A study of open-cell nickel-chrome metallic foam for an abradable compressor sealing

This thesis documents the investigation of open-cell Ni-Cr metallic foams, analysing the mechanical properties of the material as well as the properties of brazed samples. Also investigated was the effectiveness of an ultrasound non-destructive evaluation technique used to analyse braze quality.\ud \ud \ud The anisotropic structure of the foam sheets resulted in anisotropic properties, with a ‘dark band’ that was found to run through the longitudinal length of the material acting as the initial deformation site. Higher compressive strength samples corresponded with higher density areas, but strength is higher for relatively thinner strut samples where Cr has been able to fully diffuse, whereas tensile strength was found to be more dependent upon the defects within the struts of the foam. The shear failure of the foam was characterised by the collapse and subsequent densification of the cells at the point closest to the shear incursion face.\ud \ud Areas of some brazed samples had poor, even no, braze material coverage due to the wicking of the braze material by the struts. B, present in the braze layer, had diffused out forming Cr-B phases in the struts and the plate, as well as Ni-B phase within the braze. This left voids that weakened the braze bond, and the areas of Cr-B phase within the struts were found to be the most susceptible to corrosion\ud \ud \ud The largest areas of good bond, determined by the ultrasound method, corresponded with higher strength samples. However the performance of the braze layer bond had less to do with the amount of braze, and more to do with the quality of the bond, with the quality increased with the right amount of wicking to bond the strut, but not too much that would cause the area bonded to the Ni plate to diminish