A new process for design and manufacture of tailor-made functionally graded composites through friction stir additive manufacturing
Graphical abstract
Introduction
Functionally graded materials (FGM) are heterogeneous materials characterized by a gradual variation in properties over volume. FGM can withstand very high thermal gradients, making them suitable for use in high-stress structures such as space plane bodies [1]. They have the ability to inhibit crack propagation that makes them useful in defense applications as penetration resistant materials used for armor plates [2] and bullet-proof vests [3]. Wang et al. [4] showed that thermal buckling, a design feature of hypersonic vehicles, can be achieved by FGM. The FGM are also suitable for biomedical applications wherein tailored mechanical properties are required to achieve biomechanical performance in patient-specific implants [5]. Other areas of application include medicine, energy, cutting tools, insert coatings, automobile engine components, turbine blades, heat exchangers etc. Several methods, such as powder metallurgy [6], centrifugal method [7], chemical and physical vapor deposition techniques, solid free-form fabrication (SFF) techniques [8], laser deposition methods [9], selective laser sintering, 3-d printing, selective laser melting [10] etc. are all used for the fabrication of FGM [11]. FGMs are also produced by casting route, e.g., Singh and Singh [12] developed Al/Al2O3 composite as an FGM by using an alternative reinforced fused deposition modelling pattern in investment casting process. Vapor deposition techniques can also be used for depositing thin surface coatings but are not suitable for producing bulk FGM [13]. The Powder metallurgy technique produces discrete gradients and thus gives rise to a step-wise structure. The Centrifugal method gives a continuous grading but limits structures to cylindrical shapes and type of gradient that can be produced [14]. SFF techniques have an edge over other methods as they are capable of producing complex shapes and have greater design freedom, since parts are made directly from CAD data but are characterized by a poor surface finish and this necessitates secondary finishing operations [15].
Welding processes have an inherent feature of creating localized functional grade in micro or micro scale. Weld metal and the heat affected zone have functionally graded microstructures that facilitate distributive mechanical properties [16]. The buttering layer in hardfacing applications is an example of macro − functionally graded material wherein the butter layer has an intermediate thermal expansion to that of the substrate and hardfacing [17]. Recently, friction stirring has been introduced as an additive manufacturing process to control microstructure [18]. The friction stir processing (FSP) in also a mean to produce metal matrix composites, e.g. Al/SiC surface composites [19], [20] wherein a localized plastic deformation is produced by forcing a non-consumable revolving tool on the workpiece surface. The FSP is controlled mainly by rotation and traversing speeds of the tool. Kumar et al. [21] and Panaskar et al. [22] used friction stir processing for nano-composite layering. The FSP can be used in manufacture of functionally graded composite material (FGCM) [23], [24], [25] wherein reinforcement particles are packed in a longitudinal groove along which friction stirring is conducted. Friction stirring based FGCM manufacturing as reported in the existing literature is limited to creating functional grades in the vicinity of the weld nugget and compositional, microstructural and property gradients are not predictive and controllable. Such control is essential for FGM to act as a reliable component in certain assemblies.
Previous investigations in FGM manufacture have primarily employed conventional methods of characterization that include qualitative (i.e. microscopy) and quantitative (i.e. hardness measurement) methods. However, FGM in general and FGCM in the present case, in order to ascertain efficacy of process and product, must be evaluated for change in localized mechanical properties such as Young’s modulus, strain hardening exponent, yield stress etc. Such measurement is possible with advance techniques such as Digital Image Correlation (DIC), in which images of specimens under loading are taken at frequent time intervals and the relative displacement of image features from one image to another is determined and used to obtain strain maps. Some authors, e.g. Leitão et al. [26] determined local constitutive properties of aluminum friction stir welds using digital image correlation.
Keeping observations as forgoing in consideration, the overall objective of this investigation is to develop and characterize a new method of tailor-made FGM manufacture based on additive friction stirring. It is aimed to achieve a pre-defined property gradient of over a given length through systemic change in volume fraction of reinforcement particles. Three specific objectives of the investigation incudes (i) evaluation of the efficacy of the developed process in achieving gradients in mechanical properties, namely, hardness, Young’s modulus, stain hardening exponent, and yield stress (ii) quantification of localized change in mechanical properties and constitutive parameters of FGM using digital image correlation, and (iii) elucidate a process mechanism of the developed method through correlating the effect of changes in process parameters and corresponding microstructures. The following section describes the process model of the proposed approach that is followed by descriptions of the materials and method. The result and discussion are presented in final section.
Section snippets
Process model
In order to get compositional variation through FSP, blind holes are pre-drilled in a linear fashion in a plate such that the center-to-center distance changes in successive holes. The hole drilling electric discharge machine is used to blind-holes of precise diameter and depth. The holes are filled-up with reinforcement particles. Subsequently, single/multi-pass friction stirring is carried out by a traversing stirring tool along line connecting centers of the holes, as shown in Fig. 1 (a). A
Material and methods
In the present study, commercial pure Aluminum with a thickness of 6 mm was chosen as the base plate and TiC particles of average size 44 μm were used as reinforcement. The chemical composition of the baseplate is shown in Table 1.
The tool pin was cylindrical and made of H13 tool steel with a diameter of 6.2 mm, length of 5.2 mm, and shoulder diameter of 27 mm. The tool rotation was set at 1000 rpm and traverse speed was 50 mm/min. Holes of diameter 1 mm were drilled to a depth of 5.2 mm, over a length
Visual appearance and particle distribution
The top surface of the one pass friction stirred sample in Fig. 4(a) shows inadequate mixing in retreating side (RS) compared to advancing side (AS). The inadequate mixing leaves voids at nugget boundary at plate surface and beneath the pin as can be seen in longitudinal cross-section. The size of voids becomes smaller from left to right (i.e. maximum to minimum volume fraction of reinforcement particles). When friction stirred samples were repeated with one more pass in same direction, as
Conclusions
An innovative process for the manufacture of tailor-made functionally graded composites through friction stirring is presented wherein volume fraction is controlled using a mathematical model that, along with favorable process conditions, lead to property gradients. The following conclusions are drawn from the present investigation:
- i)
A systematic approach of controlling volume fraction of reinforcement particles by managing their distribution in a matrix combined with the ability of friction
Acknowledgements
The authors acknowledge JWRI, Osaka University for providing facilities for a part of this work under Sakura Science Program of Japan Science and Technology Agency (JST) and JWRI International Joint Research Collaborators (JIJReC) program 2016. Acknowledgements are also due to Japan International Cooperation Agency (JICA) for their support provided to one of the authors. Special acknowledgment is due to Mr. Masuo Ito, JWRI, Osaka University, for help in EBSD work.
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