Simulation of Extrusion Replacement with Wire+arc Based Additive Manufacturing

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23rd Sep 2019 Computer Science Reference this

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A LITERATURE REVIEW ON

 

Simulation of extrusion replacement with wire+arc based additive manufacturing.

 

Table of Contents

1. INTRODUCTION

1.1 Background

1.2 Scope

2. LITERATURE REVIEW

2.1 Additive Manufacturing

2.1.1 Additive Manufacturing in Metals

2.2 Wire Arc Additive Manufacturing

2.2.1 Classification of WAAM Process

2.2.2 Robotic WAAM System

2.2.3 Common Defects in WAAM Fabricated Components

2.2.4 Methods for Quality Improvement in WAAM Process

2.3 Numerical Simulation in Additive Manufacturing

2.3.1 Modelling with Finite Elements

3. CONCLUSION

3.1 Research Gaps

3.2 Research Questions

REFERENCES

TABLE OF FIGURES

Figure 1. Additive Manufacturing Process.  (Gibson et al., 2010)

Figure 2. Working Principle of Additive Manufacturing. (Kruth et al., 1998)

Figure 3. Classification of AM for Metals. (Ding et al., 2015)

Figure 4. WAAM system design concepts, University of Wollongong.

Figure 5. Defects in material relation in WAAM process. (Wu et al., 2018)

Figure 6. Bead on plate model’s temperature field a) Goldak b) Proposed model (Montevecchi et al., 2016).

Figure 7. Geometry of a WAAM multilayer wall sample (Ding et al., 2014)

Figure 8. Mesh of steady state thermal model. (Ding et al., 2014)

Figure 9.  Distortion comparison between the efficient “engineering” and the transient thermomechanical models (Ding et al., 2014)

 

TABLE OF EQUATIONS

Equation 1. Base material power density function…………………………………………………….13

Equation 2. Filler material power density function ………………………………………………….13

LIST OF THE TABLES

Table 1. The various wire feed process (Karnunakaran et al., 2010)

Table 2. Comparison of various WAAM techniques (Wu et al., 2018)

1. INTRODUCTION 

1.1 Background

Additive manufacturing (AM) is revolutionising the way engineers work. AM, also known as 3D printing, manufactures components using advanced engineering designs tools. (Wong and Hernandez, 2012). Present-day industries have an ever-increasing demand of sustainable, low cost, and an environmentally friendly manufacturing process compared to the conventional manufacturing process which often require large amount of machining. (Guo and Leu, 2013)

Thus, AM has become an important and revolutionary industrial process for the manufacture of intricate metal work pieces. The beads of welds are layered one upon the other for manufacturing the 3D elements. (Lockett et al., 2017)

Additive manufacturing enables to form and the pioneer standard producing process, conjointly adding potential to enrich ancient processes. Therefore, AM is receiving more and more attention and efforts all around the world because there is tremendous interest worldwide in evaluating the potential of AM as a useful and possibly disruptive technology. (Kruth, et al., 1998) The capacity to explore the potential use of AM can also lead to the emergence of innovations for light structures.

Wohlers T. (2010) proposed that AM technology has been growing rapidly from the past twenty years having gained more request for products and services, which in turn led to an increase in Wire Arc based Additive Manufacturing process, in the last years. Thus, there has been a growing interest for research in the metal additive manufacturing field.

 1.2 Scope

The thesis aims at investigating an improved method of Additive Manufacturing using the Wire-Arc based technique for structural components with the help of computer simulation. Furthermore, it also includes conducting and completing parametric studies and developing simple design implications, followed by building models, modifying them and interpreting results.

To establish material properties from elsewhere to develop a materially nonlinear and geometrically nonlinear finite element model using continuum models, i.e.

•          Large displacements

•          Material plasticity

2. LITERATURE REVIEW

In this, a review of the main fields related with the topic of the thesis is presented. It begins with a principle concepts of additive manufacturing in detail and application of this technology in metals. Moreover, the Wire and Arc Additive Manufacturing process and Simulation of Wire Arc additive manufacturing is reviewed.

 2.1 Additive Manufacturing

Additive manufacturing (AM), is defined by ASTM as the “process of joining materials to make objects, usually layer by layer, from 3D CAD data”. (F2792-10, 2018)

AM is used in a broad set of final parts, which includes models for verification, better understanding of concepts, arrangements with properties related to industrial application. (Guo and Leu, 2013). This process is developed based on the rapid prototype concept, in order to rapidly build parts and components designed by engineers and test their performance.

