Development and Characterization of Functionally Graded Materials Using Hybrid Layered Manufacturing

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1 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India Development and Characterization of Functionally Graded Materials Using Hybrid Layered Manufacturing Sajan Kapil 1,Pravin M. Kulkarni 2*, K.P.Karunakaran 4, Prathmesh Joshi 3 1 Indian Institute of Technology Bombay, ,sajan.kapil@iitb.ac.in 2* Indian Institute of Technology Bombay,400076,kulkarnipravin@iitb.ac.in 3 Indian Institute of Technology Bombay,400076,prathamesh1729@gmail.com 4 Indian Institute of Technology Bombay,400076,karuna@iitb.ac.in Abstract: A multi-layer functionally graded cast aluminium alloy is developed using Hybrid Layered Manufacturing (HLM). The system hardware is composed of a 3-axis CNC machine integrated with a Gag metal Arc Welding (GMAW) deposition unit. Properties of the functionally graded materials were investigated both at macro and micro length-scales. Realization of Functionally Gradient object is done by Directionally Heat Conductive (DHC) objects. Direction dependent variations in microstructure were studied using Optical Microscopy. Mechanical properties were tested using uni-axial tensile testing and micro-hardness indentation. The microstructure showed a distinct variation along the layer thickness. Coarse microstructures were seen towards the bottom layers whereas fine microstructures were found towards the top layers. Hardness variations within a layer are explained with respect to the observed microstructure gradients. A significant difference in heat conduction is observed in the two s. Keywords: Functionally Gradient Material, Gas Meta Arc Weld Deposition, Additive/ Subtractive/ Hybrid Manufacturing, Directionally Heat Conductive Objects 1. Introduction Hybrid Layered Manufacturing (HLM) realizes shapes using Gas Metal Arc Weld (GMAW) deposition. Such additively manufactured matrix inherently has anisotropy. While majority of the applications require homogeneity, some applications demand anisotropy in a desired manner. Moreover it may be desired to also have spatially varying property as in Functionally Gradient Materials. It is possible to achieve the desired matrix by the appropriate selection of the HLM process parameters. In order to understand the influence of these process parameters, it is required to assess the nature and extent of anisotropy of the matrix under various build conditions. The most common layered manufacturing processes are stereo lithography, laser engineered net shaping (LENS), laminated object manufacturing,selective laser sintering (SLS), 3D cladding, 3D printing, andshape deposition manufacturing (SDM). Non-metallic materials are the most produced parts by Rapid Prototyping (RP) systems.jandric et al. describe that the most RP systems that make metallic parts are making objects that are porous, have weak bonds between layers and consequently, have structural weakness. In contrast in HLM, the focus is on matrixintegrity during GMAW deposition and surface quality during machining. As the above competing processes are purely additive they have to use thin layers and hence are slow. HLM, being hybrid, isfaster by an order of magnitude as it deposits thick layers. Furthermore, HLM is more economical and safer by order of magnitude. A number of researchers have contributed an effort to develop this process; however the main focus of many researchers was on equipment setup and possible application of HLM [2]. But these processes have less material integrity, poor quality and mainly used in prototyping and form fit testing. Some experiments have been reported on the quality of the parts produced and the optimum processing parameters of HLM process [2-5].However there are very few reports on application of HLM for FGM. Surya S. [6] has reported the application of HLM in fabrication of composite dies (FGM) 429-1

2 Development and Characterization of Functionally Graded Materials Using Hybrid Layered Manufacturing with itscase made of tool steel and its core made of mild steel. The ally dependent property of functionally graded materials (FGM) has been explained by different researchers. It has been discussed that this change in properties can be caused by position-dependent variations in constituent elements, non-uniform microstructure or atomic order. In position-dependent chemical composition the gradient can be defined by the socalled transition function c i (x, y, z) which describes the concentration of the component c i as a function of position [2]. The variations in microstructure can be explained with microstructure gradient which influences tensile, hardness and macro properties such as heat conductivity, specific heat, density etc. of the material in different s. Today s technology in materials design and processing has enabled fabrication of functionally graded materials that can endure very high temperatures and large temperature gradients. However the involvement of layered manufacturing