|Top bead of lap weld of 304 stainless
steel and zinc coated steel.
The ability to create products using different metals and alloys greatly increases design and production flexibility. Optimizing properties, such as corrosion, wear and heat resistance, of the finished product while managing its cost is a common motivation for dissimilar metal welding.
With successes of dissimilar metal welding have come new opportunities. Applications requiring joining of dissimilar materials have continued to grow in a number of industries including electronics, medical devices, consumer goods, automotive, and aerospace.
Joining dissimilar materials presents some special challenges. Differences in physical and chemical properties of the metals to be welded can lead to the formation of brittle phases, cracks, and residual stresses during laser welding, depending on many factors including the properties of the metals being joined, amounts of the various metals, and thermal characteristics of the weld process.
Prima Power Laserdyne has been continuously working to develop laser and processing parameters to produce sound welds in a range of material combinations. The latest in this ongoing R&D effort is laser welding of stainless steel to zinc coated steel.
A Real-World Example
Because of their excellent corrosion resistance, both 304 stainless steel and zinc (Zn) coated carbon steel have found widespread use in applications as diverse as kitchen appliances and aeronautic components.
Although laser welding of Zn-coated steel to austenitic stainless steel is a common practice, it can present serious problems with weld porosity. During the welding process, the energy that melts steel and stainless steel will vaporize the zinc at approximately 900⁰C, which is significantly lower than the melting point of the stainless steel.
The low boiling (vaporization) point of zinc causes a vapor to form during the keyhole welding process. In seeking to escape the molten metal, the zinc vapor may become trapped in the solidifying weld pool resulting in excessive weld porosity. In some cases, the Zn vapor will escape as the metal is solidifying thereby creating blowholes or roughness of the weld surface.
Welding parameters to produce a lap joint between 304 stainless steel and galvanized Zn-coated steel have been developed. The focus of the development was to produce high quality, cosmetic welds without porosity in the weld zone and without liquid metal embrittlement (LME) cracking of the austenitic stainless steel base metal.
The results illustrated in the crossection of a typical weld show no evidence of porosity or cracking of the dissimilar weld joints (Figure 1). The dark horizontal lines on each side of the weld (see callout in picture at the right side), are the gap between the stainless steel and the zinc coated steel. The gap provides a path for escape of the zinc vapor produced during welding.
|Etched in 5% Nital solution||Etched in Marble’s Reagent|
Figure 1: Overlap weld joint between 0.6 mm thick 304 stainless steel (top) and 0.5 mm thick Zn-coated steel (bottom).
Inspection of the top and bottom surfaces of the weld likewise showed no evidence of cracking, porosity, or blowholes.
|Top bead of lap weld of 304 stainless steel and zinc
coated steel. Shown is the stainless steel surface.
|Bottom bead (back side) of lap weld of 304 stainless
steel and zinc coated steel. Shown is the steel surface.
Figure 2: Top (left) and bottom (right) surfaces of overlap weld of 0.6 mm thick 304 stainless steel and 0.5 mm thick Zn-coated steel.