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Development Of Process Simulation Models For Metal Laser Sintering

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Moving point heat sources have been utilised throughout different aspects of engineering over the past century, however it is the modelling of these moving point heat sources that offer visibility into what is actually taking place within the process. Rosenthal’s analytical model was developed in the 1930’s and allows for exploration and understanding of the fundamental behaviour that occurs during the fabrication process. The equation is valid for a moving reference frame (e,y,z) which has its origin at the point source and travels with the weld beam. The derivation of the Rosenthal equation is dependent on various simplified assumptions, which puts the accuracy of the equation at question.

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T=T_o+Q/2πkr exp(-V(r+ε)/2α)

Further developmental work was completed on the Rosenthal model which lead to Goldak’s continuum model, a double ellipsoid model for a welding heat source. This model has the ability to analyse thermal history of both shallow and deep penetration welds or asymmetrical welds (Goldak et al., 1984). To date this is still one of the most widely used models for laser heating. Despite both Rosenthal and Goldak’s model being initiated in a welding context, they both still hold their relevance today. The Goldak model contains various parameters that are extremely flexible however they do need to be adapted for the problem under investigation. At present, the fundamentals of the welding process are being adapted and utilised for Additive Manufacturing (AM) processes.

AM is a versatile technology that can be used throughout the product development process for manufacturing prototypes, tooling and fully functional end parts. AM allows for intricate geometrical features to be achieved which otherwise would not be possible via traditional manufacturing methods. AM is a much more sustainable in contrast to casting, machining or moulding as there is less energy and raw material being wasted. There are less dedicated tooling requirements for AM processes as they build 3D components by adding successive layers of feedstock material that fuse together creating merged components. AM has seven categories as collectively defined by the British Standard Institute (BSI), International Organisation of Standardisation (ISO) and American Society for Testing and Materials (ASTM) and the ASTM International Committee F42. Despite having seven processes only four of these are suitable for metallic applications, Powder Bed Fusion (PBF), Directed Energy Deposition (DED), Powder Jetting and Sheet Lamination. DED allows for metallic parts to be produced using a metallic filler, this involves metallic filler wire to be fed into a molten metal pool which is generated using an electric arc as a heat source. The process of PBF and DED which both require a moving point heat source will be investigated herein.

Aforementioned, the Rosenthal and Goldak model both hold great importance within the field of AM. (Promoppatum et al., 2017) applied the Rosenthal model comparing analytical and numerical predictions of thermal history and solidification microstructure of Inconel 718 parts produced from PBF. The author indicates that Rosenthal’s analytical model provides a swift estimation within a number of minutes into the thermal characteristics of PBF. This investigation used the Rosenthal model alongside a Finite Element (FE) model, both were compared to examine similarities and dissimilarities in their predictive capabilities. It was noted that the Rosenthal and FE model offer like and rational thermal and microstructural results at a low energy input. The Rosenthal model is useful for quick estimations however when using a high energy input it should be used with caution as heat losses are overlooked (Promoppatum et al., 2017).

Developmental work has been completed on Goldak’s model by (Hodge et al., 2014) producing a multi-physics FE code; Diablo. This model was produced for the use of laser melting of powder and allows for the evolving temperatures that are dependent on the material, state of the material, specified heating and the configuration to be determined. The code was written by the Lawrence Livermore National Laboratory with its primary focus being on nonlinear structural mechanics and heat transfer. DIABLO is capable of representing the radiation transport of laser energy into a powder, along with the phase change that occurs from powder to a consolidated material taking into account the latent heat of melting and is achieved via a volumetric heat source used in the model (Hodge et al., 2014).

