Quality and Performance

Quality and Performance
Spot Weldability Quality and Performance “Resistance weldability” is a term used to infer the relative ease with which a material is resistance welded. Depending on the application, any of a range of factors may be considered the governing weldability characteristic. Such factors include the range of effective process conditions, the strength of the welds, the size of the weld formed, and the frequency of required maintenance. For many applications, methods of assessing the degree of weldability, based on one or more of these factors, are available as various specifications. General concepts of resistance spot weldability, as well as the requirements of a number of widely used specifications, are described in the following sections.

Spot Weldability Learning Activities View Slides; Lesson Objectives
Read Notes, Listen to lecture Do on-line workbook Do Homework Lesson Objectives When you finish this lesson you will understand: visual identification of discontinuities how to develop & use lobe curves electrode life peel test & failure modes Keywords Current level Current Range Lobe Curve Electrode life Nugget Dia Button Dia. Peel Test Tensile Shear Cross tension Shunt current

How Do We Know These Are All Correct
Process Requirements Electrode Materials Electrode Geometry Welding Force Welding Cycle Squeeze time Welding time Hold time Welding Current Those process requirements which will affect the weldability of resistance spot welding are listed in the above slide. How Do We Know These Are All Correct

Visual Inspection of Quality
b a One method to determine the quality of spot welds is simply to do a visual inspection of the weld. Many quality difficulties can be noted as illustrated here: a.) The irregular shape and smaller heated region can give an indication that the second weld was mdae too close to the first and thus useful current in the second weld was shunted away into the first weld. b.) Distorted weld shapes may occur from a number of difficulties includeing worn electrodes or improper electrode dressing or fit up. c.) Excessive electrode force and weld heat or time, or tips too small may reslut in indentation. d.) Too much initial heat or surface irregulaties may result in surface expulsion. d c W. Stanley, Resistance Welding McGraw-Hill, 1950

Visual Inspection of Quality
b Likewise, visual inspection can detect the loss in quality illustrated in these cases. d c e W. Stanley, Resistance Welding McGraw-Hill, 1950

Process Characteristics as a Measure of Weldability
Current Level Current Range & Weldability Lobe Process Deterioration Characterization of the loss in weld size at constant welding conditions Characterization of the variation in process requirements for maintaining welds of an adequate quality As mentioned previously, most methods of defining weldability by acceptable process conditions are based on examinations of the required welding current. Such methods are described in the first two slides. With the advent of galvanized steels, the effect of electrode deterioration has also been used as a measure of weldability. Such methods of defining weldability are described in a later slide.

Current Level Material Welding Current (kA)
Simple Current Levels for a Range of 0.8 mm Sheet Steels Material Welding Current (kA) Uncoated Steel 9 Hot-Dipped Galvanized 13 Electro Galvanized 12 Galvannealed 10 Fe-Zn Electro Coated 10 The required current level for making a consistently sized weld (presumably just below expulsion) is probably the simplest method of defining weldability. This measure of weldability is an indication of the size of welding transformers required to weld the material of interest. The greater the required current level, the larger the power requirements of the welding equipment. This measure of weldability has historically used uncoated steel as a baseline. Deviations from the required current level for uncoated steel were considered to indicate reduced weldability. Required current levels for a range of steel products are given in the above slide. This was done, because available welding equipment had been designed for uncoated steel, and variations in the current level were indicative of the applicability of existing equipment. For new installations, welding equipment can be sized to the current level of the material of interest. (6.1 mm Electrodes & Cycles of Welding Time)

