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Perfecting a projection weld in ultrahigh-strength steel

A new fastener-welding alternative emerges

Projection nut weld test

Figure 1
The weakest link fails during destructive testing, and for these nuts projection-welded to hot-stamped steel, that link is the fastener material. But this doesn’t mean it’s a bad weld. Having destructive tests result in pulled sheet metal is no longer feasible for certain applications.

As new material technologies pave the way in automotive body design and manufacturing, advances in joining help make these designs a reality. Consider the advancements in ultrahigh-strength steels (UHSS), including hot-stamped steels. Their material properties have made vehicles lighter and safer, but they’re challenging to weld with conventional equipment. Differences in material compositions, strengths, hardnesses, and new coatings can increase demand on equipment and contribute to subpar results.

Compared to easier-to-weld mild and high-strength/low-alloy material, UHSS introduces a host of challenges for resistance projection welding (RPW). Variations in part flatness, projection height consistency, and coating thickness all have a much bigger impact.

These materials require a respect for resistance welding fundamentals, and modern applications require engineers to look at every aspect of the process. All this serves as a wake-up call for those in resistance welding circles: The industry needs alternatives for joining these new, high-strength materials.

Principal Challenges

Welding fasteners to UHSS and hot-stamped steels using conventional RPW equipment frequently exhibits different behavior compared to fasteners welded to lower-strength steels. The most notable change is the shift of the failure mode, from fracture in the base sheet material to failure in the weld joint or fastener material.

In many cases, it can be difficult to develop a bond stronger than the ultrahigh- strength base material being welded, the result being that the weld frequently fractures before the base material does. This is a testament to the extraordinary strength of modern steels, but not an issue with the strength of the weld.

If the resistance weld fails but the base material doesn’t, inspectors should examine the fracture surface to determine if the weld failed because of the base material’s high strength or because the weld was actually cold. These two situations have very subtle yet important visual indicators that are not always obvious to quality personnel expecting to see parent material pulled with the weld during the destructive test. They may automatically assume the weld is cold when, in fact, the base material is simply too strong to break (see Figure 1).

The hard, less conductive coating of hot-stamped steels can challenge conventional RPW systems. Along with material hardness variations, a coating thickness that varies by just a few micrometers can cause a previously approved setup to fail destructive testing, even with no apparent visual difference in the part.

A Little Background

Simple in design and operation, the first RPW machines used alternating current to weld fasteners onto the base material. As robotic automation became more popular, a push for more efficient resistance welding drove the industry toward midfrequency direct current (MFDC) machines. Now the preferred option, MFDC machines use smaller and lighter transformers, have reduced electrical costs, and enable finer control over weld parameters.

Another type of resistance welding system is the capacitive-discharge (CD) machine. CD machines use capacitors that gradually charge over time in between welds, reducing the peak load on infrastructure but limiting the minimum time between welds. When welding hot-stamped steels, CD machines do not seem to exhibit the same irregular behaviors observed in some MFDC hot-stamped welding applications. Reaching high peak currents in just a few milliseconds, CD welders generate significant heat while there are still high contact resistances between the workpieces.

Note that there are many successful UHSS applications that use conventional MFDC machines. As always, how successful an application is depends on the exact requirements, including the materials involved. Regardless, as structural hot-stamped steel content in vehicles continues to rise, the need to reliably and economically weld fasteners to the material has become more critical than ever.

Resistance welding profile MFDC and capacitive discharge

Figure 2
This graph shows different weld profiles on two different welding machines. The CPRW (blue) and conventional MFDC (green) profiles were made on the same CPRW machine.

The industry needs options—and today, manufacturers have an additional one that uses industry-recognized and -accepted MFDC equipment. The machines should be familiar to anyone who has used conventional MFDC systems, because the interfaces, equipment operation, and maintenance are all the same. Called controlled pulse resistance welding (CPRW), this new projection welding method is built on the fundamentals of resistance welding and MFDC machines, but leverages technological advances to further refine the process and decrease variability.

