Fracture and fatigue life of Al-based MMCs machined at different conditions
Introduction
Materials subjected to fluctuating stress fail at much lower stress compared to static fracture stress, which is commonly known as fatigue failure. This fact is particularly imperative during designing aerospace, automobile and biomedical components [6], [48], [34]. Fatigue failure mechanisms of composite materials is very intricate due to the presence of reinforcements which induce sever anisotropic properties of these materials and failure occur due to matrix cracking, reinforcement breakage and interfacial de-bonding of reinforcing particles. The diverse failure modes in addition to inborn anisotropies, complicated stress fields, and non-linear properties of composites restrict the understanding of fatigue properties of metal matrix composites (MMCs) [3], [28]. Generally, fatigue tests of materials are performed on specimens fabricated with near ideal surface finish such as after polishing which may remove surface defects/inhomogeneity that might reduce the overall fatigue performance [31]. However, from practical applications point of view, it is very expensive as well as difficult to fabricate polished/surface defect free parts towards that. Thus, the effect of machining parameters on fabricated specimens subjected to fatigue is very important [32]. The machined specimens have two main features that must be well-defined and controlled: geometric irregularities and metallurgical variations of the surface. This second feature is designated as surface integrity which was usually computed exclusively by surface cracks, but recent investigations widen the perception by all the changes implemented by a machining process such as, residual stress, roughness, hardness and microstructure of the machined surface layers [1]. These represent the quality of the machined surface and subsurface, and become very important towards machined components which endure high static and dynamic stresses [17].
It is reported that particle size, particle content and matrix aging states considerably influence particle fracture rate in MMCs under cyclic loading. Reinforced particles affect the threshold levels and fatigue crack growth rates in A1-SiCp MMCs which is partly arouse due to crack closure, defection and trapping; and to some extent, crack bridging [20], [39], [44]. As fatigue crack propagates, particle fracture delicately rests on maximum stress intensity. The occurrence of particle fracture is usually insignificant at smaller crack sizes but it rises with the rise of maximum stress intensity and becomes constant during fast fracture [20], [39], [44]. Li et al. [21] mentioned that smaller SiC particles (4.5 μm) displayed higher resistance to fatigue crack compare to the composite with 20 μm SiC particles and unreinforced alloy. The crack in 4.5 μm reinforced SiC particles induced higher level of fatigue crack closure which reduce crack growth. Shang et al. [39] found that aluminium matrix composites with larger sized reinforcements fracture preferentially at low stress intensity variation. Similarly, Kumai et al. [20] reported that larger particles had a greater propensity for fracture in 6061 alloy reinforced with SiC particles. Sugimura and Suresh [44] noticed that the probability of particle fracture rises with the rise of reinforcement volume fraction in fatigue crack growth in cast A1-3.5%Cu reinforced with varying volume fractions of SiC particles. This behaviour was attributed to higher restraint levels of matrix together with higher volume content of reinforcing particles. Arsenault et al. [2] found that underage material had a greater propensity of particle fracture than that of overaged microstructure because of lower matrix fracture strain in later material. Luk et al. [23] noticed that low cycle fatigue life of SiC particle reinforced 2009 aluminium alloy composites was equivalent within experimental scatter in both T4 and T6 conditions. While investigating the influences of particle size, volume fraction and matrix strength collectively on stress-controlled fatigue life and particle fracture during fatigue crack growth in SiC particle reinforced 2124 aluminium alloy, Hall et al., [13] found that the strength and fatigue life of MMCs rise with the reduction of particle size and increase of volume content of reinforcing particles. The particle fracture incidents raised with the rise of reinforcement size and content, crack tip stress intensity and matrix strength.
Fatigue crack nucleation and transmission can be attributed to surface integrity in most cases. Reed and Viens reported [35] a correlation between surface residual stresses and apparent fatigue limit of titanium, that is, tensile residual stresses decreased the fatigue strength and compressive residual stresses increased it. This correlation was later confirmed by Koster et al. [19] for Ti-6A1-4V where the specimens were fabricated by grinding and shaping and, as machined samples exhibit higher fatigue strength than stress-relieved samples with identical surface topography. Matsumoto et al. [24] studied the effect of fly cutting and grinding on the fatigue strength of hardened AISI 4340 steel in tension under load control. Compressive residual stress were generated all the cases though the stresses penetrated deeper when the fly cutter was used. Therefore, fly cutting induced higher fatigue strength than that of grinding. Sharman et al. [40] noticed higher tension–tension fatigue strength (475 MPa) in turned Ti–45Al–2Nb–2Mn + 0.8 vol%TiB2 alloy compare to that of electro-chemical machined and electro-discharge textured samples. This was due to higher compressive surface residual stresses generated during turning the workpiece material.
