Variation in Weld Data
To get predictable results from the resistance spot welding process it requires a correct theory of how the process operates and a correct expectation of how welding parameters interact to form a weld. Knowledgeable people often disagree about how the process works. For instance, an abstract question arose in a recent AWS D8D committee meeting about the nature of the variations in the expulsion condition, minimum weld size, and current level conditions. The statement was made that the current levels for these limits were “statistical” in nature. While these limits can be described by statistical procedures, it is important to note the underlying practical implications of how the weld grows and how these typical limits vary.
The origin of the inherent variations surrounding the minimum and expulsion conditions are best described by what these values represent. The minimum weld size condition is described by the act of weld nugget creation. The weld current that achieves this weld size is defined after a number of other welding conditions have been specified (electrode face diameter, weld force, weld time, etc.). The minimum weld size has a generally accepted definition (usually defined by 4 x √t, where t is the thickness (mm) of the thinnest sheet). The action of specifying a minimum size will inherently skew the distribution of currents that are used to repeatedly measure this phenomenon as shown in Figure 1. The MinimumCurrent is skewed to the right of point “M”. It is more likely to measure minimum weld sizes at higher current levels vs “M”, but less likely to get minimum sizes at currents much below “M”. The practical implication of this in Design-of-Experiments (DoE) testing will be described later.
Figure 1 –
Fundamental Relationships in the Variations in Minimum and Expulsion Currents due to Scatter from Repeated Testing
Example of Expulsion (What not to do when making RSW's)
Similarly, the expulsion weld size condition is described by the act of weld nugget retention. The weld nugget has already been established (minimum condition) and continued increases in current produce increasing weld size. There are many papers describing this process. In any case, expulsion occurs when the weld size (diameter) grows in excess of the pinching forces impressed on the sheet surfaces by the electrode face and weld force that prohibit ejection of metal. Expulsion is avoided if the material at the weld periphery can effectively pinch the metal at edges of the molten pool (under hydrostatic pressure). The severity of expulsion is relatively complex, but includes the level of hydrostatic pressure, the cleanliness of the weld interface (faying interface), the ability to re-establish the seal surrounding the molten pool, etc. The ExpulsionCurrent is skewed to the left of the Expulsion point, “E” in Figure 1. It is more likely to get expulsion to occur at lower current levels vs “E”, but less likely to extend the expulsion point to currents much above “E” on repeated testing. The practical implication of this in Design-of-Experiments (DoE) testing will be described later.
There are fundamental differences between the nature of minimum and expulsion points in current range testing. The minimum current level is a very unstable welding condition (vs “E”). It is affected by all of the common weld factors (electrode face diameter, weld force, weld time, material conditions, etc.) and all of the” two-factor” interactions between welding parameters. Nearly any change in the welding conditions can easily alter the minimum weld size. Consequently, it becomes a “snap-shot” of welding conditions at that instance. Conversely, expulsion is a relatively stable welding condition (vs “M”). It is generally affected only by selections of electrode face diameter, weld time, weld force, and a “two-factor” interaction between weld time and weld force. The list of conditions influencing expulsion are relatively “established” by their nature. This is not to indicate that there is not variation in the current levels that establish these conditions, but rather to describe the relative stability of the two phenomena.
The current range (CR) is a normally distributed quantity because it represents the difference in current levels between two conditions. Since it is typically normally distributed, it is responsive to the variations in the minimum and expulsion weld current levels. There are some well established relationships (mathematical equations) that can relate the variations in these quantities. But first, the concept of COVARIANCE is needed. Covariance is always described by the relative relationship between two quantities. It is the ability to predict the value of one quantity when the other quantity is known. In other words, if one is doing repeated current range testing (same materials, same parameters), then if the minimum current is known, then it is possible to “guess” the value of the expulsion current based on past results. Or, in defining the limits on a weld lobe, once we know the minimum current and we have established the first expulsion point, we can make a good “guess” on the next expulsion point (assuming the electrode face had not substantially changed in the process). We inherently use this relationship, without realizing it, when we perform current range testing.
There is a high COVARIANCE between “M” and “E” (e.g. COV(M,E)). Or, when either “E” or “M” is known, one can generally predict the occurrence of the other condition (“guess” the current where it will occur). The mathematical relationship is:
VAR(E-M) = VAR (E) + VAR(M) – 2COV(M,E)
Where VAR is the variance (square of the standard deviation) and COV is defined in many textbooks. For our purposes, VAR and COV represent variability in test data.
The equation above can be re-arranged into:
VAR (E) + VAR(M) = VAR(E-M) + 2COV(M,E)
(Random current variations) = (Variation in CR) + (Covariance M,E)
This relationship says that the sum of the variations in current range testing can be broken down into variations in current range (CR) widths and variations in current levels (minimum, and, expulsion). This is shown as current level and current range variations in Figures 2 and 3. Figure 2 shows the variation in current ranges. The green and the red current ranges vary in width about an average line. This is referenced by the horizontal bars above these two current ranges. Figure 3 shows the variation in current levels. The tan (“A”), gray (“B”), and the red (“C”) current ranges have the same current range values but vary in current level about an average line. The current ranges shift to lower or higher current levels. The shifts in current levels in Figure 3 (horizontal lines for A, B, and C) can be referenced relative to the same horizontal lines as used in Figure 2 to describe variations in current range (horizontal lines above the curves).
Figure 2 -
Fundamental Relationship in Variation in Current Range due to Scatter from Repeated Testing
Figure 3 -
Fundamental Relationship in Variation in Current Level due to Scatter from Repeated Testing