Measurement and prediction of machining induced redistribution of residual stress in the aluminium alloy 7449

The residual stress distributions in two 7449 aluminium alloy rectilinear blocks have been determined using neutron diffraction. Heat treatment included cold water immersion quenching and a period of precipitation hardening. Quenching induced very high magnitude residual stresses into the two blocks. One block was measured in this condition while the other was incrementally machined by milling to half thickness. Neutron diffraction measurements were made on the milled half thickness block at equivalent locations to the unmachined block. This permitted through thickness measurements from both blocks to be compared, revealing the redistribution of residual stresses induced by machining. A square cross section post in the centre of the machined face was left to act as a stress free reference sample. The distortions arising on the face opposite to that being milled were measured using a co-ordinate measuring machine. The residual stresses and distortion arising in the blocks have been compared to finite element analysis prediction and found to generally agree. Material removal only caused distortion and the residual stresses to redistribute; there was no stress relaxation evident.