as changes in microstructure have a great effect on the mechanical properties of the weld joint. Grain size variation tells about the tensile strength of weld joint and grain boundaries tell the state of fracture of the joint. At low temperature fine grain size exhibits greater yield strength than the course grain size. The large the temperature and strains, the weld interface causes change in hardness. The value of hardness gives information about the metallurgical changes that occurs during welding. Hardness values are sensitive to friction pressure, upset pressure, heat input etc.
