MANUFACTURING OF COMPLEX ALUMINIUM COMPONENTS BY HYBRID

LAYERED MANUFACTURING PROCESS

Sajan Kapil, Ankit Desai, Dhirendra Rana, Pravin Kulkarni, Viren Sangwan, Hari Vithasth Yagani,

Fisseha Legesse, K. P. Karunakaran

Department of Mechanical Engineering,Indian Institute of Technology Bombay, India

Abstract:

Hybrid Layered Manufacturing (HLM) is a Rapid Manufacturing (RM)process of metals which combines

the best features of additive and subtractive manufacturing techniques.The integration was carried out in

such a manner that the weld-deposition can act as an additional features to an existing CNC machine

without disturbing its other capabilities. Same as other RM processes, HLM also proceedsina layer by

layer manner. CAD model of the desired component is sliced into thin layers and tool path is generated for

each layer.Thedeposition of metal according to the generated tool path is done using a Gas Metal Arc

Welding (GMAW) gun. After the deposition of near net shape, the machining process is used for providing

thedimensional accuracy.This process has been explored for manufacturing of the complex parts made of

Aluminium, Mild steel and Tool steel. Inconel and Titanium will be addressed soon. The process

parameters such as welding current, stepover andtorch speed have been optimized to achieve the desired

thickness of the layers and spatter free welding. Different tool path generation for area filling have been

studied andadaptive slicing method has beenused for overhang features.The capability of HLM has been

proved by manufacturing an impeller having very complex geometry with use of 1.2 mm wire of Al-Si-5.

Currently GTAW is being explored and Laser deposition also will be taken up soon. These three deposition

methods will have their unique applications due to their different characteristics of cost, speed and

precision.

Keywords: Rapid Prototyping, Additive Manufacturing, Layered Manufacturing,Slicing, Area Filling,

GMAW

1 INTRODUCTION

Rapid Prototyping (RP) is a relatively old technology with the earliest references as old as 25 years ago. It was in the late 1980s that this term grabbed the attention of researchers worldwide. In 1988 the first commercial Rapid Prototyping system was born. The development was closely related to the development of applications of computers in industry. RP was a follow up of the development in CAD, CAM and CNC technologies.Rapid Manufacturing (RM), also known as Layered Manufacturing (LM), is a totally automatic process of manufacturing objects directly from their CAD models without the use of any tooling specific to the geometry of the objects being produced. RP adopts a divide-and-conquer approach in which the complex 3D object is split into several 2D slices that are simple to manufacture. Furthermore, as the object grows from bottom up, the chances of collisions are eliminated. In rapid manufacturing using deposition, the metal is deposited only in the required regions in a layer-by-layer manner. The material can be fed either in the form of wire or powder. The deposition technologies employ laser, electron beam or electric arc as the sources of thermal energy for melting the

metal, in the order of their present popularity [1]. Table 01 lists various existing technologies in each category.

