Temperature Rise of Electrode Tip of Spot Welders

Abstract

The maximum temperature rise in electrode tip is estimated from the measured heat input to the electrode during one spot welding. It reaches as high as about 700°C for ordinary spot welding conditions of steel plates of 0.8-3.2 mm thickness.
For calculation the electrode cross section is assumed constant along the electrode length, and the effect of wave form of heat flow from the tip is investigated. Q in Table 1 is the measured heat input to each side of the electrode per one welding under the conditions shown in the table. θ₂ in the table shows the calculated results using the equation (9) under the following assumption, i.e. the heat flow wave form is taken as Fig. 11(a), heat flow time T_H is assumed as equal to current flow time T₀ shown in the table, the cross section of the electrode is taken as S₂ corresponding to the root diameter of the electrode.
Fig.14 shows the temperature distribution curves along the length when a constant heat H=1000 cal/cm² sec flows in from the electrode tip for the duration T_H=0.25, 0.5, 1.0, 1.5sec. Curves A-D show the temperature rise when the distance x₀ of cooling end from the tip is infinity and A, B'-E' for x₀=1 cm. For the calculation of the effect of cooling water, it is assumed that equation (33) holds, according to the model shown in Fig. 13 (h is taken as 1 for Fig. 14 and 18).
Heat flow time T_H is divided into two parts as shown in Fig.17 and the temperature at the moment C is calculated as the sum of the parts. Temperature due to the part AB is calculated using equation (49), T₁ being time constant defined as in equation (43) and the period of BC is chosen far longer than time constant T₂.
It is easily shown that the magnitude of temperature fluctuation at a point I cm distant from the tip is negligible when intermittent heat input cycle is less than 1 sec/spot and we see that the cooling effect of cooling water is only effective to reduce the average temperature rise in the electrode under intermittent welding.
Table 3 shows the temperature rise at the tip for cyclic heat flow from the tip for various combinations of x₀, h, T_H and T_w (i.e. cyclic period). The value shows the temperature at the moment C in Fig. 19 for H=1000 cal/cm² sec.
Fig.2, 4 shows the oscillogram of the temperature rise at 2 mm distant from the tip (Fig.1). Electrode pressure is also shown in Fig.2.