AM when associated with the conventional process has more advantage because of its capability to produce components with high geometrical complexity. The materials produced have been significantly improved with better performances with lighter weight, leading to lower fuel consumption and reduction in cost (Guo and Leu, 2013). Since it is very effective in producing high geometrical complexity, it reduces the components required to complete an assembly. Thereby, eliminating the need for joining and forming process.

Guo and Leu (2013) proposed that with the required material properties, AM can produce prototype parts for carrying out manufacturing of the final products and operate assessment for the different quantities of the respective product. Currently, the direct fabrication of functional end-use products has become the main trend of AM technology.

The following are the steps in AM process (Gibson, Rosen, & Stucker, 2010)

Figure 1. Additive Manufacturing Process.  (Gibson et al., 2010)

The first steps involves converting CAD file to a stereolithography (STL) file. The drawing made in CAD file is divided and sliced so that information can be printed from the respective layers. (Wong and Hernandez, 2012)

The CAD model is obtained by making a 3D scan, a product redesign or the 3D model is downloaded. The CAD model is then converted to STL/AMF file which is used for 3D-printing, rapid prototyping and computer-aided manufacture. Next up, the CAD drawing created needs to be sliced. The slicing of the CAD model can be done by using slicing software. Slicing of 3D models is usually done so that the 3D printer gets the required information from the CAD drawing. (Kruth, Leu and Nakagawa, 1998)

The slicing of the CAD model can be done by using slicing software. Slicing of 3D models is usually done so that the 3D printer gets the required information from the CAD drawing.

The general working principle of AM is schematically represented in figure 2:

Figure 2. Working Principle of Additive Manufacturing. (Kruth et al., 1998)

2.1.1 Additive Manufacturing in Metals

AM has many advantages such as a wide range of deposition range (low to high), near net shape, therefore minimizing the loss of material and reducing cost of manufacturing, minimum conventional machining time, better structural integrity. This can replace manufacturing components made of expensive materials with complex machining so that the expected waste is minimised. (Mehnen et al. 2010).

When the casting process is in the layer by layer process, it is designated as pattern based or indirect process otherwise it is a direct process which approaches to rapid tooling where patterns are not manufactured. Instead, additive processes are used such that direct tools can be produced.

Manufacturing systems are divided into three broad categories: (Frazier, 2014)

(i)                  powder bed systems

(ii)               powder feed systems, and

(iii)             wire feed systems

Ding et al., (2015) put forward that the powder-feed process is beneficial for manufacturing small size elements with high geometrical accuracy. While, wire-feed systems contribute to cleaner environment approach when compared to wire feed approach that impose hazard to the operators of the AM process.

Further, the classification is based on energy sources (Ding et al. 2015):

(i)                  electron beam

(ii)               arc welding

(iii)             laser based

Figure 3 represents the classification of Additive Manufacturing for Metals.

Figure 3. Classification of AM for Metals. (Ding et al., 2015)

Table 1 presents various existing technologies of wire feed process (Karunakaran et al., 2010).

Table 1. The various wire feed process. (Karnunakaran et al., 2010)

ENERGY SOURCE

WIRE FEED PROCESS

Laser

Direct metal deposition (DMD)

Directed light fabrication (DLF)

Laser additive manufacturing (LAM)

Laser based direct metal deposition (LBDMD)

Rapid direct metal deposition

Electron beam

Electron beam free forming

Arc

Hybrid Layered Manufacturing

Hybrid plasma deposition and milling (HPDM)

Shape deposition manufacturing

 

 

2.2 Wire Arc Additive Manufacturing

Wire Arc based additive manufacturing has fascinated engineers from the industrial manufacturing sector in recent years. This is due to the feasibility of manufacturing high deposition rate metal components on a large scale with the reduction in the cost of equipment, properties, which makes it environmentally friendly and high material utilization. (Ding et al., 2015)

Wu et al. (2018) stated that compelling development has been made of the WAAM process along with the progress in the microstructure and mechanical properties of fabricated components. With the tremendous growth in Wire Arc based additive manufacturing, an increasing range of materials have been correlated with the process and applications.