1.1 Hybrid Layered Manufacturing

CNC machining, the subtractive manufacturing method, is the most accurate process capable of producing objects out of any material. However, it requires human intervention for producing the cutter path and it is difficult or impossible to realize certain features through machining. Furthermore, a variety of tools are required and a large portion of the raw material goes waste as chips in CNC machining. The difficulty in developing foolproof CAPP systems for subtractive manufacturing led to the development of additive or generative manufacturing methods popularly known as Rapid Prototyping. Essentially RP is a CNC machine with an embedded CAPP system for generative manufacturing. Compression of product development cycle, feasibility of small lot production and better quality of design through more design iterations are the significant benefits of RP. Subtractive processes can produce good quality parts but are slow; although the material removal by itself is fast, human efforts required for cutter path generation is the bottleneck. On the other hand, additive processes are fast but produce poor quality parts. Hybrid processes judiciously combine the advantages of both these approaches while carefully filtering out their limitations. The additive process focuses on speed while ensuring the desired material integrity. The resulting object is only near-net as no attention to the geometric quality is paid at the time of building it in layers. The inherently fast CNC machining, the subtractive process that follows, ensures the desired geometric quality. In HLM, the near-net shape is obtained with total automation. However, the subsequent two stages of stress relieving/ heat treatment and finish-machining have fair amount of human intervention. In other words, HLM does not strive to achieve total automation but aims at optimal/ economical level of automation.Depending on the geometric complexity, the deposition may be in planar or non-planar layers; similarly deposition and finish-machining may involve 2.5-5 axis kinematics. 1.2 Slicing In Additive Manufacturing to create any part the 3D CAD model is first divided in several 2D slices or slabs. Several commercial RP machines use Uniform Slicing of the 0th order edge approximation [2]. As the name suggests, all slices have uniform thickness and have vertical edges irrespective of the geometry. However this results in inaccurate build with a staircase effect due to the nature of slicing. One method of increasing the accuracy would be to use very thin slices, this may not always be possible and in the event that it is possible, it will definitely increase the build time. In an attempt to improve the accuracy without decreasing the slice thickness led to the concept of Adaptive Slicing. In this method, the CAD model is sliced with varying layer thicknesses. The region of the object with a higher gradient with respect to the build direction is sliced with a smaller layer thickness and vice versa. If the current layer overhangs the previous one beyond a limit then adaptive layer thickness method can be utilized as shown in Figure 01. 1.3 Area Filling Several area-filling tool-path strategies are availablesuch as zigzag or direction-parallel filling, contour-parallel or spiral filling. Depending on the strategy employed the build time, cost, surface quality, warpage, strength and stiffness of the product will change. The best tool-path strategy for HLM aims to optimize the heat dissipation during deposition, mechanical characteristics of the final shape and the fabrication time and ensures uniform deposition. The surface quality of the near-net shape is not of great importance in HLM as it subsequently undergoes a much more accurate finish-machining operation, but integrity of the interior is very important.In most cases, contour-parallel strategy is preferred for HLM because it can provide: Effective heat dissipation, Less number of arc switch on/offs, Reduced warpage as toolpath

direction changes continuously, Better geometrical quality of object by following geometrical trend of boundaries. The contour parallel strategy for a random boundary is shown in Figure 02.

2 OPTIMIZATION OF PROCESS

To ensure maximum utilization and minimum wastage of material in HLM, the process parameters should be optimally chosen. This needs the understanding of effects of process parameters and their inter-dependencies. Modeling of weld deposition can provide the knowledge of influence of process parameters. This is done by modeling for single bead deposition and verifying experimentally. Even for multi-bead deposition the same can be done. During welding deposition three types of flow take place; current flow, heat flow and mass transfer. The parameters controlling these flows are divided into following three classifications:

Primary parameters

Secondary parameters

Distinct level parameters

Voltage, current, torch speed are the primary parameters for weld deposition. These can the easily measured and continuously adjusted. These parameters are used most commonly to change the weld characteristics. The formation of bead geometry (bead height, bead width, penetration) depends on these primary parameters. The height and width of bead are responsible for weld geometry and penetration influences the strength of the weld. The secondary parameters do not directly affect the bead formation but can be changed over a wide range. These affect on the primary parameters which control the bead geometry. Secondary parameters like are difficult to measure and control. Electrode stick out, nozzle angle, nozzle distance from base plate are considered as secondary parameters. Distinct level parameters remain unchanged during a particular welding session. These are filler wire material, size, welding current type, polarity, shielding gas composition, flow rate etc. These parameters are selected depending on the deposition material. For example, Argon gas with 5% to 20% CO2 is used for mild steel welding, but for Aluminium welding 100% Argon gas is used. The wire diameter also controls the heat input and deposition resolution. Thinner wire is preferred for lesser heat input and better resolution of deposition.

2.1 Bead Modeling

GMAW is the most popular process for joining in welding application. In HLM, GMAW is used for metal deposition by cladding. Data of cladding by GMAW for different metals are not well established. Therefore, some preliminary experiments on welding parameters in cladding were carried out. In pulsed synergic GMAW, the geometry of weld bead depends on torch speed and wire speed. Wire speed can be controlled by controlling the current flow as current and wire speed has a monotonous relationship in synergic control. The relation of current and wire speed of 1.2mm Al-Si wire is shown in the Figure 03. In HLM, the layers are built by depositing weld bead overlapped. Face milling decide the layer thickness after milling. The ratio of volume of the layer after face milling and volume of metal deposition is known as yield. Yield denotes the material utilization of the process. Yield and layer thickness are the main output parameters in HLM.

