Investigations on additive manufacturing of metallic materials by micro-plasma transferred arc powder deposition process

Abstract

Additive manufacturing (AM) is a bottom-up approach based manufacturing philosophy in which a product is manufactured directly from its computer-aided design (CAD) model by depositing the material in thin successive layers in such a way that good mechanical properties, dimensional accuracy and surface finish are achieved along with sound metallurgical bonding between the deposited layers. AM processes are generally material-efficient because they incur a small loss of the product material than subtractive, primary accretion (i.e. casting, powder metallurgy), and deformative type manufacturing processes. Being material-efficient makes them energy saving and consequently environment friendly as well. They offer following worth-mentioning advantages overother processes: (i) ability to additively manufacture a part of complex geometry made of diverse materials such as polymer, composites or metallic materials; (ii) ability to economically repair damaged components or products which are very expensive, complex and require longer delivery time; (iii) ability and more flexibility to add delicate features to an existing component; (iv) ability to modify surfaces of a product by coating and/or texturing; (v) reduced design-to-market time and material procurement time and consequently reduced cost of the manufactured product (Nikam et al., 2016). Therefore, AM has generated a lot of research interest and industry expectations in the recent times.Some worth-mentioning applications of AM include rapid prototyping (RP), rapid tooling (RT), rapid manufacturing (RM) of an actual part, surface modification, surface coating, repairing and remanufacturing. Type of energy source used in AM processes is one of the most important criteria distinguishing between them. Arc [such as gas tungsten arc (GTA) or plasma transferred arc (PTA)] and high-energy beam [such as laser or electron beam] are the commonly usedheat source. Arc-based AM processes have some major advantages such as higher deposition efficiency, lower capital and maintenance costs but they yield poor deposition quality having higher dilution, porosity and oxides and are energy inefficient. High-energy beam-based AM processes have more focused and precisely controlled heat source than the arc-based AM processes. However, they suffer from major drawbacks such as poor energy conversion efficiency and higher capital cost, operating cost, and maintenance cost. Suryakumar et al. (2011) reported that the deposition rate achieved by high-energy beam-based AM processes is of the order of 2-10 g/min whereas arc-based AM processes can achieve it in the range of 50-130 g/min. Jhavar et al. (2014) developed micro-plasma wiredeposition (μ-PTAWD) process to bridge the gaps between capabilities of arc-based and high-energy beam-based AM processes. But, this process cannot be used for good quality deposition of those materials which are very difficult to be drawn in the form of wire (i.e. hard and/or brittle metals and alloys, refractory materials, ceramics, some composites, functionally graded materials). Moreover, use of deposition material in powdered form enables (i) attainment of higher deposition rate; (ii) better metallurgical bond between different deposition layers as well as deposition and the substrate materials; and (iii) better control over the deposition geometry. Therefore, this present work is aimed to develop an energy-efficient and cost-effective process referred to as micro-plasma transferred arc powder deposition (μ-PTAPD) process for various AM applications of the metallic materials with following research objectives: • Todevelop experimental apparatus for μ-PTAPD process for various AM applications of the metallic materials with programming movement of the micro-plasma deposition head along x, y, and z axes by microcontroller. • To study the characteristics of single-layer multi-track coating of Stellite powder on steel substrate by μ-PTAPD process. • To compare the capabilities of μ-PTAPD process with laser and PTA-based processes for coating of Stellite. • To study the characteristics of multi-layer single-track deposition of titanium alloy by μ-PTAPD process using continuous and dwell-time modes. • To study the influence of dimple and spot texturing by μ-PTAPD process on HSS toolin machining of titanium alloy and to compare the performance of textured HSS tools with non-textured tool in terms of machining forces, temperature and wear of the tool, chip formation, and workpiece surface roughness. • To develop a mathematical model of dilution of deposition by μ-PTAPD process.Experimental Apparatus Figures 1a and 1b show schematic diagram and photograph of the experimental apparatus respectively for μ-PTAPD process developed by integrating the following (i) micro-plasma power supply system capable of supplying constant value of DC voltage 22 volts and provision to vary