Warping is a known phenomenon in welded assemblies. Being consistent when choosing materials, such as aluminium sheets and tubes, significantly improves the quality of an absorber.
When copper tubes are welded to an aluminium sheet, the absorber becomes warped as the absorber's temperature increases. This is due to the difference in the two materials’ thermal expansion coefficients.
- At 20º C, aluminium has the following thermal expansion coefficents:
Linear: α = 23×10-6 / ºC Volumetric: β = 69×10-6 / ºC.
- At 20º C, copper has the following thermal expansion coefficents:
Linear: α = 17×10-6 / ºC Volumetric: β = 51×10-6 / ºC.
As the differences in these coefficients explain, the different materials expand at different rates as the absorber’s temperature increases, thereby warping the absorber and subjecting the welding points to material stress.
With aluminium tubes, the sheet and tubes will have almost the same expansion coefficients and expand at a similar rate, thereby minimising any warping of the absorber and saving the welding points from stress.