A model has to be developed to predict the layer thickness and yield from the geometry of the weld bead. This model may help in identifying the optimal process parameters to maximize yield and minimize heat input.

2.2 Slant Features in 2.5 Axis

In HLM, no support material is used to manufacture overhang parts. This is one of the major limitations of HLM. But some complex geometry such as slant walls can be manufactured using 2.5-Axis deposition. This is limited to a certain angle. For mild steel the angle is 30° from vertical. For Al-Si material the angle was not still found out. So, we did some simple experiments to find out the maximum possible slant angle of wall. We started tilting the wall from 15° from vertical and slowly increased the angle up to that point where the previous layer cannot hold the weld droplet and weld droplet falls down.

3 CHARACTERIZATION OF THE PROCESS

Deposition of metal is done by using GMAW. So, the common welding defects are also visible in metal deposition. These are; non homogeneous structures, porosity and anisotropic properties. High porosity, non-uniform hardness and higher surface residual stresses are main problems in HLM. These problems restrict HLM to be used in industrial applications. Porosity affects the strength of material. Entrapment of gases inside the layers at the time of welding generates porosity. This problem occurs due to insufficient pressure of Inert gas (Ar for Al, Ar+CO2 for Mild Steel). To avoid this problem the gas pressure is kept 10 times than the wire diameter. As we are using 1.2mm diameter Al-Si wire, so we used the gas supply at the rate of 12 lit/min Residual stresses are also a major problem in HLM. It reduces the fatigue life of the object. Residual stresses caused by differential heating and cooling. Some of the sources of residual stress are; contraction stress, stress due to higher surface cooling, stress due to phase transformation. The techniques, used to remove residual stresses are:

Heat treatment

Vibratory stress relief

Pneumatic hammer peening

Shot peening

Hot Iso-static Pressing (HIP)

Using clamping and unclamping method

Here we are more concentrated on building up critical features by HLM. So, no detailed experiments have been done in the field of characterization. Only the tensile strength has been measured to understand the strength of the deposition. Till now we had done the single bead and multi bead deposition experiments to build up fully filled layer of deposition. But to build up a complete object several layers should be deposited. Two consecutive layers must be joined fully. The penetration of welding takes responsibility to join two consecutive layers. The surface of single layer deposition has scallops. And the upper surface may have oxidized during welding. These may cause problem for penetration of the deposited layer and porosity between two layers. So, we face mill the base layer up to desired height to ensure flat and non-oxidized surface suitable for next deposition. These definitely causes lose of material, but it is very less compared to the deposited material. Thus by depositing one by one layer the complete object is built up.

4 EXPERIMENTS AND RESULT

4.1 Bead Deposition Experiments

To find out the relationship between input and output parameters of single bead formation, experiments were carried out for different values wire and torch speeds. The wire was Aluminium-Silicon alloy (Al-Si) (ER4043) of 1.2 mm diameter. ‘Fronius TPS 4000’weld-deposition unit was used. The current was varied in the range of 50A to 150A. This is directly related to wire speed range 2.4 to 6.8 m/min. the torch speed was varied from 600m/min to 1000m/min.

A pattern shown in the Figure 04 was deposited on a substrate of 160mm x 100mm x 20mm for various combinations of wire and torch speeds. The pattern is such that the starting beads preheat the substrate so that the beads at the end are representative of the actual working conditions. So, the bead among last three beads is used for the measurements. One such deposition corresponding to a wire speed of 5.4m/min and torch speed of 750m/min is shown in Figure 04.