Modelling allows for an increased understanding to address known problems within processes. This has evolved from Rosenthal’s basic analytical model to present commercial multi-physic FE software packages. (Lavery et al., 2014) stated that thermal modelling of melting and solidification, residual stress modelling, topological and shape optimization are the main areas commercial software packages are concentrating on. Modelling is both necessary and exceptionally useful for both welding and AM processes. In situ measurements are often difficult or almost impossible to obtain near the melt pool due to the nature of the processes and the localised heating produced. Therefore, modelling constitutes the only efficient method of obtaining the essential information in order to make predictions about the end part and thermal cycles prompted during these processes (Peyre et al., 2008).

Welding is a millennia embedded technology that marries forging, diffusion, friction or melting of the faying faces of parts producing continuity between them. Aforementioned, AM like welding involves the use of an energy source such as laser, electron beam, electric arc or plasma. It also requires one or several feedstock including powder, wire and ribbon. Finally it requires spatial displacement, typically provided by a CNC stage or a multi-axis robot (Marya and Hascoet, 2017).

From a metallurgical perspective, AM is very similar to fusion welding as it calls upon a multitude of complex and interacting physical phenomena. Heat and mass transfer, phase changes alongside a number of process variables associated to the moving point heat source, its paths, and added metal feed rate are commonalities. All of which control the deposition dimensions, aspect ratios and properties including any defects that may occur (Marya and Hascoet, 2017).

Along with the complexity of AM comes it uncertainties. One technical challenge yet to be solved is the ongoing uninterrupted thermal cycles which occurs as a result of the heat source moving away from previously deposited metal. Ongoing and future work is required within the sector to produce industry compliant components that are reliable in terms of physical and mechanical properties. The evolution over the past century from welding to AM has accommodated for an increase in knowledge and understanding regarding thermal management.

Aforesaid, the evolution from welding to AM has occurred expeditiously. However, one method of manufacturing an entire component from the deposition of a weld metal that has been practised since the 1920’s and is now being used as the basis for Wire Arc Additive Manufacturing (WAAM). This technology received its name by Cranfield University as they are among the most active research institutes examining the technology. (Derekar, 2018). WAAM is more advantageous when compared to traditional manufacturing methods as it has a better buy-to-fly (BTF) ratio. BTF is the weight ratio between the raw material used for a component and the weight of the component itself (Arcam, 2018). Furthermore, WAAM is theoretically viewed as having no dimensional limits in terms of a manufacturing and financial technique. WAAM is more economically viable in contrast to PBF where the cost of the material is exceptionally high, (Cunningham et al., 2018) compared the cost per kilogram in wire and powder in 2016. Ti-6Al-4V cost £120.00/kg and £280.00/kg, Inconel 718 cost £58.00/kg and £80.00/kg, Inconel 625 cost £49.00/kg and £80.00/kg, and Stainless Steel 316L cost £12.00/kg and £40.00/kg for wire and powder respectively.

Aforementioned, the process of producing a near net shape from welding has been in practise nearly 100 years, this is more commonly known as WAAM and often mistook as being an emerging AM technology within the last 15 years. According to (Derekar, 2018) the evolution of the WAAM process can be divided into three periods. A patent was filed by (Baker, 1920) for the formation “of superposed layers of metal deposition” achieving various configurations. This method enabled Baker to produce parts from ornamental welding (Figure 1). In order to achieve an equal width of deposit, Baker manipulated the electrode forming an endless spiral, further manipulation took place to achieve an equal depth by forming a continuous helix of deposited metal.

Following the work of Baker a new patent was filed by (Shockey, 1930). This invention allowed for a layer or layers of molten metal to be deposited, it could also deposit from beginning to end in a continuous spiral manner. Essentially an automatic welder, this invention deposits a single layer or bead of molten metal of a predetermined size and composition. Shockey stated that the number of turns per inch of cross feed allows each bead as it is being deposited to overlay the previous bead by approximately one third.

Later work by (Ujiie, 1971) focussed on developing a methodology whilst utilising apparatus to allow for the fabrication of a thick walled cylinder, substantially circular in cross section such as a pressure vessel solely from deposited weld metal (Figure 2). Further work was completed by Ujiie focusing on the deposition rate and developed a three wire electrode gas metal arc welding technique.

