Evaluation of cutting force and surface roughness in the dry turning of Al–Mg2Si in-situ metal matrix composite inoculated with bismuth using DOE approach
Graphical abstract
Introduction
Metal-matrix composites (MMCs) are developed to satisfy the demand for materials with high strength and toughness and capable to be used effectively in adverse conditions. MMCs have attracted the attention of automotive and aerospace industries which are looking for materials with low density and, appropriate mechanical properties such as high strength and stiffness in order to reduce weight and fuel consumption [1]. Particulate metal-matrix composites (PMMC) offer some advantages, such as better manufacturability, when compared to fiber metal-matrix composites. Even though components made from PMMCs are produced to near net shape, some secondary operations are needed to achieve the required dimensions and surface finish [2]. The main problem associated with the machining of MMCs is the high rate of tool wear because of the ceramic reinforcing particles which act as an abrasive and result in tool wear and short tool life thereby increasing the overall cost of production. The alteration of tool geometry caused some defect on the machined surface and these defects and their distribution play an important role in determining some mechanical properties such as creep and fatigue that consequently affect the performance of the machined components during service [3], [4].
Although it has been reported that cutting speed and volume fraction of reinforcing particles affect the surface quality, feed rate is the most influential factor [5]. The previous investigations based on Taguchi method explore that tool geometry (flank wear) and cutting force are affected by the cutting speed and most significant factor which influence the surface roughness is feed rate [6], [7], [8]. The findings supported that Taguchi method and Analysis of Variance (ANOVA) are powerful techniques to develop predictive models [9]. Most of the ex-situ/in-situ MMCs such as Al–SiC suffer from thermodynamic instability of interfaces among the ceramic reinforcement/matrix and poor wettability of the reinforcement [10]. Therefore, in recent years, in-situ composite such as Al–Mg2Si in which reinforcements are synthesized internally in the matrix during the composite solidification has attracted many researchers. Moreover, in situ fabrication provides more homogeneous distribution of the dispersed phase particles. Mg2Si compounds are prone to the formation of undesirable coarse Chinese script and brittle dendritic morphologies, which would deteriorate the mechanical properties of the materials. Thus, several researches have been done to modify the deleterious morphology of Mg2Si particles with addition of strontium (Sr) [11], [12], [13], phosphorous (P) [14], yttrium (Y) [15], antimony (Sb) [13], [16], manganese (Mn) [17], lithium (Li) [18], mischmetal [19], and cerium (Ce) [20]. Moreover, bismuth (Bi) is used as an alloying element in wrought aluminium alloys to promote chip breaking and helping tool lubrication [21]. Bi is also added to Al–Mg alloys to prevent embrittlement by sodium [22], and to disrupt the formation of oxide defects in Al alloys [23]. It has also been shown that the presence of Bi has a refinement effect on the morphology of eutectic silicon and Mg2Si reinforcement particles [24], [25], [26], [27], [28]. Additionally, Bi-containing work-piece showed the lowest cutting force and better surface roughness in Al–11Si–2Cu cast alloy in comparison to Sb and Sr containing work-pieces [29]. Moreover, it has been found that cutting force decreased and chip fragility increased by adding Bi to the 220 Al–2%Cu–1.3%Si–0.4%Mg alloy [30]. In terms of Al–Mg2Si in-situ composite, most previous studies focused on its microstructure and mechanical properties. There is a lack of knowledge about machinability characteristics of Al–Mg2Si composite especially with addition of Bi. Furthermore, the design of experiment (DOE) methodology was used to achieve this objective. The analysis of the effects of each variable and their reciprocal interaction on the machinability characteristic provided the necessary information needed for the machining of Al–Mg2Si composite casting.
Section snippets
Work-piece fabrication
A commercially available Al–11Si–2Cu alloy, pure aluminium and pure magnesium were used as starting materials for adjusting the chemical composition. The materials were melted in an induction furnace to fabricate the composite ingot with chemical composition given in Table 1. A 1 kg ingot was then re-melted with a melt temperature of 750 ± 5 °C. The molten metal was degassed using hexachloroethance tablets. After degassing, the desired amount of pure bismuth (99.99 wt.%) in the form of metallic
Microstructure analysis
Fig. 1 shows the elemental mapping of fabricated composite without Bi addition indicating that the particles are composed of Mg and Si elements, which are distributed in aluminium matrix. Moreover, based on the EDS profile shown in Fig. 1 the atomic ratio of Mg is almost twice that of the Si element in order to form the molecular structure of Mg2Si particle. Optical and SEM micrographs of Al–20Mg2Si work-piece is shown in Fig. 2(a) and (b) and the coarse polyhedral Mg2Si particles in the
Conclusion
The multilevel factorial design was used to examine the effect of cutting speed (70–210 m/min), feed rate (0.1–0.2 mm/rev) and Bi addition on cutting force and surface roughness of Al–20%Mg2Si in-situ composite. The obtained results were analyzed using Analysis of Variance. The following conclusion can be drawn:
- 1.
The recommended optimum cutting conditions in machining of Al–20%Mg2Si composite is found to be: cutting speed at 210 m/min and feed rate at 0.1 mm/rev in the presence of Bi.
- 2.
According to the
Acknowledgments
Financial support from the Ministry of Higher Education Malaysia (MOHE) and Universiti Teknologi Malaysia (UTM) through the Fundamental Research Grant Scheme and Research University Grant Scheme are gratefully acknowledged.
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