MICROSTRUCTURE STUDY OF AN AL-CU SYSTEM COLD WELDED UNDER PRESSURE BY MEANS OF 'CUT-WELDING' TECHNIQUE

摘要

The use of light alloys, such as aluminium, magnesiumand titanium, is constantly growing in industrial applications,most of them are used in the automotive (Al and Mg) and aerospaceindustry (Al and Ti).The present study is focused on the metallurgical applicationsof the welding process of light alloys. Moreover a mechanicaldesign for cold cut welding of light alloys has been proposed.With this respect, the device designed involves the possibilityof cutting the surface oxidation layers off which presence mainlycauses a bad welding for low temperature processes. Theoretically,the device has all the conditions required for a correct andefficient cold welding. Experimentally, some aluminium-copperwelding systems have been made by means of this device and it waspossible to establish the goodness of the welded Al-Cu binary system.The welded samples have been investigated by means of EDAX(electron Device Analyser by X-Rays) technique together with SEM(Scanning Electron Microscope).Thus, it was possible to establish that the Cu atoms diffuse till6 #mu# m into the aluminium region, whereas the Al atoms reachthe depth of 1.5-2.0 #mu# m within the copper region. These resultsclearly show the possibility of obtaining welded samples by meansof the designed cut welding device.A heat treatment at 500 deg C/Ih for half of the welded sampleshas been performed. Then the two conditions have been compared.The post-welding heat treatment promoted inter-diffusionphenomena, improving the efficiency of the cold welded joints.The microscopic investigations performed along the cold cutwelded joints of the binary system Al-Cu showed the possibilityto get welded Al-Cu specimens by means of the cold cut weldingtechnique. Moreover the successful welding of the Al-Cu binarysystem showed the goodness of the designed device.The main results got by means of this welding techniques can besummarised as follow:I.No heat-treated specimensa) The atoms of copper have been diffused into the aluminium matrixand it spread Out over a mean depth of 6pm; whereas the diffusiondepth of the aluminium into the copper was only of 2 #mu# m. Thisdifferent behaviour depends upon the different atomic dimensionsof the two elements; the aluminium atoms have a higher diameterthan the copper ones, thus they can diffuse with less ease. Thisholds the more the lower is the temperature involved in the weldingprocess. In this case, the copper acted as atomic binding in thecold cut welding procedure.b) The adhesion between the two materials was uniform through thewhole joint perimeterc) EDAX analysis along the interface did not revealed any second phase in it.2.Annealed specimensa) BSE signal highlighted zones with different atomicnumber contrail. Every zone was related to a different meanconcentration of Al and Cu atoms there present; these steep zonesindicate interdivisional phenomena wider and more complex to the previous case.b) The mean diffusion region spread symmetrically out,within the two materials, over a depth of 12pm.c) Yet, in this case no trace of second phase along the inter-face was revealed.d) Thus, the results can be summarised as follow:The post annealing treatment strongly induced diffusion phenomena,yielding better results. Moreover the diffusion does not seem tobe affected by the amount of aluminium surface layers peeled outand this holds for the heat treated and for the other ones. Thus,the lower tolerance of 0.05mm (i.e. 50 #mu# m) was enough to setgoing the adhesion and diffusion phenomena between the two materials.