technologies on ally conductive materials is almost none. But few research works have been reported on modelling steady state heat conduction in graded materials [7, 8]. This paper will discuss the fabrication of ally conductive material using hybrid layered manufacturing (HLM) and variations of mechanical properties and metallographic analysis in different build s. Different reported studies done on layer by layer manufacturing have conflicting results about the mechanical properties and microstructure gradient of the different weld zones [3-5]. Song et al. [3] has stated that, the micro structure of the upper area of the solid contains enlarged grains with some dendrites whereas the lower area has relatively small grains however the report made by Gharibshahiyan et al.[5] state that the higher the heat input let to coarsening which is more pronounced in HAZ. In addition, the hardness results reported by Song et.al [3] and Jandric et.al [1] are contradicted to each other. Song et al. reported hardness values to decrease gradually from bottom of deposit to the top, while Jandric et al. reported a maximum hardness at the top deposited layer and a slight decreasing trend towards the middle and bottom layers. 2. Experimental Details For the fabrication and characterization of an object through HLM, process parameters such as slice thickness, current intensity, wire feed rate, stepover increment etc. are to be chosen and optimized to minimize the number of defects and material wastage due to improper setting of these parameters. Weldingparameters used in fabrication of experimental samples are tabulated in table 2.1. Table 2.1Welding parameters Current (A) 70 Torch speed(mm/min) 850 Wire feed 3.4 rate(m/min) Welding wire 1.2 diameter (mm) Gas flow rate(l/min) Shielding gas 82%Ar+18%CO 2 for steel,100%ar for Al-Si 2.1 Fabrication of DHC objects A multi-layer functionally gradient aluminium silicon alloy is developed using Hybrid Layered Manufacturing (HLM). A Cold metal transfer (CMT) 2700 welding machine and a CNC machine were used in deposition and subtraction. A rectangular sample of size 50x50x22mm was produced by depositing 1mm layer thickness to a total of 22 layers. An air gap of 250 µm was intentionally provided between two adjacent deposited layers. The step over increment and heat intensity was adjusted and provided to avoid distortion and complete fusion between two consecutive layers. Figure 1 show the CAD model of the sample with air gaps between each strip of layers. A calibration setup was established to calibrate a thermocouple and find the corresponding curvefit correlation based on See beck effect. This is a crucial step that must be done in order to get an equation of curve that fit to the temperature versus voltage graph

3 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India sides of the sample. The two thermocouples were placed at a distance of 6mm from the edge of the specimen and the third thermocouple was place at thecenter of the sample. Thermocouple number 1, 2 and 3 are attached to the heat input side; the other thermocouples 4, 5 and 6 are attached to the output side. The thermocouples on the input and output sides were connected to the selector switch for sequential selection and recording of the millivolt readings. Figure 1 CAD model of a ally conductive sample The multimeter reading in millivolt corresponding to the difference in temperature between the surroundings (ice) and the bath was recorded. The measured bath temperatures values (T B ) were plotted on y-axis against the corresponding reading of emf (millivolt) values on Y-axis, figure 2. The slope and intercept of the curve-fitted were found from the equation of the graph. The heater plate assembly is made up of mica sheets in which Nichrome sheet is packed with upper and lower mica sheets for stabilizing and avoiding damage of the thermocouples. 2.2 Fabrication of Samples for Experimental Analysis Three axis Cartesian coordinate system was adopted to identify the of deposition and the cross section of the samples for mechanical testing, ally heat conductivity test and metallographic analysis. The Y-axis (torch ) is taken in the long travel of the weld torch, Z-axis is taken perpendicular to the surface of the substrate (weld ) and X-axis is taken in the stepover increments. Figure 4 illustrates the coordinate system used in deposition and characterization. Figure 2 Calibration Curve Directionally Heat Conduction test The main objective of this experiment was to measure the al conductivity of the sample explained above. Small variation in the air gap width due to spattering of molten metal to adjacent weld beads is ignored. For the measurement of thermal conductivity (K) what is required is to have a one dimensional heat flow through the rectangular specimen. Figure 3 Schematic diagram of experimental setup As shown in figure 3, sixthermocouples are used to measure the temperature difference on the heat input and output side of opposite faces of the sample. Three small holes,for three thermocouples