R. Baker (1925) proposed that Wire and arc additive manufacturing (WAAM) process is combination of an electric arc as heat resource with wire feeding, to create 3D component.

In the contrary to traditional subtractive manufacturing, WAAM proves to be a promising candidate for extensively used materials like nickel, titanium, aluminium and steel, because of its ability to reduce the overall time and post-matching time by 40-60% and 15-20% respectively. It is also preferred over traditional subtractive manufacturing for fabricating the metal components with complex geometry, that are larger in size and are expensive.  

2.2.1 Classification of WAAM Process

The three types of Wire Arc based additive manufacturing depending on the nature of heat sources are:

  • Gas Metal Arc Welding (GMAW)-based. (Ding et al., 2011)
  • Gas Tungsten Arc Welding (GTAW)-based. (Dickens et al., 1992)
  • Plasma Arc -Welding (PAW)-based. (Spencer et al.,1998)

GMAW has 2-3 times a higher rate of deposition compared to GTAW-based or PAW-based methods. As electric current acts directly on GMAW-based WAAM and generates more weld fumes it is less stable as compared to the other types of WAAM. The rate of production and the condition in which it is manufactured will be decided based on the WAAM techniques selected. (Wu et al., 2018).

 The Comparison of various WAAM techniques is shown in Table 2: (Wu et al., 2018)

Table 2. Comparison of various WAAM techniques. (Wu et al., 2018)

WAAM

ENERGY SOURCE

FEATURES

GTAW-Based

GTAW

Non-consumable electrode; Separate wire feed process

Rate of deposition: 1-2kg/hour;

Wire and torch rotation are needed;

GMAW-based

GMAW

Consumable wire electrode;

Rate of deposition: 3-4kg/hour;

Poor arc stability, spatter

Cold metal transfer (CMT)

Reciprocating consumable wire electrode;

Rate of deposition: 2-3kg/hour

zero spatter, heat input is low, high process tolerance

Tandem GMAW

Two consumable wires electrodes;

Typical deposition: 6-8kg/hour;

Easy mixing to control composition for long-range-ordered alloy manufacturing

PAW-based

PAW

Non-consumable electrode;

Separate wire feed process;

Rate of deposition: 2-4kg/hour;

Wire and torch rotation are needed;

 

 

2.2.2 Robotic WAAM System

Wu et al, 2018 states that most WAAM systems use an articulated industrial robot as the motion mechanism.

There are two designs available:

The first design provides shielding for inert gas with the help of an enclosed chamber.

The second design uses linear rail positioned robot using the local gas shielding mechanism which is present currently or can be designed to increase the overall envelop. Very large metal structures can be assembled up to large dimension because of its proficiency.

Figure 4 shows an example of this design of WAAM system, used for the research and development at the University of Wollongong (UOW).

Figure 4. WAAM system design concepts, University of Wollongong.

 

There are three steps for manufacturing a part of WAAM:

(i)                  Process planning

(ii)               Deposition

(iii)             Post processing

.

Fabrication with high geometrical accuracy can be obtained by developing the desired robot motions and welding parameters by 3D slicing of CAD model (is done so that 3D drawing effectively translates into something a 3D printer can understand) and software with effective programmes. (Ding et al., 2015a, 2015b, 2016)

3D slicing and programming software is used to reduce the faults or defects of the potential process established on the welding deposition model for fabricating the components. The defects can be avoided by automated path planning and optimization of the process put forth by 3D slicing and programming software. (Ding et al., 2015)

In order to improve the material deposition efficiency (defined as the ratio of the real area of geometry to the deposited area), Ding et al. (2015) created an algorithm based on medial axis transformation, which can increase 2.4 times the material deposition efficiency.

Sensors such as welding signals, metal transfer behaviour (Geng et al., 2017), deposited bead geometry and interpass temperature in WAAM systems can help in manufacturing products of better quality and supporting in-process monitoring. In a preliminary study Zhang et al. (2016) developed a dedicated control technology for WAAM which included the CAD model processing into vector-based programming, path planning strategy and the control of deposition process parameters. A special control of the parameters has been given to both start and end of each pass where has been identified a higher and lower height, respectively.

 

2.2.3 Common Defects in WAAM Fabricated Components

Wu et al. (2018) stated that there are a few additive manufacturing defects even though the mechanical properties of WAAM is efficient in comparison with the traditional methods which are required to be considered for the various field of applications.