Then a dial gauge with 10 micron accuracy was mounted with the machine. With the help of dial gauge the Z value of top point of the most uniform deposited bead from last three layers and the Z value of the base plate surface was measured. The difference between these two Z values gives the height of the bead by which the bead geometry can be calculated. To compare the geometry of bead with the calculated one, co-ordinates of eight more points (four points from both sides of the top point with 3mm-5mm gap in X direction) on the bead were measured. These points gave a parabola curve, which show the bead geometry. Such experiments were carried out for a combination of 9 torch speeds and 11 wire speeds (total 99 experiments). The Figure 05shows the variation of bead height with varying current and torch speed. With increasing torch speed the height of bead decreases in constant current or wire feed rate. The bead height increases with increase of current in constant torch speed. The Figure 06 shows the variation of bead width with different torch speed and varying current. In constant torch speed the bead width increases wing increasing current or wire feed rate. The bead width decreases with increasing torch speed in constant current. 4.2 Slant Feature Experiments The angles given to the deposited walls are; 15°, 20°, 25°, 30°, 35°, 37.5°, 40°, 42.5° 45°, 47.5° respectively as shown in the Figure 07. The walls given angle 45° and more almost fallen down and touched the base. From this, we can come to a conclusion that HLM can built slant wall of Al-Si material with maximum 42.5° angle with vertical. In any object, where the angle of slant wall is less than 42.5°, can be manufactured by HLM. Now overhang parts are not impossible in HLM. Until the angle crosses 42.5°, simple 2.5-Axis deposition can built the part. If it is more, then there is 5-Axis deposition to bring in role. 4.3 Experiments for Mechanical Properties

The layer of material is deposited by overlapping more than one bead and the object is manufactured by depositing layers one by one. So, there is always a chance for strength variation in different weld directions. But for a functional object this type of characterization is undesirable. The overlapping beads as well as two layers must be joined such a way that the strength in all directions would be same. Already we had chosen the welding parameters and step over increment, which are optimal for deposition. We made a block (70x60x50 mm³) by depositing layers. We cut out tensile specimens from three different welding directions; torch direction, welding direction and vertical direction. Here from the Figure 08, we can say that the torch direction is along X-axis, welding direction is along Y-axis and vertical direction is along Z axis. We cut out total 11 tensile specimens (4 from torch direction, 4 from welding direction and 3 from vertical direction) by milling. The specification of the specimen is given in Figure 09. The tensile test of the specimens has been done in Universal Testing Machine (Lloyd Instruments-LS 100 PLUS) which is shown in Figure 10. The results of the tensile testing are given bellow in Table 02 At 38°C, ultimate tensile strength of AL-Si 4043 is 165 MPa. From the results, it is found that the tensile strength of deposited material is almost same as the material welding property. Also, strengths in different directions are nearly same; there is no variation with direction. The weld directions will not affect the quality of the object manufactured by HLM. So, the parameters are acceptable for HLM.From the tensile test it is clear that the tensile strengths in all directions are nearly same. So the inert quality of HLM manufactured object is same in all positions. The parameters chosen are acceptable to build up objects.

Now we can use the same parameters to manufacture HLM objects. The parameters are most optimal in case of speed, complete deposition and quality of the object in HLM.

5 CASE STUDY: AN IMPELLER

Here manufacturing an impeller using 3-axis Hybrid Layered Manufacturing is explained. The CAD model of the impeller is shown in Figure 11. Slicing for 3-Axis deposition are conventional slices. These slices are required to deposit the impeller on 3-Axis machine. The uniform layer thickness is 1mm. After slicing, the next step is to generate the contouring tool paths. If the contouring is to be done using a vertical axis, then a pattern finishing strategy results in the simple tool path in Figure 12. Following deposition along the contour the next step is to fill the interior area using area-filling tool paths. The most important parameter in this process is the step over value. The ideal step over value for superior weld deposition of Al (Al-Si-5) of 1.2 Diameter, with 90 Amps current, is approximately 2.5mm. This number is arrived after some experiments measuring the overlap of depositions on adjacent paths. Figure 13 shows some deposited layers with different step over value, it can be observe that the toolpath for 2.5mm step over having the most uniform deposition and layer thickness.The near-net shapes are generated after depositing the contouring and area-filling tool paths for each layer as shown in Fig 14.Near net shapes are deposited, Figure 15 and 16 shows the near-net shape of a scaled impeller made on 3-Axis. The deposition of each layer is followed by a face-milling operation. Figure 17 shows the final impeller after machining on 5-axis CNC. During the entire manufacturing process, the build time and amount of material consumed was recorded. The saving in build time is approximately 72% and that in material is approximately 63% which we can see from Table 03 and 04.