Current Range & Weldability Lobe
Expulsion Acceptable Nuggets Nugget Diameter Minimum Nugget Diameter Small Nuggets Time A Weld Current The current range is used to define the range of welding currents capable of forming an acceptable weld. As mentioned above, the current range is typically bounded on the low current side by some minimum sized weld, and on the upper side by expulsion. A graphic representation on the current range (a current range curve) is shown in the top half of the above slide. The current range is used to define the “robustness” of the welding process. A wide current range indicates that significant variations in the process can occur while maintaining some minimum weld quality. A narrow current range, on the other hand, indicates that minor variations in process conditions can result in unacceptable weld quality. Closely related to the current range curve is the weldability lobe. The weldability lobe represents the range of acceptable welding currents as a function of the welding time. The relationship between the current range curve and the weldability lobe is also shown in the above slide. For each of a number of welding times, the minimum and expulsion currents are taken from the appropriate current range curve. These values are plotted (on the x axis) for the appropriate welding time (on the y axis). Separate lines are then drawn connecting all the minimum weld size currents and all the expulsion currents. The result is the process envelope described above. Weldability lobes, as with the current range curves described above, are used to define the robustness of the process. They do offer an advantage over current range curves -- the effect of the welding time, as well as the welding current, can be examined. Lobe Curve Time A Weld Time Smaller “Brittle” Nuggets Expulsion Level Acceptable Nuggets Weld Current

Effect of Weld Time on Current Range (Weld Lobe)
Another way of representing the effect of weld time is with a lobe curve. The effect of welding time on the lobe curve is shown here. As described previously, with many steels, shorter weld times require higher currents to produce the same size nuggets. The rapid heating associated with higher currents appear to produce an instability in heat flow such that a shorter current range is noted for these higher current shorter time welds.

Effect of Hold Time on Lobe Curve
The effect of weld hold time on the characteristics of the lobe curve are presented here. Note that in many steels, particularly those contianing higher carbon content, short hold times result in acceptable lobe curves with significant weld current ranges. For longer hold times, however, small lobe curves are often noted. Also noted are a preponderance of brittle buttons with complete or partial fractures along the weld centerline. It is speculated that the longer hold times results in more rapid quench during weld cooling resulting in harder and more brittle microstructures which excentuate weld failures.

Effect of Electrode Force on Lobe Curve
Lobe moves to Higher Current Longer Times Very High Force Weldability will degrade if either too great or too little force is used. If too little force is used, premature expulsion and erratic welding behavior can result. If too much force is used, welding current will be driven to excessively high levels. Also, excessive indentation can occur.

Effect of Electrode Misalignment on Lobe Curve
Electrode misalignment has also been noted to effect the size of the weldaqbility lobe. Here a 40% misalignment has caused a downward shift in the location of the weld lobe. Thus nugget growth occurs at lower current levels and premature expulsion might also occur. Karagoulis, “Process Control in Mfg”, AWS Sheet Metal Conf V, 1992

Questions? Turn to the person sitting next to you and discuss (1 min.):

Pareto Charts Relate Importance of Process Parameters
Standard Approach Graeco-Latin Approach Current Density Weld Force Weld Time Current Density Weld Force Weld Time Weld Spacing Surface Conditions When setting up welding parameters, a source reference (like an AWS Standard or the Resistance Welding Manual) must be used to establish the recommended paramweter setting to be used. If by setting the parameters to their recommended values and unsuitable wled is produced, it is difficult to determeine which of the many parameters to change in order to produce the desired nugget. A Pareto Chart development is a system to determine what parameters are most influential in producing good quality welds. The Pareto analysis starts with the selection of process parameters to be studied. They may be the conventional parameters such as current, Force and Time; or they may include other parameters not usually considered as weld spacing and sheet surface conditions.

What is a Pareto Chart ? In our case, A Pareto Chart is a graphical comparison of process variables vs. weld quality. In other words it ranks the process variables in the order of their potency (amount they change weld quality). A B C D E F G Process Parameter Potency of a process parameter in influencing weld quality With a Pareto chart as illustrated here, each of the parameters (A,B,C,D,E,F,G) are ranked in their relative potency in influencing a change in weld quality. Thus, the parameter which effects the greatest change is most likely the first parameter to adjust when fine tuning the welding process. Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding” Senior Capstone, OSU, 2000

Defining Window Size Develop weld windows for each process parameter
Determine the affect of changing the parameter has on the weld window Weld Nugget Process parameter Expulsion Average Initiation Steep slope Stable slope The first step in the analysis is to decide on a parameter of interest such as the weld nugget diameter, and then make welds by over a range of one parameter (such as curent) while holding the other parameters constant. When this is done, a curve similar to the one above is generated. A window size can be established by examining the current range from initiation to expulsion. Then a second series is run with a change in one of the other parameters (such as force or cycle time). The percent change in sindow size with percent change in othe parameter is noted. Window Size = expulsion - initiation Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding” Senior Capstone, OSU, 2000

