Nickel-based superalloys are widely applied in aero-engine key components because of their excellent chemical and mechanical properties at elevated temperatures [1]. Inconel 718, the most popular nickel-based superalloy, is usually used in manufacturing the hot sections of aero-engines, especially for turbine discs because of its superior strength at around 650°C. However, it is also regarded as one of the most difficult-to-cut materials due to its low thermal conductivity and high-temperature strength and hardness [2, 3]. When machining Inconel 718, severe work hardening, high cutting forces and temperatures often occur [4]. Besides, welding and adhesion between the alloy and tool material further exacerbate machining difficulties [5, 6]. As a result, cutting tools wear rapidly, which affects and damages the machined surface integrity characteristics, including external topographical parameters, mechanical properties, and metallurgical states.
Various works have focus on investigating the effects of cutting parameters and tool wear on the machined surface integrity of nickel-based superalloys. Several articles reviewed and summarized the recent advances of surface integrity in machining of titanium and nickel alloys [7–9]. It is well known that the alteration of surface integrity is associated with the coupled thermo–mechanical effects, which is affected by the cutting parameters, tool structure, tool material, tool wear, and so on [10, 11]. Ren et al. [12] proposed a hybrid model coupled thermo–mechanical effects for predicting residual stress and microstructure modification in turning of Inconel 718, they found that both the microhardness and grain size decreased at larger cutting forces. Arrazola et al. [13] found that due to the additional thermo–mechanical stresses, more surface defects were found in machining with worn tools, especially when flank wear width exceeded 0.15 mm. Based on the chip formation, tool wear and tool edge radius, a mechanistic force model was proposed by Orra et al. [14], the simulated results showed that increase in flank wear width led to dramatic increase in the cutting force. Same observation was also reported by Kamdani et al. [15] when milling of Inconel 718.
Except for the cutting force and temperature, tool wear also has a significant effect on the surface topography. M'Saoubi et al. [16] reported that during turning of Inconel 718, a wavier and worse surface was generated from a worn coated carbide tool. Zhang et al. [17] established a simulation model that considered tool flank wear for the prediction of surface topography during the ball-end milling process. The experimental results concerning the milled surface topography are consistent with the model predictions. Jafarian et al. [18] found that an increase in the machining time could result in a high Ra value when turning Inconel 718, and the effect of tool wear on Ra could be negligible within 120 s turning time. Conversely, it has been reported that a lower Ra value is achieved with slightly worn tools, this is because the wear scars on the tool flank surface reduces the peak heights of the machined surface profiles which acts as a wiper [19, 20]. In other words, slight tool wear would make the tool–workpiece contact surface more adaptable. Unfortunately, a dramatic increase of the Ra value would inevitably occur at the end of the tool lifetime due to the large tool wear and its fractured cutting edges.
During machining process, the induced residual stress and work hardening are associated with the coupled thermo–mechanical effects [1]. High thermal gradients tend to generate tensile stress and softening phenomena at small depths below the surface, whereas the extrusion effect produced by mechanical loads contribute to induce compressive residual stress and work hardening [21–24]. The final residual stress and microhardness depend on the thermal or mechanical effects which one playing the dominant role [25, 26]. Normally, tool wear increases the enhanced thermal effects more than the mechanical effect, thus tensile residual stresses are induced especially at the surface. Aspinwall et al. [27] and Soo et al. [28] found that when milling with new and worn tools, the surface tensile residual stress of Inconel 718 increased with tool wear due to the dominant thermal effects. Peng et al. [29] reported experimental investigations on the induced residual stress distributions with new, semi-worn, and worn tools. The results showed that for worn tools, the tensile residual stresses were distributed within approximately 50 µm beneath the machined surface, while the residual stresses distributed within the range of 50–300 µm were compressive. Niaki and Mears obtained similar conclusions when turning of Inconel 718 [30], large tensile residual stresses were generated at 0.05 mm distance from the surface and then small compressive residual stress occurred.
The effects of tool wear on the microhardness of nickel-based superalloys have been discussed in several works. General trends indicate that the microhardness increases with tool wear due to severe plastic deformation, nose extrusion, and friction between tool–workpiece interface [31, 32]. Zhuang et al. [33] obtained the in-depth microhardness distributions under different flank wears when dry turning of Inconel 718, a work-hardened layer of 0.23 mm thickness was found under the notch wear width and depth of 0.2 mm and 0.12 mm, respectively. Hood et al. [34] found that when turning RR1000 superalloy, compared with the new tool, the worn tools could produce a deeper work-hardened layer and a higher microhardness value of 50 HK. The microstructure alteration induced by tool wear has been analyzed using different micro-examination techniques. M'Saoubi et al. [16] found that the misorientation value was larger when turning Inconel 718 using worn tools compared with new tools. Zhou et al. [35] found that when turning Inconel 718 with new and worn tools, the thickness of subsurface plastic deformation layer increased with the tool flank wear, which could be explained by the increased thermo–mechanical loads. Similar results were found by Agmell et al. [36], the relative strain was large when using worn tools.
Most published works have focused on the surface integrity of Inconel 718 during turning process, but few considered the high-speed end milling process. This study evaluates the effects of tool wear on the surface integrity during milling of Inconel 718. The machined surface integrity characteristics under various flank wear conditions were examined based on the surface roughness and topography in the surface, residual stress, microhardness, and microstructure in the subsurface. Moreover, the cutting force and temperature were analyzed to further explain the mechanisms of surface integrity characteristics.