Some manufacturing parts due to intense environmental conditions are exposed to defects like high residual stress level, porosity, cracking and delamination. It should be made sure that these defects are avoided as they can cause other failure modes such as high temperature fatigue.

Following are some reasons which can cause defects in WAAM (Wu et al., 2017)

(i)                  Accumulation of heat can lead to thermal deformation.

(ii)               The strategy for the programming is poor.

(iii)             The parameter set up is not as per acceptable standards, will cause instability in weld pool dynamics.

(iv)              Machine malfunction and environmental significance (e.g. gas contamination)

As shown in Figure 5, defects for different materials are illustrated:

Figure 5. Defects and  material relation in WAAM process. (Wu et al., 2018)

The figure explains that for Titanium alloy, there is sever oxidation followed by residual stress and deformation, porosity for aluminium alloy is maximum followed by crack. Steel has severe deformation and crack along with poor roughness surface. Nickel alloy has severe cracks and poor oxidation large deformation, residual stresses and cracks typically occur in Bimetal.

 

2.2.4 Methods for Quality Improvement in WAAM Process

Generally, to improve the properties of materials, eliminate deformation and residual stress, reducing porosity, post processing treatment for WAAM parts is required. Most of the problems has an impact on quality of deposition can be eliminated by convenient application of post process. To improve the quality of WAAM, numerous post-processing treatments in manufacturing technologies have been proclaimed recently. (Wu et al., 2018)

For accomplishing improvement in quality, it is crucial to have a comprehensive understanding of different materials, the ideal process set up, parameter of the component and post processing methods. Three fundamental perspective considered are feedstock optimization, manufacturing process, and postprocess treatment. To guarantee the consistent quality and to reduce the defects, the deposition of the material should have sensible welding WAAM process. 

As WAAM matures as a commercial manufacturing process, improvement of an economically accessible WAAM framework for metal parts is an interdisciplinary test, which coordinates physical welding process advancement, materials engineering and thermo-mechanical building, mechatronic and control framework structure.

 

2.3 Numerical Simulation in Additive Manufacturing

2.3.1 Modelling with Finite Elements

Many studies, from the past few years have been published with direct modelling of physical phenomena that has been implemented in additive manufacturing. These theories are collaborated and interrelated together. The final geometry of parts in powder projection depends mainly on the following:

  • The evolution of the welding local geometry during the manufacturing;
  • The displacements and the inherent stains induced by the manufacturing

The welding of local geometry depends upon the dimensions of the molten pool that the laser forms on the substrate. Displacements and residual strains are usually temperature gradient dependent and thermos-mechanical property of the material used.

Residual stresses and distortion for the manufactured element are one of the main defects of WAAM. Thus, finite element (FE) modelling can be used to improve the quality and enhance the process since large scale simulation are not very effective using traditional model.

Montevecchi et al., 2016, presented research on simulation of the WAAM process using modified Goldak model using original definition of heat flow.

Goldak model does not allow to consider the actual power distribution between filler and base metal. Indeed, in GMAW, there are two ways in which arc power is transferred to the molten pool: direct transfer from electric arc to the base metal and filler metal melting energy transferred by means of beads enthalpy.

Power distribution between filler and base metal cannot be determined by using Goldak method. Therefore, there are two ways in GMAW by which the power is transmitted i.e, Direct transfer where the melting energy is transferred by means of bead enthalpy directly from electric arc to the base metal.

The main theory is to get different power for filler and base material. The base material receives the total power using Goldak Gaussian distribution and the rest of the power is dispersed with patterns that are consistent over the filler material. This will give the results of the steep temperature gradient which will give out the exact heat required by the filler material as proposed by Goldak heat source. (Goldak et al., 1986)

The proposed power density function is presented

                                              

qb=63Qbff,rππaf,rbcexp3x2af,r2+y2b2+z2c2

                                  (1)

                                              

qw=QwVel

                                                                                                (2)

Qw   and Qb  are the total power delivered respectively to the filler and base metal.

ff,r

= Ellipsoidal distribution factor

b = Ellipsoid y semi axis

c = Ellipsoid z semi axis (front)

Figure 6. Bead on plate model’s temperature field a) Goldak b) Proposed model (Montevecchi et al., 2016).