It is well known that cracks due to fatigue usually start from free surface [21] as it undergoes maximum loading and thus the free surfaces generated from diverse machining processes show wide range of fatigue behaviour [49]. Therefore, machining induced residual stresses, surface morphology microstructure (phases, plastic deformation, tears, voids, pits, burrs, cracks etc.), microhardness and notch-like surface irregularities affect the fatigue strength significantly [10]. In addition, machined surfaces became work-hardened severely and contain micro-cracks which eventually affect the fatigue behaviour of specimens. Taylor and Clancy [45] explored high-cycle fatigue behaviour of En19 steel specimens fabricated by polishing, grinding, milling and subsequent heat treatment to eliminate residual stress. The fatigue limit data were presented as a function of roughness through Kitagawa-type charts. It was reported that the fracture mechanics methods are beneficial for comparatively low roughness, when the surfaces can be modelled as a series of short cracks. A notch-based methodology is more suitable for higher roughness.
Probabilistic terms are used to represent the fatigue performance of mechanical components because of stochastic nature of this parameter. As evident in literature, wider deviation in fatigue results are presented in many investigations (typically 20%) for same Ra value and this demands the relevance of relying merely on surface roughness Ra. Novovic et al. [26] studied effect of surface topography/integrity generated from traditional and non-traditional machining on the fatigue performance based on data available in literature. It was noted that lower roughness gave longer fatigue life, but fatigue life was dependent on workpiece residual stress and surface microstructure, rather than roughness when roughness (Ra) in the range 2.5–5 μm Ra. machined surface roughness in excess of 0.1 μm Ra has a strong influence on fatigue life when there was no residual stress. Bayomi and Abd El-Latif [5] correlated different surface roughness parameters with fatigue endurance limit of an aluminium alloy where Ra and Rq, rather than spatial parameters have noticeable effect on endurance limit. Residual stress in the surface is often a superior indicator of fatigue behaviour than surface topography when the range surface roughness, Ra is 2.5–5 μm. Pramanik et al. [30] reported that non-traditional machining methods such as, electrical discharge machining on MMCs have varied effect on workpiece surface integrity and therefore fatigue performance depends on the material removal mechanism.
An extensive review of literature indicates that the research on the effect of machining parameters on fatigue behaviour of materials is still at early stage. Fatigue strength of machined components depends on workpiece material, machining type and conditions. So far, the information on this area is available only for very few alloys such as, titanium, steel, aluminium and nickel alloys which include few isolated machining parameters as reviewed by Hakami et al. [12] and Pramanik et al. [31]. Pramanik et al. [32] investigated the contribution of different parameters on fatigue life of machined MMC components. As per their findings, the interacting effects of particle size/feed and particles size/speed considerably affect the fatigue life of machined MMC components. The combined effect of these contributions is much more pronounced than the summation of individual contributions of specific parameters due to synergy effect. Therefore, the modelling of fatigue life based on particle size, machining speed and feed becomes very complex. Larger particles and lower feeds induce more defects on machined MMC surfaces while the speed does not have a noticeable effect. Therefore, fatigue life increases with the decrease in particle size and increase in feed. However, fatigue life does not change with speed variation. The propagation of fatigue cracks is arrested or deflected by reinforcing particles. The larger particles possess a higher capability to stop crack propagation. Therefore, machined MMC specimens reinforced with smaller or without any particles often separated completely as opposed to the counterparts containing larger reinforcing particles [32].
Having said that, there is no details information on synergized interactions of interacting machining parameters on fatigue behaviour such as, fatigue life, crack propagation and fractured surface of MMCs though these materials are emerged as an important candidate for many high performance applications. In view of that, the objective of the present study is to investigate the effect of interacting machining parameters on flexural fatigue behaviour of MMCs reinforced with different reinforcement sizes. The results will benefit researchers as well professionals to increase the understanding of fatigue behaviour of MMC parts produced by machining.
Section snippets
Material and methodology
Two different types of MMCs were investigated in this study, namely: 0.7 µm and 13 µm SiC (10 vol%) reinforced 6061 aluminium matrix composite. The MMC slabs was obtained through in-situ casting technique followed by hot-isostatic pressing and acquired commercially from Aerospace Metal Composites Limited, UK. These slabs were used as feedstock materials to fabricate specimens for flexural fatigue testing with the help of milling and drilling, which was carried out on a Leadwell V-30″ CNC
Nature of failure
On the completion of fatigue test, visual check on specimens were carried out and consolidated in Fig. 4 for comparison purpose. Irrespective of specimens, initial observation is that, fatigue crack starts at the edge of the drilled hole on flat surface which was intended. After crack initiation, it starts to propagate along flat surface as well as along the depth of drilled hole. The failure due to fatigue occurs along transverse axis of the specimens. Crack propagation length in transverse
Discussion
Overall fatigue life involves two modules: time to start crack and time to propagate crack. In a defect free test specimens, the time to start fatigue crack inhabits a substantial portion of the entire fatigue life [29]. For example, in samples of high static strength without notch (i.e., aluminium alloys), start of crack generally inhabits 75–95% of the entire fatigue life [11]. The portion of time comprised of the start of crack also rises with the rise static tensile strength. The tensile
Conclusion
The above investigation on the effect of machining speed and feed, and the size of reinforcement on the fatigue behaviour of MMCs can be concluded in following points.
Though the machining process leaves traces on newly generated surface, fatigue cracks don’t follow those traces. Crack propagation is arrested or deflected by reinforced particles. The bigger particles possess a higher capability to stop crack propagation though those are more prone to facture. Therefore, machined MMC specimens
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