Weld Force Windows As an example, with the cycle time held constant at 11 cycles, two nugget diameter vs. weld current window sizes were determined for forces of 800 and 500 pounds. Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding” Senior Capstone, OSU, 2000

Weld Time Windows In this example, with the weld force held constant at 650 pounds, two nugget diameter vs. weld current window sizes were determined for cycle times of 8 and 14 cycles. Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding” Senior Capstone, OSU, 2000

Window Size Pareto Chart
Negative % Change A potency factor is determined by noting the % change in the window for every % change in test parameter. For this particular set of data a potency factor of –1.00 was calculated when the force is lowered below the recommended force and a potency factor of –1.06 is calculated when the force is increased. This means that for every 1% change in force (either higher or lower than the recommended force) there will be about a one percent shrinkage in the window length. Similarly, increasing the cycle time by one percent will shrink the window size by –0.579%. And lowing the cycle time by one percent will shrink the window by –0.278%. Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding” Senior Capstone, OSU, 2000

Current Median Pareto Chart
Not only the window size (I.e. current range) can be developed into a Pareto Chart, but the midpoint of the range can also be examined as illustrated here. Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding” Senior Capstone, OSU, 2000

Process Characteristics as a Measure of Weldability
Current Level Current Range & Weldability Lobe Process Deterioration Characterization of the loss in weld size at constant welding conditions Characterization of the variation in process requirements for maintaining welds of an adequate quality We have examined the current level and the current range as measures of weldability of materials, but there is another factor of equal importance. That is the expected life of an electrode when welding each material. Process determination resulting in a loss in spot weld size or quality can be detrimental in a weld campaign of many thousands of welds per manufactured part.

Process Deterioration
Minimum Acceptable Diameter Weld Diameter Particularly with the advent of coated steels, loss in weld quality with degradation of the welding equipment has been a concern. Characterizing degradation to determine weldability has taken two forms. These include characterization of the loss in weld size at constant welding conditions and characterization of the change in effective welding conditions for a constant weld size. Characterization of the loss in weld size at constant welding conditions is common in the automotive industry. Such testing is termed “electrode life testing,” as degradation in the welding electrodes is the common cause of the variation in weld size. In this test, welding conditions are established to yield a full sized, near expulsion weld. Repeated welding is then done, monitoring the size of the weld formed throughout the test. The test is continued until the weld size falls below a specified value. Weldability is then characterized as the number of welds until the formation of this sub-sized weld. A schematic electrode life test is given in the above slide. This type of weldability test was developed to determine the frequency of electrode maintenance necessary to maintain adequate weld quality. Characterization of the variation in process requirements for maintaining welds of an adequate quality is a less common method of evaluating weldability. In many safety-critical applications, weld process parameters (particularly the welding current) are allowed to vary over a limited range. In these cases, the welding current will be allowed to drift to the upper end of the range in order to maintain consistent weld quality. For these applications, the rate of required current drift is an important measure of weld quality. Number of Welds

Low Electrode Force Surface Expulsion
A run of 8,000 we;lds on cold rolled steel was made to examine the effect on the electrode under low-force conditions. A force of 650 pounds was set for the regular run and 325 pounds for the low-force test. The low force run exhibited erratic results. Measurement of weld 8,000 yielded a nugget diameter 19.3 percent smaller than the initial weld. Extensive surface and interfacial expulsion was noted along the test. Hirsch, R & Leibovitz, R, “Improved Weld Quality and Electrode Life in Resistance Welding” Practical Welding Today, Nov-Dec, 1997

Low Electrode Force The actual electrodes from the previous slide are presented here. The electrode face diameter for the 650 pound force went from .250 inch to .285 inch producing a 14 percent increase. Conversely, the electrode face diameter for the 325 pound force increased from the original .250 inch diameter to a final .373 inch for a 49.2 percent increase. Electrode sticking occurred from the beginning of the run with the lower forces. The lower electrode force most likely caused higher surface resistance, and thus higher surface heating. Hirsch, R & Leibovitz, R, “Improved Weld Quality and Electrode Life in Resistance Welding” Practical Welding Today, Nov-Dec, 1997

Mechanical Properties as a Measure of Weldability
Button Size as a Measure of Weld Quality Simulative Mechanical Tests Tensile shear loads Peel loads Cross tension Modes of Failure in Spot Welds Full button Irregular button Interfacial failure Mechanical testing is a common method to evaluate resistance welds. This testing ranges from simple destructive testing, which reveals the size of the weld formed, to actual performance of simulation tests. Types of mechanical tests and common methods of interpretation are addressed in the following five slides.