After carrying out the validation, the proposed model had almost similar result compared to the experimental result which gives higher accuracy as that compared to traditional methods. Therefore, WAAM process is accurately simulated using the process proposed by the author.

Ding et al., (2013) examined the stress evolution of WAAM process during heating cycles using thermomechanical FE model. Using thermal cycles of WAAM process, residual stress at the peak temperature can be determined. After this theory, FE model was developed. (Teng et al., 2003, Song et al., 2005)

To help understand and optimise the process, finite element (FE) models are commonly used; however, the conventional transient models are not efficient for simulating a large-scale WAAM process. In this paper, the stress evolution during the thermal cycles of the WAAM process was investigated with the help of a transient thermomechanical FE model

A detailed mechanical model was developed, encouraged by Camilleri’s research (Camilleri et al., 2004, 2007) which was based on different processes of welding. His theory explained that the nodal response of the plastic flow will determine the thermal load that has to be applied to each node.

The dimensions of the model are shown in the figure 7  

Figure 7. Geometry of a WAAM multilayer wall sample (Ding et al., 2014)

The GMAW process is considered forming a 5mm width and height of 2mm with centreline if base plate being deposited with the four-layer wall. To provide a high rate of deposition and input low heat Fronius cold metal transfer (CMT) was taken as the power source.

ABAQUS® was used for designing the finite element model. The distribution of temperature was first calculated by using coupled transient thermocouple simulations and later the mechanical analysis was carried out.

Usually high-density uniform mesh is not required in the steady-state thermal model when compared to transient thermal model. In the heating area small elements were used to get temperature at higher gradient as shown in Figure 8. Course mesh is used in the model as temperature gradient was much less. Forced convection/diffusion brick elements (DCC3D8) were utilised in this model.

Figure 8. Mesh of steady state thermal model. (Ding et al., 2014) 

Research showed that the residual stress of a point can be determined by the maximum temperature experienced by the material in the WAAM process. Based on this theory, mechanical model with simplified properties was developed. The same properties used in the transient model was provided the in simplified mechanical model, i.e., having same boundary conditions and mechanical material properties.

Figure 9.  Distortion comparison between the efficient “engineering” and the transient thermomechanical models. (Ding et al., 2014)

It was assumed that as the number of layers added to the component kept on increasing the significant distortion increased which is represented in Figure 9. But it was found that increase in the number of layers, reduced the prediction of the model thereby increasing the rigidity of the component.

Ding et al., (2014) concluded that “The distortion and residual stress predictions from the efficient “engineering” FE model were validated against a conventional transient model and experimental result. It has been proved that the new model can provide accurate residual stress and distortion predictions as a full transient solution. Therefore, this model could be utilised in real engineering applications to help optimise the process by providing thermomechanical predictions of large-scale WAAM components efficiently.”

3. CONCLUSION

A comprehensive review of the detailed Wire Arc additive manufacturing has been presented, which gives an insight on the mechanical properties of  components, microstructure, defects in the WAAM process and post-processing treatment to improve the defects in the WAAM process. (Wu et al., 2018).

Wire Arc Additive Manufacturing when compared to powder based Additive Manufacturing for a given layer of thickness, it is noticed that it takes much longer for components to be manufactured in powder-based method. Also, the outcome of the structural properties is not as that expected.

Elsewhere, the properties exhibited by the Wire based method is affirmative to structures such as ductility with reduced lead time i.e., time between the initial and the completion of a production process. It uses electric arc as the heat source and wire than powder-based method. The deposition rates in WAAM ranges from low to high. Also, there is considerable reduction in the use of material as it produces near-net shape, which in turn reduces the machining time. WAAM is capable of manufacturing components with low to medium complexity with good structural integrity. (Hoefer et al., 2018)

Since WAAM provides high flexibility, it provides many opportunities for manufacturing large light structures as it can be tailored because of the unique approach of WAAM. Experimentally tested new geometries have been produced that provides material with properties that are flexible for ready-to-use parts. Thermo-mechanical conduct of WAAM can be understood by using Finite Element Model. Potential of AM can be analysed by combining virtual product design, CFD, FEM and the different analysis tool for numerical calculation along with the proper understanding of optimized design principles.

3.1 Research Gaps

Wire arc based additive manufacturing is not yet developed commercially even though there has been considerable advancement along the past few years. This is caused because CAD model inputs cannot be set effectively due to lack of automated process planner. 