current from 0.1 to 20 A with an increment of 0.1 A. It can supply power both in continuous and pulsed modes, (ii) in-house developed powder feeding system to ensure an uninterrupted supply of the deposition material in powdered form with particle size ranging from 20 to 200 μm. It consists of a hopper to store the powder which is supplied to the deposition head by means of pressurized argon supplied atconstant flow rate of 0.5 Nl/min. Mass flow rate of the powder can be varied by changing rotation of a metering shaft driven by DC motor, (iii) in-house developed deposition head consisting of the μ-plasma torch surrounded by 4 equi-spaced inclined nozzles placed at its periphery. It also provides argon shielding-gas during the deposition process to protect the molten pool against oxidation, and (iv) arduino based microcontroller to control movement of the deposition head along X, Y and Z axes. (a) (b)2. Study on Coating of Stellite 6 on AISI 4130 Steel Substrate Coatings play significant roles to improve performance and life of those parts which operate in adverse environments and require resistance to different types of wear such as fretting, surface fatigue, corrosion, erosion, abrasion, adhesion, and diffusion. Stellite is a cobalt-based alloy which exhibits very good resistance to erosive and corrosive wear particularly, at higher operating temperature due to intermetallic compounds and carbides formed in its coating (Luo et al., 2012). The experiments were conducted in four stages with objectives to (i) identify optimum values of important parameters of μ-PTAPD process for single-layer multi-track coating of powdered Stellite 6 (with particle size ranging from 50 to 106 μm) on AISI 4130 steel substrate; and (ii) to compare its capabilities with laser-based and PTA-based deposition processes for Stellite coating in terms of dilution, deposition thickness, microstructure, secondary dendritic arm spacing(SDAS), micro-hardness and abrasive wear resistance. In the 1st stage, pilot experiments were conducted varying six significant parameters of μ-PTAPD process namely micro-plasma power, travel speed of worktable, powder mass flow rate, shielding gas flow rate, plasma gas flow rate and stand-off distance to identify their values for the main experiments which will ensure continuous uniform single-layer single-track deposition of Stellite. The identified values for the main experiments were: 407; 418; and 429 W for micro-plasma power, 80; 100; and 125 mm/min for travel speed of worktable, 1.7; 2.9; and 3.5 g/min for powder mass flow rate, 3.5 normal liter per minute for shielding gas flow rate, 0.3 normal liter per minute for plasma gas flow rate and 8 mm for stand-off distance. Twenty-seven main experiments were performed in the 2nd stage to identify optimum values of micro-plasma power (as 407 W), travel speed of worktable (as 125 mm/min), powder mass flow rate (as 3.5 g/min) to ensure minimum energy consumption and dilution of single-layer single-track deposition of Stellite by μ-PTAPD process. Four experiments wereconducted in the 3rd stage using 10%; 20%; 30%; and 40% overlapping between two successive tracks and using the identified optimum values from the main experiments in single-layer multi-track deposition of Stellite to identify optimum value of overlapping considering minimum dilution and maximum deposition height as selection criteria. These experiments found 30% overlapping as the optimum value. These identified optimum values were used in the 4th stage experimentation to compare the considered characteristics of single-layer multi-track coatings of Stellite manufactured by μ-PTAPD, Nd-YAG laser-based, and PTA-based deposition processes. The parameters used in Nd-YAG laser-based deposition included: power: 2 kW, spot size: 4 mm, travel speed: 480mm/min, and powder mass flow rate: 11 g/min. The parameters used in PTA-based deposition were: current: 95 A, voltage: 22.5 V, travel speed: 180 mm/min, powder mass flow rate: 21 g/min; plasma gas (argon) flow rate: 2.1 Nl/min, and shielding (argon) gas flow rate: 2.5 Nl/min.Fig. 2: Optical micrographs showing cross-section of the coatings of Stellite-6 manufactured by (a) μ-PTAPD; (b) laser deposition; and (c) PTAD processes. Table 1: Mean values of SDAS, cooling rate, dilution and coating thickness for different processes of Stellite coating. Stellite coating process SDAS ‘λ’ (μm) Cooling rate ‘R’ (oC/s) Dilution (%) Coating thickness (mm) μ-PTAPD 1.74 6.69 x 103 6.3 0.7 Laser deposition 1.72 6.93 x 103 5.8 0.9 PTAD 6.79 1.12 x 102 21.5 2.6 2.1 Some Significant Results • The optical images of the Stellite coatings by μ-PTAPD (Fig. 2a) and laser-based (Fig. 2b) deposition processes reveal that they have good surface appearance, smaller HAZ, excellent metallurgical bond with the substrate and are free from the defects such as cracks and porosity. In contrast, optical image of the Stellite coating by PTAD process(Figure 2c) shows presence of blowholes and cracks which may be due to trapped