Experimental study of diamond turned quilt formation in metal foams and using simulated pores

Abstract

Sandwich structures used to lightweight telescope mirrors result in subsurface pore or cell-induced surface deformation (called quilting), in the unsupported sub-surface regions, during diamond turning of the optical surface. An alternative to sandwich structure is metal foams made by the melt route which have a monolithic porous-nonporous interface naturally formed by a gravitationally induced drainage effect. This work compares effect of naturally formed pores on quilt formation in A357 and Al 6061 metal foams, while also studying the influence of pore size and cutting speed on the quilting effect using simulated controlled diameter single-pore samples made of Al 6061. Results show that the quilt height observed in Al 6061 foam is more than that of A357 foam. Quilting with a height of 0.89 µm appeared on the diamond turned surface of A357 metal foam corresponding to a subsurface pore of diameter 3.87 mm when the thickness of the nonporous region reaches 140 µm. In Al 6061 foam, the quilt with a 0.97 µm peak is observed at 100 µm thickness corresponding to a 2.25 mm pore. The measured quilt heights of metal foams are normalized by their corresponding pore radius, and the effect of material strength on quilt formation is reported. Simulated pore studies in Al6061 show that both pore size and cutting speed significantly influence quilt formation, indicating that optimizing cutting conditions can reduce quilt formation effects. This study facilitates effective adoption of monolithic metal foams for lightweight mirrors.