Properties And Failure Analysis Of Electrodes Made Of Different Materials For Spot Welding

With economic development, improving people’s living standards, China’s automobile industry has received an unprecedented rapid development. Galvanized steel are widely used in the manufacture of automotive body due to its excellent corrosion protection properties, such as the chassis, the outer cover, front cabin, streams boxes, doors and other parts. Galvanized steel in some models use reaches 100%. Currently, galvanized steel in car body and various components are still welded by spot-welding which is excellent in its unique superiority. In resistance welding electrode is the main supplies. The main use CuCrZr alloy for manufacturing. Part other use Al2O3 dispersion strengthened copper and other alloy copper for manufacturing. there are some problems in spot-welding galvanized steel by CuCrZr alloy electrodes. Mainly molten zinc is easy to adhere to the electrode surface of the electrode, making working surface alloying, reducing the thermal conductivity properties of electrodes, accelerating electrode wear resulting in lower productivity and higher costs. The Al2O3 dispersion strengthened copper alloy electrode is expensive, so it has less application currently. To solve these problems, researchers have developed a variety of CuCrZr alloy coated electrode that improved the life of the electrodes. But there are also some issues for coating electrode, just like preparation is relatively complex, and coating electrode after coating loss is equivalent to ordinary CuCrZr alloy electrode. To this end, the subject select CuNiCoBe and CuNiBe alloy making electrode for spot-welding to make comparison test with CuCrZr and Al2O3 dispersion strengthened copper alloy electrode for spot-welding galvanized steel; analyzed contact resistance between the different electrodes and the galvanized steel during spot-welding process; measured the temperature changes in the spot welding for electrodes of different materials and compare thermal performance of the electrodes; make failure analysis of the electrode by Super depth of field microscopy,SEM, EDS, XRD, spectrometers and other instruments. The result shows:Although the conductivity of CuNiBe and CuNiCo Be alloy is lower than CuCrZr and Al2O3 dispersion strengthened copper alloy, the magnitude relation on the contact resistance between the electrode and the steel plate is quite because contacts resistance between the electrode and the steel sheet depends on the galvanized steel.In spot welding process, temperatures of four electrode are rising rapidly and will soon enter the dynamic stabilization phase. On thermal performance is concerned, CuCrZr, Al2O3 dispersion strengthened copper alloy electrode is superior to CuNiBe and CuNiCoBe alloy electrode.But the peak temperatures of the electrodes from the recent end face are measured at about 160℃ to 170℃, there is no significant difference.According to AWS/SAED8.9M: 2002, under the same process for spot-welding on galvanized steel, the life of four electrode of CuCrZr, Al2O3 dispersion strengthened copper, CuNiBe and CuNiCoBe is that: 1600 times; 4300 times; 3700 times; 3500 times.The failure analysis of the four electrode shows that: The abrasion of the upper surface electrode is severe than the lower electrode; And the diameter of end face for upper surface electrode is larger; There is "pitting" and "self-healing" phenomenon in the spot-welding process for four kinds of electrodes. Four electrodes are all alloyed in varying degrees and the major product is Cu-Zn alloy.The main phases are CuZn、(CuZn)、Cu5Zn8、Cu1.05Zn0.95. The fastest rate of alloying for electrodes is CuCrZr, Al2O3 dispersion strengthened copper alloy and CuNiCoBe electrodes followed, the rate of alloying for CuNiBe electrode is minimized.Microhardness of end face for four electrodes after the failure shows that: The change in hardness of Al2O3 dispersion strengthened copper alloy electrodes is to a lesser extent. The hardness of CuNiBe alloy electrode and CuNiCoBe alloy electrode after fail is still high due to their high hardness. Thus tendency of plastic deformation for these three electrodes is relatively small. And the hardness of CuCrZr alloy electrode is minimum and the end faces are more prone to plastic deformation.The failure of four electrodes results by the interaction of worn, plastic deformation, pitting corrosion, alloying and other factors.Because of the differernce of material and organizational status, action levels above factors lead to failure in different electrode materials play is not the same, so the life is not same.