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The size effect caused by scaling effects in micro milling play an important role in the material removal mechanism and the underlying mechanics during machining process. The size of the ploughing region where material elastically deforms and recovers to its original position after the tool passes cannot be neglected in micro milling. Consequently, high friction and stress is induced between the cutting tool and the materials which in turn makes the cutting edge the most loaded portion of micro endmills (Li et al., 2011).Therefore unpredictable tool life and

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the premature failure of micro endmills have been recognised as the major barriers of micro milling (Tansel et al., 1998).

The flank wear is considered as the main wear criterion in conventional milling process, but it becomes less significant in micro milling process. The wear mechanisms and failure types in micro milling are diverse and complex. Additionally, tool wear measurement is challenging due to miniaturised size of endmills in micro milling. A literature survey shows that the current state-of-the-art lacks generic tool wear criteria and assessment methods in micro milling. Some attempts in investigating tool wear mechanisms in the micro milling of engineering materials are summarised below.

Tansel et al. (Tansel et al., 1998) studied the wear mechanism of micro endmills when machining aluminium and mild steel and concluded that tool wear was mainly caused by fatigue and stress induced chip-clogging breakage. Another subsequent study (Tansel et al., 2000) established a relationship between tool wear and cutting force characteristic based on a neural- network method in micro milling of aluminium and steel. A steady increase in cutting force with the progression of tool wear was observed in the machining of soft materials made of aluminium, which caused a loss in effectiveness/sharpness of the cutting edge. The wear condition could be estimated accurately by observing variation in cutting force. Conversely, a sudden increase in cutting force occurring before tool failure was induced in the machining of hard material (steel) leading to the difficulty in estimating tool wear.

Rahman et al. (Rahman, Senthil Kumar and Prakash, 2001) also investigated the effect of development of tool wear on cutting forces in the micro machining of pure copper using a micro grain carbide endmill. Increasing cutting speed from 35m/min to 45m/min was found to accelerate the tool wear significantly. Non-uniform wear and chip adhesion were observed on both major and minor cutting edges. This resulted in an increase of cutting forces and led to an early tool failure.

The abrasion wear which was found to be the dominant wear mode during the micro milling of copper 101 using a tungsten carbide tool was reported by Filiz et al. (Filiz et al., 2007) (Figure 2.14). The stress acting on the cutting edge was found to dramatically increased when the material was elastically removed due to the size effect at low feed per tooth. Therefore, the greatest tool wear in the form of flank wear was experienced at the lowest feed per tooth. An improvement in tool wear was found when cutting speed increases from 40 m/min to 120 m/min at all feed per tooth, which was in contrast to results reported by Rahman et al. (Rahman, Senthil

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Kumar and Prakash, 2001). The reduction in tool diameter was utilised as a measure of tool wear.

Figure 2.14 SEM images of severely worn endmill. (Filiz et al., 2007)

Ucun et al. (Ucun, Aslantas and Bedir, 2013) compared the performance of endmills with different coatings (AlTiN, TiAl&AlCrN, AlCrN, TiAl&WC/C and DLC) during the micro milling of Inconel 718 super alloy. Effective tool diameter and cutting edge corner were measured to quantitatively define tool wear condition (Figure 2.15). Flank wear due to abrasion and chipping were reported as the dominant wear modes. Also, local fractures resulting from fatigue and burr formation were observed due to the excessive friction on a part where smeared by workpiece material, and this was dominated by the size effect at small uncut chip thickness. DLC and TiAlN&WC/C coatings exhibited better wear resistance compared to others.

Figure 2.15 Tool wear criteria: (a) effective tool diameter; (b) cutting edge radius. (Ucun, Aslantas and Bedir, 2013)

Imran et al. (Imran et al., 2014) stated that the abrasion was initially experienced on a TiN coated tool in wet drilling, followed by cyclic workpiece adhesion onto the cutting edge. Finally,

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the micro chipping which gradually increased the edge blunting was observed near the cutting edge due to the diffusion of workpiece materials into the WC-Co binder with the assistance of adhesion effect.

Overall, previous studies have investigated tool wear mechanisms-based only on the experimental methods. Moreover, diverse wear mechanisms have been reported in the machining of different engineering materials but only a few publications have investigated wear mechanisms during the machining of MMCs reinforced with nanoparticles.

When compared to the non-conventional machining methods mentioned in previous sections, the relatively high dimensionally accuracy and the flexible and controllable nature make micro machining a promising method to produce MMCs parts with complex 3D features and high surface quality. However it is believed that the enhanced mechanical properties of MMCs would make machining process more challengeable than that in conventional machining process. Therefore, it is crucial to comprehensively understand the material removal mechanism in micro machining of nano MMCs.

2.5 Finite element modelling on machining of MMCs