gasses and varying contraction during the solidification. It also indicates larger HAZ which is caused by more amount of heat used in PTAD process.It can be observed from Table 1 that μ-PTAPD and laser-based deposition processes are capable of manufacturing coatings of thickness less than 1 mm with lower dilution, finer dendritic structure, and smaller SDAS value than the coating manufactured by PTAD. Smaller SDAS result in finer dendritic structure due to higher cooling rate in μ-PTAPD (6.69 x 103 oC/s) and laser deposition (6.93 x 103 oC/s) processes.Phase analysis of Stellite coatings manufactured by all three processes by XRD revealed presence of ε-Co having HCP crystal structure and α-Co having FCC crystal structure mixed with chromium-rich carbides (Cr23C6, Cr7C3), and tungsten containing complex carbide (W2C). These carbides are responsible for higher hardness and wear resistance of Stellite coating.Evaluation of micro-hardness profile revealed that Stellite coating by μ-PTAPD and laser-based deposition processes had almost similar micro-hardness i.e. 553 and 551 HV respectively which is much higher than the coating manufactured by PTAD process (501 HV). This is due to higher cooling rates in μ-PTAPD and laser-based deposition which result in formation of finer carbides which impart higher micro-hardness whereas, lower cooling rate in PTAD process (i.e. 1.12 x 102 oC/s) results in formation of a coarser carbides and higher heat input results in higher dilution (i.e. 21.5%).Coatings manufactured by laser-based and μ-PTAPD processes showed lower wear volume than the coating manufactured by PTAD process for all the values of sliding distance (Fig. 3) due to formation of finer carbides, lower dilution and higher micro-hardness of Stellite coatings. Wear volume of PTAD manufactured coating increases drastically after 400 m sliding distance due to extensive ploughing of the coating.3. Study on Multi-layer Single-track Deposition of Ti-6Al-4V Higher strength-to-weight ratio, fracture toughness and excellent biocompatibility and corrosion resistance of titanium and its alloys have led to their extensive and varied applications in biomedical, aerospace, power generation, gas turbines, automotive etc. (Mahamood and Akinlabi, 2017). The experimental study was conducted in three stages to (i) identify optimum values of six influential parameters of μ-PTAPD process (i.e. micro-plasma power, travel speed of deposition head, powder mass flow rate, shielding gas flowrate, plasma gas flow rate and stand-off distance) for multi-layer single-track deposition of Ti-6Al-4V on the substrate of same material; and (ii) study their effects on deposition characteristics, tensile properties, microstructure evolution, microhardness, and wear characteristics. Pilot experiments were conducted in the 1st stage to identify those feasible values of six considered parameters of μ-PTAPD process for the main experiments which will ensure continuous single-layer single-track deposition of Ti-6Al-4V powder on the substrate of same material. The identified values for the main experiments were: 418; 429; and 440 W for micro-plasma power, 52; 57; and 62 mm/min for travel speed of the deposition head, 1.5; 2.1; and 2.7 g/min for powder mass flow rate, 5 Nl/min for shielding gas (i.e. argon) flow rate, 0.3 Nl/min for plasma gas (i.e. argon) flow rate and 10 mm for stand-off-distance. Twenty-seven main experiments were conducted in the 2nd stage by varying micro-plasma power, powder mass flow rate and travel speed of the deposition head to identify their optimum values considering minimum energy consumption aspects. Identified optimum values were: micro-plasma power as 418 W; powder mass flow rate as 2.7 g/min; and travel speed of deposition head as 62 mm/min. In the 3rd stage of experimentation, thin wall structures of Ti-6Al-4V were made by moving the deposition head in following two ways for its multi-layer single-track deposition by μ-PTAPD process using the optimum values of the six parameters identified from the main experiments: (i) continuous deposition: depositing the successive layers both in forward and backwarddirection movement of the deposition head; and (ii) dwell-time deposition: depositing the successive layers only in the forward direction movement of the deposition head only when the previously deposited layer cools down to 100oC with temperature being monitored by an infrared pyrometer. 