CONCLUSION The HLM process which uses GMAW is particularly suited for the rapid manufacture of comparable quality objects at considerably lower cost.As HLM does not have any support mechanism, building components with undercuts/overhang is achieved using adaptive slicing method. This is a unique feature of HLM. For deposition of the layer, uniform slicing of the 0th order edge approximation is adequate.The layer thickness can be change by following the face milling operation after deposition.By the experiments it has been found for aluminum a slant feature up to 42.50 can be achieved without any support mechanism. It is also found that the tensile strength of deposited material is almost same as the material welding property and strengths in different directions are nearly same; there is no variation with direction. While building a geometry with high undercuts, if the current layer overhangs the previous one beyond a limit (42.50 for Al), the material flows down before it solidifies. To avoid this problem, one can suitably tilt the substrate using a 5-Axis kinematics.

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Table 01 Various deposition-based RM processes according the energy source used [2]

Energy Source Name

Laser

Direct Metal Deposition (DMD) [3] Directed Light Fabrication (DLF) [4] Laser Additive Manufacturing (LAM) [5] Laser Aided Manufacturing Process (LAMP) [6] Laser Based Additive Manufacturing (LBAM) [7-9] Laser Based Direct Metal Deposition (LBDMD) [10] Laser Engineered Net Shaping (LENS) [11] Rapid Direct Metal Deposition [12]

Electron Beam Electron Beam Freeforming[13]

Arc

3D Micro Welding (3DMW) [14-16] 3D Welding [17] 3D Welding and Milling [18-20] Hybrid Layered Manufacturing (HLM) [21,22] Hybrid Plasma Deposition and Milling (HPDM) [23] Micro-Plasma Arc Welding (MPAW) [24] Shape Deposition Manufacturing (SDM) [25]

Fig 01 Adaptive layer thickness method[2]

Fig 02 Contour parallel strategy

Figure 03 Current vs Wire speed graph for 1.2 mm Al-Si wire

Figure 04 CAD model and weld deposition on substrate

Figure 05 Surface plot of Bead Height (h)

Figure 06 Surface plot of Bead Width (w)

Angle: 15°, 20°, 25°, 30°, 35°

Angle: 37.5°, 40°, 42.5° 45°, 47.5°

Figure 07 slant walls with different angles

Figure 08 Toolpath and block made by HLM using torch speed 1000mm/min and current 90Amp

Figure 09 Specification of tensile test specimen

Figure 10 Universal testing machine and samples before and after testing

Table 02 Tensile strength of HLM object in different directions

Directions

Yield Stress (MPa) Ultimate Stress (MPa)

Torch direction

(X)

X1 59.12 170.308

X2 59.99 170.65

X3 52.49 148.41

X4 85.99 169.28

Average Value(X)

64.3975 164.662

Weld direction

(Y)

Y1 86.4 162.25

Y2 58.45 170.09

Y3 45.5 158.29

Y4 65.85 176.16

Average Value(Y)

64.05 166.697

Vertical direction

(Z)

Z1 66.87 147.02

Z2 55.33 162.35

Z3 48.86 174.49

Average Value(Z)

57.02 161.286

Figure 11 CAD model of Impeller

Figure 12 3-axis depositiontoolpath of the outer contours of the impeller. The red vertical lines show the tool axis/ torch direction.

Figure 13 Experiments for selection of step over value Figure 14 3-axis deposition tool path of the area filling

of the impeller.

Figure 15 Intermediate step during near net deposition Figure 16 Final near net shape of the impeller

Table 03 Time Data

Deposition Time 4hr

Finish Machining 27hr

Total 31hr

Machining from cylindrical block 110hr

Table 04 Material Data

Final Part 1.015 Kg

Near-Net Part 1.826 Kg

Al Cylinder (170 x 80) 4.901 Kg

Material saved as compared to purely subtractive process

4.901 - 1.826 = 3.075 Kg

Figure 17 Impeller after machining