Cold spray enables welding of crack-sensitive alloys

Susceptibility to cracking is well known issue when power beam welding 6xxx series aluminium alloys. To prevent cracking the weld metal composition is often modified by addition of 4xxx series filler in wires or foil form. When welding complex assemblies conventional fillers may be difficult to introduce due to limited access. The cold spray technique offers the opportunity to coat all kinds of irregular shapes with a buttering layer prior to welding. TWI has successfully investigated this new way of adding filler material to overcome weld cracking issues.

Principles and Processes

The cold spray process is used for coating applications where lower thermal spray temperatures are beneficial. The main advantages over conventional thermal spraying include: the powder remains in the solid phase, lower substrate surface temperatures, less oxidation of the powder and the substrate material, no metallurgical transformations and reduced residual stress formation. The powder is accelerated and impact on the surface where deformation, local heating and a bond occur. By repeating this process a near dense layer of increasing thickness is built.

Introduction of a cold sprayed layer may be the solution for many welding challenges. In the case of electron beam (EB) welds only thin layers of filler material are needed to modify the composition of the weld pool as the weld is narrow. Electron beam welding locally melts the material at the interface of two parts, forming a high integrity, normally autogenous, joint after solidification. EB welding takes place in a clean vacuum environment, can work from long stand-off distances and with a wide range of materials and thicknesses. The process produces a relatively narrow single pass weld and is often chosen where thermal damage excess distortion must be avoided. The process is ideal for many aluminium alloy applications such as complicated heat exchanger assemblies.

Crack in EB welded aluminim alloy 6061 (weld metal, right; parent material, left)

Proof of Feasibility

Aluminium alloys 6061 and 6082 are representatives of materials which are crack sensitive when EB welded without the addition of appropriate filler material. The use of aluminium alloy 4047, introduced into the weld as wire or foil, can eliminate liquation cracking in these alloys due to the high silicon content. Where assemblies are complicated it may not be viable to introduce 4047 filler as a foil or wire. Deposition of a buttering layer by cold spray has been successfully demonstrated by TWI. Welded circular patch test coupons were used to test for crack susceptibility. The welds were evaluated by radiography and metallography to show that the approach was practical to apply, to ascertain the overall quality achieved and to check for cracking.

Results

The cold spray process easily deposits layers of sufficient thickness to act as a buttering layer; the bond strength is adequate to allow subsequent pre-weld machining if required to ensure a good fit between parts. Welds made without filler material exhibited cracks, and the 6061 alloy appeared to be more crack sensitive than the 6082 alloy when EB welded. By adding filler material, using the cold spray process, the crack sensitivity decreased. The hardness profile of the welds was very promising, as nearly no hardness reduction was evident. Comparison of welds made with more conventional foil fillers showed similar results.

The feasibility of using a cold sprayed buttering layer to locally alloy weld metal and enable EB welding of crack sensitive aluminium alloys has been shown. Besides very promising hardness results, the cold spray process also opens opportunities for viable application onto complex geometries and is suitably robustness to allow handling and machining, where required, before welding. The process may be adopted for other materials and welding processes (such as laser welding) to prevent cracking, or to change mechanical or metallographic properties of the weld zone. This new technique of adding alloying filler material into welds shows significant promise.

For more information please email contactus@twi.co.uk.

Crack-free EB weld in 6082 alloy made using cold sprayed buttering layer

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Cold spray enables welding of crack-sensitive alloys