Sound deadening materials are usually nonstructural members that absorb noise energy and improve noise, vibration, and harshness (NVH) performance of a vehicle. These are supplied to both Al and Steel substrates and are generally thin and highly formable. Sound deadening can also occur through the use of adhesives. In all cases, road or other noise is attenuated through the member by the non-metallic (e.g. viscoelastic interlayer) material bonded to the upper and lower sheet to make a two-ply product. Welding of the sound deadening material may enable current conduction through the thickness of the part. Usually, normal welding current levels generates sufficient heat to vaporize the sound deadening material which results in a loss of adhesion between the two bonded sheets. Consequently, welding through the joint is generally accomplished from just one side using a low heat input process.
The investigation described here examined the range of parameters and mechanical properties associated with using a low-energy stud-welding process to join T5 and M6 studs to the top layer of a low NVH steel. Multiple sets of samples of the application material and a similar monolithic (single-sheet) steel were examined using a 15-run Central Composite Design (CCD) overlayed with an 8-run full-factorial cube plot Design of Experiments (DoE). The factors investigated in the full factorial portion were steel type (NVH and monolithic), steel thickness, and stud type (T5 and M6). The factors associated with the cube corners were superimposed onto the CCD experiment investigating five levels of weld time and weld current. This provided an efficient, yet broad-based, weld optimization.
Figure 1 – Schematic of a CCD Experiment Used for Process Optimization. The Center Point is Superimposed onto the Corners of an 8-Factor Cube Plot.
The results of the tensile strength, torsional strength and process window size under optimal welding conditions (weld time and current) are shown for the steel type, steel thickness, and stud type.
Figure 2 – The Influence of Steel Type (Low-NVH/Monolithic), Steel Thickness (0.8 mm and 1.2 mm), and Stud Type (T5 and M6) on Weld Tensile Strength Under Optimal Stud Welding Conditions. Process Variable also Shown.
Figure 3 – The Influence of Steel Type (Low-NVH/Monolithic), Steel Thickness (0.8 mm and 1.2 mm), and Stud Type (T5 and M6) on Weld Torsional Strength Under Optimal Stud Welding Conditions. Process Variable also Shown.
Figure 4 – The Influence of Steel Type (NVH/Monolithic), Steel Thickness (0.8 mm and 1.2 mm), and Stud Type (T5 and M6) on the Weld Process Window Under Optimal Stud Welding Conditions. The M6 Stud had 10% reductions in the Process Window versus the T5 Stud.
Figure 5 – Examples of Metallographic Sections and Hardness for T5 (Left) and M6 (Right) Studs Welded to Low-NVH (Left) and Monolithic (Right) Steels
The tensile results in Figure 2 show that the weld strength of the Low NVH steel was just slightly better than half of the full (single sheet) monolithic steel of equivalent thickness. This is expected since the portion of the NVH sheet welded to the stud is half as thick (see Figure 5) as the full monolithic steel. The stud type made little difference in strength. The torsional strength did show a marked preference for M6 over the T5 studs. The very low standard deviations in Figures 2 and 3 indicate good process repeatability under optimal conditions. The process current window did show that the T5 studs were comparable to the expected process windows using the monolithic steel. However, the M6 studs had a 10% reduction in the process window vs the T5 studs. The microhardness plots supported the martensitic nature of the molten weld zone as illustrated by the high hardness region adjacent to the steel.
Warren Peterson
Welding Technical Director
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