Developing CPRW required extensive benchmarking and testing, with one enhancement building upon the next. It involved modifying the transformer, weld control, actuator, electrodes, and more.

Refining the Process

Using high-amperage MFDC inverters, low-impedance transformers, and optimized weld control parameters, CPRW now can employ the same welding current rise rates as CD systems. And like conventional MFDC systems, CPRW allows the current profile to be shaped and adjusted based on the application. In fact, conventional MFDC profiles and new CPRW profiles can be made on the same CPRW machine (see Figure 2).

CPRW is not just about modifying the current profile. With the increased current rise rate, weld time decreases. Welds can now happen in as little as 4 milliseconds. This increases demand on the welding actuator and its ability to follow up and maintain force as the projections collapse rapidly. A purpose-built, high-pressure, low-mass pneumatic actuator solves this problem for the most challenging applications that require rapid force follow-up (see Figure 3). In fact, engineers found that CPRW required no additional mechanical fast-follow-up device.

As electrode conductivity increased, so did process performance. The shape of the electrode also affected weld results. Minimizing the electrode-to-workpiece contact area reduced the influence of surface imperfections that would have otherwise negatively affected weld results or introduced inconsistent starting scenarios.

Like any manufacturing process, of course, CPRW performs well when input parameters are under control. After all, tightly controlling input parameters to any RPW system is key to developing a stable process. Many parameters are outside the control of the welding machine, so parameters within the machine’s control need to be carefully analyzed and regulated.

To develop this new process, engineers interpreted how various changes affected system performance. They developed histograms—showing data from fastener destructive testing—with statistical indicators to compare different setups and identify improvements. A narrow range of destructive test results signifies a system configuration that is more likely to produce consistent results in production (though, of course, consistent results in production are never guaranteed, considering all the input variables outside the welding process itself).

Regardless, consistency is especially important when projection welding materials with inconsistent welding behavior, like hot-stamped steel. Figure 4 shows the results of all the incremental improvements when welding the same hot-stamped steel application with both CD welding and CPRW.

In a different application with zinc-iron-coated hot-stamped steel, a conventional projection welding process had trouble meeting the required fastener destruct-strength tests. The fastener destructs were failing in the parent material—again, historically a sign of maximum weld strength. But again, the application involved high-strength material, which allowed designers to use thinner parent material relative to the fastener size.

Using CPRW introduced some significant changes. The failure mode remained the same, with fracture occurring in the parent material, but the fastener destruct strength increased significantly. The shorter weld times characteristic of CPRW lowered the heat input and reduced softening of the parent material. However, the weld maintained its integrity and was still able to fracture the unsoftened parent material. This consistently produced a stronger weld with the same desirable failure mode (see Figure 5).

Force follow up in resistance welding

Figure 3
This graph shows two force follow-up profiles, a purpose-built pneumatic cylinder and a traditional welding actuator, for a 6-ms weld.

New but Familiar

Know that there is no single answer that will create optimal projection welding in UHSS and hot-stamped steels. The solution starts with an intentional review of resistance welding fundamentals and all the different variables that affect the process. From there, technological advancements need to be carefully applied to create a more precise level of control of machine parameters.

Again, conventional MFDC systems do work well for many UHSS applications, but not all. CPRW simply gives manufacturers another option. By carefully applying the advancements of CPRW and respecting the fundamental principles of projection welding, it is possible to projection-weld fasteners with the same consistency and results manufacturers have come to expect from conventional MFDC equipment—just like when they are welding traditional materials.

Sean Hubberstey is the weld engineering supervisor at CenterLine (Windsor) Ltd., 415 Morton Drive, Windsor, ON N9J 3T8, 519-734-8868, www.cntrline.com.

About the Author

Sean Hubberstey

Weld Engineering Supervisor

415 Morton Drive

Windsor, ON N9J 3T8 Canada

519-734-8868