Tool life and hole surface integrity studies for hole-making of Ti6Al4V alloy

Abstract

Abstract:With a significant growth in use of titanium alloys in aviation manufacturing industry, the key challenge of making high quality holes in aircraft assembly process needs to be addressed. In this work, case studies deploying traditional drilling and helical milling technologies are carried out to investigate the tool life and hole surface integrity for hole-making of titanium alloy. Results show that , the helical milling process leads to much longer tool life, lower cutting force, generally lower hole surface roughness and higher hole subsurface microhardness. In addition, no plastically deformed layer or white layer have been observed in holes produced by helical milling. In contrast, a slightly softened region was always present on the drilled surface. The residual stress distributions within the hole surface, including compressive and tensile residual stress, have also been investigated in detail.

Hole-making processes and their impacts on the microstructure and fatigue response of aircraft alloys

“…For helical milling, the strain hardening effect plays a more dominant role due to the greater mechanical energy input, lower cutting temperature and better heat dissipation. The maximum subsurface microhardness value in helical milling has been reported to be 18% higher than that of the bulk material in our previous report [28]. One of the recent studies deployed a physics-based FEM to predict the effect of machining (2D orthogonal cutting) on the phase transformation and grain size growth of Ti-6Al-4V [30].…”

“…The absence of severe subsurface plastic deformation and white layer in milled coupons could be again associated to the decreased thrust and cutting forces (therefore reduced strain rate on material) of the helical milling process and the lower processing temperature. A detailed investigation on the subsurface microhardness of Ti-6Al-4V alloy subjected to drilling and helical milling processes has been previously carried out by the present authors [28]. During the drilling process, the higher temperatures generated combined with the low thermal conductivity of Ti-6Al-4V could lead to thermal softening, which in turn increased the contact area between the tool and workpiece causing a significantly deformed layer [29].…”

“…Such condition resulted in the best hole quality (least roughness) and the material microstructure is least affected. In addition, the residual stress present in the hole surface can be another factor contributing to the fatigue behavior of the alloys [28]. The tensile residual stress measured for drilled surface is expected to reduce the fatigue life of the workpiece, while the compressive residual stress measured for the milled coupon can effectively extend its fatigue life [31].…”

A comparative study of hole-making performance by coated and uncoated WC/Co cutters in helical milling of Ti/CFRP stacks

“…As much lower force and temperature are generated during this process, better hole quality can be achieved without deploying redundant post processing, hence hole-making efficiency can be greatly improved [8][9][10][11][12].…”

“…Although a few studies have deployed coated tools in helical milling of alloy or composites [12,26,27], the role of different coatings in helical milling of Ti/CFRP stack is still lacking. Since helical milling is increasingly used in aircraft assembling, it is necessary to systematically evaluate the coated tool performance and understand their degradation mechanisms in helical milling of Ti/CFRP stacks.…”