on each side, with 3mm deep are drilled into four Figure 4 Coordinate system for deposition and characterization The above coordinate system was used in deposition patterns of layers for ally conductive specimen and to identify the orientation for mechanical properties test and metallographic analysis of the samples with respect to the build s. The heat conduction was measured in parallel and perpendicularr to the air gaps in X and Y s respectively. The hardness tests were sectioned in XZ plane and measured across the sectioned area, from top to bottom layer. However the tensile specimens were machined in the XY plane and tested in the Y and the metallographic specimens were sectioned perpendicular to Y-axis and X-axis and mounted and polished in the XZ plane and XY plane respectively. Standard metallographic techniques were employed for microstructural analysis. An appropriate moulding was made for mounting and 429-3

4 Development and Characterization of Functionally Graded Materials Using Hybrid Layered Manufacturing polishing. Metallographic specimens were examined and photographed using optical microscopy. Image analysis package was employed for image analysis. 2.3 Tensile specimen fabrication Tensile of HLM has three major factors, weld, torch, and step over. Six specimen of each were built for tensile testing with four in X and Y and two in Z. Dimensions for the specimens built in XY are mm and for Z mm. Specimens were tested in EZ 50 universal testing machine. Similarly Yield and UTS of Al-Si samples along various s were prepared in the same fashion. Specimen orientation and block for mild steel and Al-Si used for tensile sample preparation are given in figure 5. (a) (b) Figure 5 (a) Plan of specimens for X and Y s (b) Plan of specimens for Z The tensile of the object through HLM was measured through ASTM A370 standard. This standard is usually applies for weldedd objects and so it was thought to be more appropriate for HLM manufactured objects. The dimensions of tensile specimens based are given in figure 6. Figure 6 Tensile specimen 2.4 Experimental Results Anisotropic Heat Conduction The experimental results for the ally conductive sample showed anisotropy heat conduction measured through the opposite sides. A heat input of 30.4 watt was provided from one side of the specimen and the temperatur re difference across the specimen was measured. A K-type thermocouple was welded to the sides of the substrate plate and was used to measure the difference in temperatures. The input and output measuredtemperatures were averaged after steady state condition was reached. The averagee temperature versus time for the two s showed a similar trend. Figure 7 shows the average temperature measured in the parallel and perpendicular to the air gaps. Fig. 7 Time versus Temperature and perpendicular to air gaps From Fourier s law, the value of thermal conductivity parallel and perpendicular to the air gaps are summarized in table 3.1. To check the uni- was measured al heat flow, the temperature on the adjacent side of the heat input side. A negligible heat flow on the sides was observed. The value of thermal conductivity was Wm -1 0 C -1, which is negligible when compared to heat flow on the other two s Tensile results Table 3.2 and 3.3 summarize the tensile test results. Based on the coordinate system explained in previous section, the samples were measured for tensile. The four mild steel samples measured in torch (Y-axis) (X-axis) and weld (Z-axis) stepover showed a fluctuation trends both in yield and ultimate values. The specimens measured in torch provide the maximum value of yield and ultimate. However the minimum value of both tensile s was measured in the weld. In contrast to Mild Steel samples, for aluminium sample, the minimum value of the yield s varied with. The yield showed a minimum result in stepover (Xultimate axis) but the minimum reading for value was in the weld (Z-axis). These variations in tensile properties with of both Mild steel and Aluminium samples indicate the capability of HLM for fabrication of functionally gradient materials. The increase in both yield and ultimate in torch for both sample, reveals the capability of HLM objects to withstand a maximum load along the layer s. However the least tensile values in the stepover s implies the extent of overlapping between one layer and next adjacent layer. It is further possible to make FGM with much higher or lower tensile 429-4

