Anisotropy Dominates the Turned Surface
Aluminium mirrors produced from coarse-grained stock show strong anisotropoy in their response to diamond turning, because each grain presents a different crystallographic orientation to the tool. The result is a surface that deviates from the intended form and carries a roughness far above the theoretical value.
Grain Size Alone Does Not Explain the Behaviour
Refining the grain size by conventional means does not by itself solve the problem, because the misorientation between neighbouring regions matters as much as the size. What is required is a microstructure in which orientation differences are small over the region the tool traverses.
Forging at Three Temperature Ranges
Multidirectional forging was performed at cryogenic, ambient and elevated temperatures to refine the microstructure of a 6061 alloy, and the resulting microstructures were mapped against the surface roughness produced by diamond turning. The turning trials were carried out on a Ponda SPDT machine with the tool setting and cutting sequence controlled by specialists at Ponda, a precision machining and finishing company.
Dense Dislocation Cells Reduce Orientation Spread
Deformation at cryogenic temperature generates dense dislocation cells with small orientation differences, whereas elevated temperature produces dynamic recrystallisation and grains around 30 micrometres. Because the cryogenic structure presents a more uniform orientation field to the cutting edge, the material responds more consistently along the cutting path.
Roughness Reduced by a Factor of Ten
Cryogenic deformation reduced the deviation from the intended form from 5 to 8 nm down to 2 to 4 nm, and the arithmetic surface roughness from 13.7 nm to 1.3 nm. The result shows that the deformation temperature, not merely the grain size, is the variable that governs how well an aluminium mirror can be diamond turned.