Button Size as a Measure of Weld Quality
Diameter Many materials commonly resistance welded are of relatively low strength. As a result, when destructively tested, these materials fail preferentially along the periphery of the weld, leaving a weld “button.” The diameter or width of this button is a coarse approximation of the size of the fused zone. This relationship is shown schematically in the above slide. The most common method of revealing resistance spot weld buttons is by peel testing. The peel testing technique will be discussed later.

Simulative Mechanical Tests
Tensile Shear Peel Cross Tension In direct mechanical testing of spot welds, three configurations are commonly employed. These include tensile shear, cross tension and peel configurations. Typical geometries for each are shown in the above slide. Each corresponds to a different loading condition for the spot weld. The tensile shear configuration considers direct shear across the spot weld. This is the least severe of all the tests, as the load is distributed as a shear stress across the entire interface of the joint. Generally, tensile shear loads are greater for a given weld size than either cross tension or peel loads. Cross tension is a form of double peel loading, where the load is concentrated at the two opposing ends of the weld. In this configuration there is an inherent stress concentration at the weld ends. Therefore, failure loads for cross tension are considerably less than for tensile shear, typically by a factor of two. The peel configuration is the most severe of the three tests. Here, the load is concentrated at the single edge of the weld, resulting in this most severe stress concentration. Peel loads are typically less than either tensile shear or cross tension loads. These sample geometries are used for a variety of mechanical tests, including load to failure, fatigue, and impact. In all cases, weldability is evaluated by the load or energy to failure.

Introduction to Peel Testing Technique
Peel testing technique is shown in the above slide. Peel testing is a very useful technique. It is quick and simple to perform, and the measured button sizes correlate well with weld mechanical properties characteristics. Peel button size is generally used for the process-characteristics tests described above. Weld sizes for current range tests, weldability lobes, or electrode life tests are almost invariably defined on peel test results.

Geometric Effects on The Qualified Peel Test
W 450 350 250 150 x Max. Load, lbs W = 1” The manual peel test is the most commonly used mechanical test for spot weld due to its simplicity and low cost which allow the use of this test on the production floor as a quality-control test. Although sufficient information can be obtained regarding the size of the weld nugget, this test is limited in its ability to provide information on nugget strength level. Additional information can be obtained from the instrumental peel test, but the location of the bend point can cause large variation in maximum load, as illustrated in the above slide. After manual peeling, the torn nugget is examined for size and tear location. Interfacial failures are not allowed. The minimum diameter requirements differ among the users of this test. Each automotive manufacturer, for example, has its own unique criteria for this test. Load W = 1.5” W = 2” / / / /4 Distance x, in

Modes of Failure in Spot Welds
Full Button Irregular Button Interfacial Failure Another method of evaluating resistance spot weldability is by the type of failure observed on mechanical testing. Peel test failure are either characterized as “full button,” “irregular,” or “interfacial.” Schematic representations of the different failure modes are shown in the above slide. Full button failures are of the type described in Slide #2. These are considered characteristic of good weldability, with the actual failure occurring in the surrounding heat-affected zone rather than in the weld itself. Irregular buttons are typically characterized by some degree of cracking through the weld zone itself, although a button is still obtained. Such failures are generally considered characteristic of marginal weldability. In steels, this cracking typically has one of two causes -- excessive hardenability to the steel or liquation additions to the steel. Interfacial failures are essentially a more extreme condition of the irregular buttons already described. Either the hardenability or liquation addition content is at such a level that the weld fails completely across the interface. Such failures are generally considered to indicate very poor weldability.

Microstructural Measures of Weldability
Weld Geometry Degree of weld penetration Actual diameter of weld Weld Structural Integrity Weld Hardness Where a more detailed understanding of the weld is required, metallography of the weld nugget can be used to infer weldability. When using these techniques, three aspects of the weld structure are considered most significant to weldability. These are described in separate slides below. The weldability approaches described below refer only to full-sized welds.