The effect of residual stress and metallurgical phases on end mechanical properties was previously overlooked until a shape melted pressure vessel underwent crack failure (Derekar, 2018). Since then great focus has been placed on the importance of these issues and their overall effect on the end quality and reliability of the part. Computational developments have accommodated for advancements within 3D welding, however it has instigated a reinvention curve as the introductory technique was completely different from the traditional process which involved manual and machine controlled processes. Despite new computational efforts enabling innovative testing techniques accommodating for safer structures, it is an additional complexity to pass the test criteria.

During the 1990’s joint research by Rolls Royce and Cranfield University took place investigating aerospace parts produced from Shape Metal Deposition (SMD) using Ti-6Al-4V and Inconel 718 alloys. This research led to numerous theoretical and practical modelling approaches within the field. Areas of study to date within the field include design, process variation, residual stress, forming appearance, interlayer rolling and its effect on microstructure, mechanical properties and residual stress, Cold Metal Transfer (CMT), fatigue failure and toughness. These areas have helped researchers and practitioners develop a better understanding of the process.

At present technical challenges still exist within the process of WAAM. WAAM has lower accuracy in contrast to PBF (±0.2 mm against ±0.04 mm) (Venturini et al., 2016), the process does not have the ability to produce extremely complex parts however it is suitable for the production of very large parts up to meters, whilst having a level of medium complexity such as a stiffened aeronautical panel. WAAM is very attractive to the aerospace industry as a huge reduction of BTF ratio can be achieved. WAAM differs from traditional production methods for aerospace components where subtractive methods generate vast amounts of raw material that cannot be used again; therefore, the use of WAAM within the aerospace industry can have a large economic impact upon a process and final part. Wire based AM has a higher material utilisation in contrast to powder based AM as the entirety of the wire is fed into the molten pool (Xiong et al., 2016). The surface finish of WAAM components is typically not compatible with functional surfaces, therefore post processing is required. WAAM can also be extremely advantageous in terms of deposition rates in contrast to PBF, ranging from 1kg/hr – 4kg/hr for steel (Venturini et al., 2016). Due to the hardware set up of WAAM in contrast to PBF, the initial set up cost is much lower as it consists of a Cartesian or robotic device to position the welding torch, a shielding gas source and a wire feeder unit. WAAM has the additional benefit of being able to alternate between additive and machining procedures, increasing efficiency and usability.

In order to fully elucidate the fundamentals of WAAM the behaviour of a single bead, multilayer structure needs to be examined. A particular focus needs to be given to forming appearance, design, residual stress, process variations and strategic tool path planning (Derekar, 2018).

The forming appearance of a part allows one to appreciate the metal behaviour of the deposited layer. Controlling the deposition of metal at the weld start and end is achievable via a parametric study preventing any undesirable defects. (Derekar, 2018) states that when a heat sink is present at the start of a weld bead is a result of unrestrained flow of the weld and leads to the formation of wrinkles. The author indicates that in order to rectify this issue, start and stop strategies are undergoing development to achieve a smooth part profile. (Xiong et al., 2016) demonstrated that the forming appearance of deposited parts can be enhanced efficiently via control strategies. In multi-layer deposition processes for fabricating metal parts, the difference in height between the arc striking and extinguishing area increases alongside the number of deposited layers. This leads to issues regarding process stability and terminating depositions that are to follow. In order to control this when depositing closed path parts, arc striking and extinguishing area within the same layer were superposed to accommodate for a difference in height. To eradicate the metal knob n overlapping surfaces, it was suggested by (Xiong et al., 2016) that the deposition current is kept constant whilst increasing deposit velocity and arc voltage. Furthermore, open paths parts were controlled by alternating the direction of deposition in adjacent layers. The author proposed that if deposition was to take place in the same direction then the arc current needs to be constant while enhancing the deposit velocity and arc voltage within the arc striking area.

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