As a result, the parameter’s selection for the process cannot be done practically by a person and will need to be selected by automated process because of the complexity of geometry. Hence, the next step will be to establish a CAD automated software, for making it a true WAAM system. The Powder based process or polymer material are one of few commercially available additive manufacturing software, but cannot be applied for WAAM systems.

Process planning has the following challenges:

  • Bead Modelling
  • Slicing
  • Path Planning
  • There is a considerable lag in manual pre and post-processing even though the better accuracy and the fast approach of 3D printers.
  • The production chain in AM can be lengthy and have expensive pre- and post-processing steps e.g. to set up the model, recycling the material and support removal.

3.2 Research Questions

  • How to assess the possible impact of AM on the product in terms of technical performance economic considerations?
  • How effective is AM as compared to the other (traditional) manufacturing methods?
  • How to identify the best building strategies using simulations and systematically designed experiments?
  • What are the different approaches for improving the integration each process step in WAAM?
  • How to analyse the effect of the deposition rate for parts with very complex geometries?

REFERENCES

  • ASTM. ASTM F2792–10 standard terminology for additive manufacturing technologies
  • Ding, D., Pan, Z., Cuiuri, D. & Li, H. (2015a). A multi-bead overlapping model for robotic wire and arc additive manufacturing (WAAM). Robotics and Computer Integrated Manufacturing, 31, 101-110.
  • Ding, D., Pan, Z., Cuiuri, D. & Li, H. (2015c). A practical path planning methodology for wire and arc additive manufacturing of thin-walled structures. Robotics and Computer Integrated Manufacturing, 34, 8-19.
  • Ding, D., Pan, Z., Cuiuri, D., Li, H. & Larkin, N. (2016). Adaptive path planning for wire-feed additive manufacturing using medial axis transformation. Journal of Cleaner Production, 133, 942-952.
  • Ding, J., Colegrove, P., Mehnen, J., Williams, S., Wang, F. & Almeida, P. S. (2014a). A computationally efficient finite element model of wire and arc additive manufacture. The International Journal of Advanced Manufacturing Technology, 70(1), 227-236.
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  • Geng, H., Li, J., Xiong, J., Lin, X. & Zhang, F. (2017). Optimization of wire feed for GTAW based additive manufacturing. Journal of Materials Processing Tech, 243, 40-47.
  • Gibson, I., Rosen, D. & Stucker, B. (2015). Additive manufacturing technologies: 3D printing, rapid prototyping, and direct digital manufacturing (Second ed.). New York: Springer.
  • Guo, N. & Leu, M. C. (2013). Additive manufacturing: technology, applications and research needs. Frontiers of Mechanical Engineering, 8(3), 215-243.
  • Karunakaran, K. P., Suryakumar, S., Pushpa, V. & Akula, S. (2010a). Low cost integration of additive and subtractive processes for hybrid layered manufacturing. Robotics and Computer Integrated Manufacturing, 26(5), 490-499.
  • Kruth, J. P., Leu, M. C. & Nakagawa, T. (1998). Progress in Additive Manufacturing and Rapid Prototyping. CIRP Annals – Manufacturing Technology, 47(2), 525-540.
  • Lockett, H., Ding, J., Williams, S. & Martina, F. (2017). Design for Wire + Arc Additive Manufacture: design rules and build orientation selection. Journal of Engineering Design, 28(7-9), 568-598.
  • Montevecchi, F., Venturini, G., Scippa, A. & Campatelli, G. (2016). Finite Element Modelling of Wire-arc-additive-manufacturing Process. Procedia CIRP, 55, 109-114.
  • Song, Y.-A., Park, S. & Chae, S.-W. (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, 45(9), 1063-1069.
  • Teng, T.-L., Chang, P.-H. & Tseng, W.-C. (2003). Effect of welding sequences on residual stresses. Computers and Structures, 81(5), 273-286.
  • Wong, K. V. & Hernandez, A. (2012). A Review of Additive Manufacturing. ISRN Mechanical Engineering, 2012.
  • Xiong, J., Yin, Z. & Zhang, W. (2016). Closed-loop control of variable layer width for thin-walled parts in wire and arc additive manufacturing. Journal of Materials Processing Tech, 233, 100-106.

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