3.1 Some Significant Results • Dwell-time deposition of Ti-6Al-4V yieldedlower total wall width (3.73 mm) and higher effective wall width (3.51 mm) than that by continuous deposition (4.1 mm and 3.32 mm, respectively). This led to higher deposition efficiency (89.5%) and lowerdeposition waviness (0.11 mm) of dwell-time deposition than that given by continuous deposition (i.e. 77.2% and 0.39 mm, respectively). This implies that a component manufactured using continuous deposition will require more amount of finishing which will increase cost and wastage of the deposition material. • Optical micrograph of the continuous deposition of Ti-6Al-4V (Fig. 4a) shows inter-layer cracks and voids formed due to non-uniform thermal expansion during the solidification process. It indicates weak bonding between different deposition layers as well as the deposition and substrate because continuous deposition produces higher heat which causes higher thermal gradient between the deposition and substrate materials. In contrast, dwell-time deposition of Ti-6Al-4V (Fig. 4b) depicts that there is no inter-layer cracks and voids and has very good metallurgical bonding between different deposition layers as well as between the deposition and substrate. • SEM image of the continuous deposition (Fig. 5a) showscoarse grained microstructure having colonies of lamellar α and β phases of titanium placed within the boundaries of the big grains. This is due to slowing down of the solidification process by higher heat content in continuous depositions of Ti-6Al-4V which causes larger melt pool. When the molten material is cooled at sufficiently slow rates from the β-phase into the α-β phase region then α-phase lamellae nucleate preferentially at β grain boundaries leading to continuous α-layer along β-grain boundaries. These α-lamellae continue to grow until they reach to other α-colonies nucleated at other grain boundaries. The individual α-lamellae are separated within α-colonies by the retained β-matrix. SEM image of thedwell-time deposition (Fig. 5b) depicts basket-weave fine microstructure which is generally produced by faster cooling rate from β-transition temperature which reduces both α-lamellae thickness and α-colony size. Additionally, new α-lamellae nucleated at other grain boundaries grow perpendicularly to the existing lamellae. This leads to formation of the basket-weave microstructure. • Measurement of the lamellae width from the microstructures revealed that both continuous and dwell-time depositions of Ti-6Al-4V have smaller lamellae widths in the top portion of the deposition than that in the bottom portion. Lamellae widths of dwell-time deposition are smaller than that of continuous deposition. This is due to faster cooling rate in dwell-time deposition. • Evaluation of the tensile properties showed that dwell-time deposition of Ti-6Al-4V has higher yield and ultimate strength, and lower % elongation (i.e. 890 MPa; 930.3 MPa;and 13.2%, respectively) than that for the continuous deposition (i.e. 754 MPa; 788 MPa; and 18.2%, respectively). Examination of the fractured tensile specimen of dwell-time deposition showed fine dimple rupture while that of continuous deposition exhibited occurrence of tear ridges or elongated regions. • Dwell-time deposition of Ti-6Al-4V had higher microhardness than continuous deposition due to fine basket-weave microstructure. • Dwell-time deposition showed lower wear volume and coefficient of friction than that of continuous deposition.4. Study on Texturing of HSS Tool to Improve Machining of Titanium Alloys Machining of titanium alloys using high speed steel (HSS) tool is difficult due to their lower thermal conductivity which increases the temperature of the machining tool thus accelerating its wear. Texturing on rake face of a machining tool has recently emerged as a promising and environment friendly method to enhance removal of heat from the machining zone (Wei et al., 2017). Therefore, investigations were conducted to study influence of spot and dimple texturing by μ-PTAPD process on the rake face of a single-point machining tool made of HSS in machining of Ti-6Al-4V alloy. It was done in the following four stages: (i) In the 1st stage, pilot experiments creating single texture on shank of the HSS machining tool to identify feasible values of the variable parameters of μ-PTAPD process (i.e. micro-plasma power and exposure time for the dimple-texturing, and micro-plasma power, exposure time and powder flow rate of Stellite 6 for the spot-texturing,). The identified values for the main experiments were: 246.4; 264; and 281.6 W of micro-plasma power and 15; 30; and 45 s values of exposure time for the dimple-texturing and 264; 286; and 316 W for micro-plasma power; 6; 10; and 14 s for exposure time, 1.45; 1.76; and 2.10 g/min for powder flow rate for the spot-texturing; (ii) Nine main experiments, creating single texture in each experiment, were conducted in the 2nd stage by varying identified values from the pilot experiment and used to identify optimum values for dimple and spot texturing considering maximum aspect ratio and dilution respectively; (iii) In the 3rd stage, an array of 12 textures on rake face of the HSS machining tool were produced using the identified optimum values of the considered variable parameters. The spot-textured HSS tool was ground to make uniform size of the spots; and (iv) In the 4th stage, performance of the dimple-textured, spot-textured and non-textured HSS machining tools were compared in terms of machining forces, temperature and flank wear of the tool, chip formation, and