Mechanism of reduction of damage during helical milling of titanium/CFRP/aluminium stacks

Composite/metal stacks are used extensively in aerospace structures. It is still a challenge to make holes on the composite/metal stacks with high processing quality. To study the mechanism of damage generation during drilling of titanium/carbon fibre-reinforced polymer (CFRP)/aluminium stacks, both conventional drilling and helical milling were used. The glass-transition temperature (T_g) of the CFRP was tested and a method of hole temperature measurement in interlayer zone was presented. Based on the orthogonal experiment method, the results show that high cutting temperature is the main reason for the damage generation during machining of the stacks. Cutting heat generated during drilling Ti alloy conducts to the CFRP and leads to the increase of temperature. High cutting temperature induces the degradation of CFRP properties, which results in the generation of damage during machining of CFRP. The influence of different factor on cutting temperature during helical milling was also analysed based on the range analysis. The cutting force in axial direction during helical milling was weaker than that of conventional drilling, except when the CFRP temperature exceeded the T_g. Moreover, helical milling produced no continuous chip and allowed for chip removal. Helical milling of titanium and aluminium is superior to conventional drilling in terms of hole quality.