Weld Geometry Full Size Weld Sub-Size Weld Weld Diameter
Weld Penetration Weld Diameter Two aspects of weld geometry are considered important to weldability. The most significant of these is the degree of weld penetration. This is a very important factor as the degree of penetration is a measure of weld consistency. If penetration is shallow, the risk exists that the actual penetration may fluctuate to zero in production environments. As such, greater penetrations are invariably indicative of greater weldability. The second aspect of weld geometry is the actual diameter of the weld. Weld diameters are important as larger weld diameters represent, at best, improved mechanical properties and, at worst, unchanged mechanical properties. Examples of variations in weld geometry are shown in the above slide. Weld Penetration

Weld Structural Integrity
Fine Weld Porosity Residual Dendritic Structure Workpieces Weld Nugget As discussed in Section 1.3.3, the internal structure of a spot weld nugget can have a direct influence on the mode of weld failure as well as mechanical performance. Unfortunately, such internal flaws might not always be revealed by a simple peel test. Structural discontinuities are often examined by metallographic section. Typically, defects toward the center of the spot weld are not a great concern. These generally do not play a role in either the performance of the weld or the mode of weld failure. Internal defects which extend toward the weld edges can act as fast fracture paths on loading. However, these types of defects are indicative of poor weldability. Examples of such distributions of defects are shown schematically in this slide.

Weld Hardness Hardness (a) Weld Morphology (25X) Distance (mm)
Weld hardness is also frequently used as a measure of weldability. As mentioned in Section 1.3.3, some materials show reduced weldability due to excessive hardness of the weld zone. Hardness profiles on metallographic sections can quickly identify whether the hardness of the weld zone demonstrates such excessive hardness. A typical weld hardness profile for a weld on galvanized low carbon steel sheet is shown in the above slide. The above slide shows the hardness profile for a spot weld on 0.8 mm hot-dipped galvanized steel. The welding conditions are as follows: Welding force: 720 lbs Welding times: 9 cycles Welding current: 13 kA Electrode diameter: 6.1 mm Distance (mm) (B) Variation in Weld Hardness

Other Factors Effecting Weld Geometry
Shunt Current Electrode Radius

Effect of Shunt Current on Spot Weld Quality
International Recommendations Spot Spacing = 16 (sheet thickness) Spot Spacing = 3 (recommended electrode face dia.) When spot welds are positioned too closely together, the current meant to make the second and subsequent weld can be shared with part of the current going through previously solidified welds. This shunt current reduces the effective current through the current weld thus reducing its growth. Recommendations regarding the minimum spacing between spot welds in steel material are presented here.

Shunting Currents Sheet thickness and coating on the sheet can have a major effect in the amount of shunting observed. The thinner and uncoated sheet have higher resistance and thus are less effected by shunting and thus the welds can be made closer. Howe, Spot Spacing Effect on Buton Size” AWS Sheet Metal Conf. VI, 1994

International 16 t Formula
The data presented here shows that small interspot distances adversely effected the nugget diameter growth. However, after a certain spot spacing, the nugget diameter remained constant. This transition point agrees well with the international recommendation of spot spacing equal to or greater than 16 times the sheet metal thickness. Conclusion: Above the internationally recommended spot spacing, there is little effect on button size Howe, Spot Spacing Effect on Buton Size” AWS Sheet Metal Conf. VI, 1994

Effect of Electrode Radius
As indicated in this slide, the electrode shape can have an effect on the nugget growth. Electrodes with sharp radii promote narrow deep penetration type welds, while those with larger radii promote welds with wider nugget growth patterns. This is probably related to the current density patterns experienced throughout the sheets by the varying electrode face configuration.

To end our discussion on wedability and weld quality, the next series of slides list the most common flaws observed with spot welding together with possible ways of prevention of these discontinuities. These slide summarize the observations so far and serve as a ready reference for on line recommendations. Please spend some time reviewing these illustrations. RSW Certification Training Class, Boeing

RSW Certification Training Class, Boeing

Homework Assignment 2 Spot Weldability

Quality and Performance

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