surface roughness of the machined workpiece during turning of the Ti-6Al-4V cylindrical bar under flooded type coolant system. Other parameters selectedfor turning of Ti-6Al-4V bar were: 45 and 105 m/min as cutting speed; 0.1 mm/revolution as feed rate; and 1 mm as depth of cut (da Silva et al. 2013). Micro-plasma power as 264 W and exposure time as 45 seconds were identified as optimum values to obtain dimple-texture with high aspect ratio and approximately circular shape. They were used for producing an array of dimple texture on the rake face of the HSS machining tool and its optical image and photograph shown in Figs. 6a and 6b. Micro-plasma power as 316 W; exposure time as 14 seconds; and powder flow rate as 1.76 g/minute were identified as optimum values to achieve approximately sphere-shaped spot-textures having highdilution ratio and minimum unmolten particles attached. These values were used to produce an array of spot-textures on the rake face of the HSS tool and its optical image and photograph shown in Figs. 7a and 7b. (4.1 Some Significant Results • Use of spot-textured HSS tool in turning of Ti-6Al-4V resulted in least values of cutting force, thrust force, and tool temperature than the dimple-textured and non-textured HSS tools at different values of cutting speed. These observations can be explained with the help of Fig. 8 which schematically shows how flow of chips over the spot-textured HSS tool increases rake angle and reduces chip curl radius. Increase in rake angle reduces the cutting force whereas reduction in the chip curl radius helps in chip breaking which aidsin reduction of thrust force. Additionally, spots act as fins which enhances the heat loss to the machining environment by increasing surface area of the rake face of the spot-textured tool thus helping in further reduction of its temperature. • Spot-textured tool exhibits least amount of flank wear and adhesion of workpiece material than the dimple-textured and non-textured tools. This is due to higher temperature of the dimple-textured and non-textured tools which increases their sticking tendency for products of machining causing more adhesive wear of their flank surface. Higher temperature also reduces their hardness resulting in further wear of their flank surfaces. • Use of spot-textured tool resulted in formation of segmented chips in turning of Ti-6Al-4V whereas dimple-textured and non-textured tools formed long continuously curlingribbon-like chips. • Average surface roughness of the turned Ti-6Al-4V workpiece revealed that spot-textured HSS tool yielded minimum values of average surface roughness ‘Ra’ of the turned Ti-6Al-4V workpiece than the dimple-textured and non-textured HSS tools at both the cutting speeds. This is due to spot-textured tool having lesser temperature rise and flank wear which results in better surface finish.5. Mathematical Modeling of Dilution Following mathematical model of dilution of single-layer single-track deposition as function of μ-PTAPD process parameters and materials properties was developed using the fundamental principles of energy balance. 𝐷=(1+𝜂𝑑 𝑉𝑑 𝜌𝑠 Δ𝐻𝑠( 𝜂𝑎 𝜂𝑚 𝑃 𝑡)−(𝜂𝑑 𝑉𝑑 𝜌𝑑 Δ𝐻𝑑))−1×100 (1)where, ΔHd and ΔHs are change in enthalpies of the deposition and substrate material respectively (J/kg); ρd and ρs are densities of the deposition and substrate material respectively (kg/m3); P is micro-plasma power (Watts); Vd is volume of deposited powder (m3); t is deposition time; ηa is energy transfer efficiency (%)and ηm is melting efficiency (%). The developed mathematical model for dilution of single-layer single-track deposition was experimentally validated depositing Ti-6Al-4V powder on substrate of the same material and depositing Stellite powder on AISI 4130 steel substrate by μ-PTAPD process. The error between the predicted and experimental dilution for Ti-6Al-4V deposition on the same substrate and Stellite 6 deposition on AISI 4130 is in range from -16 to 6.41 % and -16.85 to 14.30 % respectively.6. Some Significant Conclusions • μ-PTAPD process has a capability to selectively deposit a thin and sound quality coating of Stellite on metallic substrates. It has capability to provide better techno-economic solution than the existing processes for Stellite coating. • Multi-layer single-track of deposition of Ti-6Al-4V alloy by μ-PTAPD process using dwell-time mode having better deposition characteristics, fine basket-weave microstructure, tensile properties, higher microhardness and lower wear volume and coefficient of friction. It demonstrates that μ-PTAPD process has capability to additively manufacture complex part geometry of titanium alloys. • Spot-texturing of rake face of HSS machining tool by μ-PTAPD process is an economical, effective and environment friendly method to improve machining of titanium alloys. • μ-PTAPD process is a very promising process for different additive manufacturing applications of metallic materials. It can be used for similar as well as dissimilar deposition and substrate materials.