1. Introduction The requirement for an improved corrosion prevention in today’s automotive production has led to increased fabrication of galvanized steel sheets. This also applies in particular to the passenger car type Mondeo currently produced by Ford at Genk (Belgium).The nondestructive ultrasonic testing of spot welds has been
extremely successfully applied by Ford all over Europe, and also in Taiwan, for many years now. It was at first only used as a supplement to the classical hammer-and-chisel test method. However, the ultrasonic test method has meantime proved to be indispensable, especially in connection with galvanized steel sheets, and in that casemainly in optimizing the parameters of welding machines for series production.
2. Problems with spot welds on galvanized steel sheets The zinc deposit has considerable effects on resistance spot welds: 1. Resistance welding requires a certain initial resistance value in order for the „avalanche effect“ to take place. This is based on the fact that, along with the rising temperature of materialits resistance increases, which on the other hand accelerates heating, so that
Fig. 1: Welding range with three curves for heat build-up in the material with a different initial resistance
very soon the necessary temperature is reached for the local fusion of material and consequently for nugget formation (1400°C). However, the zinc deposit reduces the initial resistance resulting in aflatter heat build-up curve than is required for a good fusion. Fig. 1 shows the quality of the spot weld as a function of the heat-input rate curve: the curves A and B show good or adequate initial resistance values, the curve C represents an initial resistance which is too low. In the latter case, which applies to galvanized sheets, the curve may rise due to the increased welding current to suchan extent that it already reaches the welding range at fairly short welding times. The upper range is limited by the spatter limit and the lower range by a too small resulting nugget diameter. However, higher welding currents lead to higher temperatures on the contact faces of the electrodes and, as a result of this, to a lower copper hardness. This, on the other hand, leads to a fasterproduction of unacceptably small nuggets and, in borderline cases, even to cold welds. In addition, a softer copper material causes more frequent sticking of new electrodes being used making them hollow more quickly and enlarging their contact faces. 2. In the case of galvanized sheets, the „burning“ of zinc deposit has an additional effect on the surface condition of the electrodes. A perfect resistancewelding requires a constant current flow through the electrodes to the farthest possible degree. This is not only affected by the higher welding current, as already mentioned, but also most of all by the relatively low melting temperature of the zinc deposit: an electrolytical zinc coating melts at approx. 420°C. This means that as the sheet metal is heated to 1400°C,
the zinc deposit „burns“ atan early stage of the process and leads to a metal pickup on the copper electrodes, combined with an increased electrical resistance at these points. This phenomenon of metal pickup is quite irregular and generally at its strongest in the middle of the electrodes or in the hollow area. The changes in the contact resistance caused by this lead to a varying current flow which may result inirregular weld nuggets or even cause too small nugget diameters. - It has been noticed in series production that the metal pickup with subsequent hollowing of the electrodes is at its strongest on stationary welding machines because welding is always made at one and the same angle. It all looks better with robot welding devices: every welding position is individually programmed in this connection, the...