5 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India by varying torch speed, wire feed rate, controlled cooling rate and tailored microstructure variations. The process parameters change the extent of microstructure hence change the tensile. Tensile of object made by HLM can also be reduced by inclusion of porosity Hardness test results To study hardness 3 samples each for mild steel and Al-Si alloy were prepared and tested for Rockwell and superficial hardness. The results of the experiments are shown in figure 8 (a) and (b). As shown in the these graphs, the specimen hardness decreases as we go from top to bottom layers in the mild steel, whereas there is negligible variation for Al-Si specimen. Table 3.2 Tensile of the HLM object in different s (Mild steel) Direction Torch (Y-axis) Stepover (X-axis) Tensile Strength Specimen (MPa) No. Yield Ultimate Weld (Z-axis) Average Table 3.3 Tensile of the HLM object in different s (Al-Si alloy) Direction Torch (Y-axis) Stepover (X-axis) Weld (Z-axis) Specimen No. Tensile Strength (MPa) Yield Ultimate Average Porosity The main problem associated with most of RM processes is porosity. Porosity not only affects the surface finish of an object but also affects the fatigue of the object. This generally appears in the object by entrapment of gases inside the layers at the time of welding. This problem occurs due to insufficient pressure of inert gas (Ar+Co 2 for Mild steel, Ar for Al). During manufacturing of samples we take care of this problem and keep gas flow above 10 l/min. In addition to this we also keep the speed of welding torch slow since hot metal can react. (a) (b) Figure 8 Variation of hardness with thermal cycles (a) Mild steel (b) (Al-Si alloy) Small number of porosities was observed in aluminium sample whereas mild steel showed no porosity. Figure 9 (a) and (b) show the microstructure of mild steel and Al-Si alloy respectively Micrographic analysis The microstructure of samples for Al-Si and mild steel specimens were comprehensively studied. It was observed that there is a good structural integrity and fusion between the weld beads. Due to the difference in thermal cycle and cooling sequence of each layer, the microstructure showed a distinct variation along the layer thickness. In HLM objects are made layer by layer weld deposition. The bottom layers are exposed to a large number of thermal cycles as compared to top layers. Moreover the top layers are naturally air quenched whereas the bottom layers go through 429-5

6 Development and Characterization of Functionally Graded Materials Using Hybrid Layered Manufacturing slow cooling. Coarse microstructuress were seen towards the bottom layers whereas fine microstructures were found towards the top layers as shown in Fig.3.4 (a) and (b). However, within each layer the bottom part was seen to be finer than the top. The change in microstructure is the result of multiple reheating of preceding beads by subsequent weld passes. The geometric and crystallographic orientation relation of α dendrites with respect to thickness and heat flow s were studied. (Not presented in this paper) (a) (b) Figure 9 (a) Fine microstructure at the top layers (b) Coarse microstructure at the bottom layer 3. Conclusions The following concluding remarks can be made from the experimental results. a. The tensile showed a higher values in the torch and lower values in weld s for both Al-Si alloy and mild steel specimens. b. Hardness decreases as we go to bottom layers in the mild steel, whereas there is negligible variation for Al-Si specimen. c. The microstructure analysis shows a distinct variation along the layer thickness. d. Coarse microstructures were seen towards the bottom layers whereas fine microstructures were found towards the top layers. e. Within each layer bottom part was seen to be finer than top part. f. The value of heat conductivity parallel and perpendicular to air gaps on the specimen was showed anisotropic thermal conductivity g. The thermal conductivity was decreased by 35% in perpendicular to the air gaps and 16% in parallel to the air gaps. References 1. Jandric, Z.,Labudovic, M.,,Kovacevic R. (2004), Effect of heat sink on microstructure of three-dimensional parts built by welding-based deposition, International Journal of Machine Tools &Manufacture, Vol. 44, pp Kieback, B., Neubrandb, A.,Riedel, H. (2003), Processing techniques for functionally graded materials, Materials Science and Engineering Vol. 362, pp Song, Y., Park, S., Choi,D., Jee, H. (2005), 3D welding and milling: Part I a direct approach for freeform fabrication of metallic prototypes, International Journal of Machine Tools & Manufacture 45(9), pp Song, Y., Park, S., Chae, S. (2005), 3D welding and milling: part II--optimization of the 3D welding process using an experimental design approach, International Journal of Machine Tools and Manufacture, Vol. 45(9), pp Gharibshahiyan, E., Raouf,A.. H., Parvin,N. Rahimian, M.(2011), The effect of microstructure on hardness and toughness of low carbon welded steel using inert gas welding, Materials and Design, Vol. 32, pp Karunakaran, K.P., Shanmuganathan, P.V., Jadhav, S.J., Bhadauria, P. and Pandey, A. (2000): Rapid prototyping of metallic parts and molds, Journal of Materials Processing Technology, Vol. 105(3), pp Gasik, M. M. (1998), Micromechanical modelling of functionally graded materials, Computational Materials Science, Vol. 13, pp Sutradhar, A.,Paulino, G. H.(2004), The simple boundary element method for transient heat conduction in functionally graded materials, Comput. Methods in Applied Mechanics and Engineering, Vol. 193 (42-44), pp