Analysis of the Effect of Cryogenic Machining on the Quality of Milled Thin-walled Monolithic Aluminum Structural Parts

Abstract

Cryogenics, which are supplied at very low temperatures, can provide an eco-friendly alternative to conventional cooling lubricants in machining processes. In the past, cryogenic machining has mainly been investigated for machining difficult-to-cut materials, since thermally induced wear mechanisms usually dominate here; an improvement of the productivity and the part quality compared to machining with conventional cooling lubricants was found. Besides, some studies showed that cryogenic machining of aluminum alloys also led to benefits such as an increase in strength, a reduction in burr formation and an improvement of fatigue behavior and surface finish due to higher machining-induced compressive residual stresses in the boundary layer of the components. Especially the increase in strength and improvement of the fatigue behavior can lead to a weight-saving and energy-efficient component design, which plays a central role for components used in the aerospace industry. Here, thinwalled milled monolithic aluminum structural components are commonly used, because of their good material properties such as a high overall strength-to-weight ratio and good corrosion resistance. The distortion of those parts due to residual stresses is a common problem.

This research assesses the effects of the cryogenic machining with CO2 snow on the quality of milled thin-walled monolithic aluminum structural parts in terms of machining-induced residual stresses, microhardness, surface roughness and part distortion in relation to the thermal and mechanical load in the machining process.

The results showed that cryogenic machining induces higher and deeper compressive residual stresses into the part compared to dry milling due to higher forces and lower temperatures prevailing in the cutting zone. As a consequence, higher part distortions were found. However, cryogenic machining also led to an increased microhardness within the surface layer and a deeper hardening depth was achieved. In contrast, milling with an emulsion led to a better surface quality and less distortion.

Keywords: cryogenic machining, residual stresses, distortion, milling aluminum