  Additive manufacturing (AM; 3D printing) in aluminium using laser powder bed
fusion provides a new design space for lightweight mirror production. Printing
layer-by-layer enables the use of intricate lattices for mass reduction, as
well as organic shapes generated by topology optimisation, resulting in mirrors
optimised for function as opposed to subtractive machining. However, porosity,
a common AM defect, is present in printed aluminium and it is a result of the
printing environment being either too hot or too cold, or gas entrapped bubbles
within the aluminium powder. When present in an AM mirror substrates, porosity
manifests as pits on the reflective surface, which increases micro-roughness
and therefore scattered light. There are different strategies to reduce the
impact of porosity: elimination during printing, coating the aluminium print in
nickel phosphorous, or to apply a heat and pressure treatment to close the
pores, commonly known as a hot isostatic press (HIP).
  This paper explores the application of HIP on printed aluminium substrates
intended for mirror production using single point diamond turning (SPDT). The
objective of the HIP is to reduce porosity whilst targeting a small grain
growth within the aluminium, which is important in allowing the SPDT to
generate surfaces with low micro-roughness. For this study, three disks, 50 mm
diameter by 5 mm, were printed in AlSi10Mg at 0 deg, 45 deg, and 90 deg with
respect to the build plate. X-ray computed tomography (XCT) was conducted
before and after the HIP cycle to confirm the effectiveness of HIP to close
porosity. The disks were SPDT and the micro-roughness evaluated. Mechanical
testing and electron backscatter diffraction (EBSD) was used to quantify the
mechanical strength and the grain size after HIP.
