Keywords Additive Manufacturing, Selective Laser Melting (SLM), Aluminium Alloys, Porosity.

Transcription

1 Additive Manufacturing by Selective Laser Melting of Al Alloys 1 Aditya Kundekar, 2 Hrishikesh Yadav, 3 Amar Pandhare 1,2 U.G. Student, 3 Associate Professor and Head 1,2,3 Department of Mechanical Engineering, Smt. Kashibai Navale College of Engineering, Pune, India Abstract: Currently, Additive Manufacturing (AM) technologies have grown their demand in market tremendously due to continuous improvement and advancement. Selective laser melting (SLM) is widely spreading and gaining popularity as an alternative manufacturing technique for complex and customised parts. SLM can directly produce final products which doesn't require much more post processing. Also, as SLM produces little waste and enables more optimal designs it adds to environmental advantages. This paper covers the detailed information about this process and its application for manufacturing aluminium alloys. Al alloys are used in various high-end applications like automotive and aerospace industries for their strength and light weight. However the use of aluminium (Al) alloys in SLM is limited due to difficulties in processing. Such as higher porosity, key holes, low surface finish, deformation, etc. In this paper, different studies are reviewed which are used to reduce porosity and optimise SLM process. To get required results compromise between the different parameters and scan strategies was achieved and used to produce parts achieving a density greater than 99%. Keywords Additive Manufacturing, Selective Laser Melting (SLM), Aluminium Alloys, Porosity. I. INTRODUCTION There is a current need to manufacture geometrically complex structures that are light-weight. Also, this has to be achieved with low cost and minimum time of manufacturing. Conventional methods are not sufficient to give this result. Thus, this demand resulted in the development of rapid prototyping also known as Additive manufacturing (AM). In which material is added layer by layer to produce 3D object unlikely in conventional method material is removed (subtractive manufacturing). SLM is one of the methods used in additive manufacturing. The development in SLM is partly motivated by the aim of finding a cleaner, efficient and more eco friendly manufacturing process. It has outstanding ecological advantages since it saves resources as the waste can be reduced to zero. Another advantage is that complex parts can be easily manufactured with this process. Also, it allows lightweight structuring with approx. 50% weight reduction. In brief, SLM has the aim of "design for performance" instead of design for manufacturing. II. ADDITIVE MANUFACTURING Additive Manufacturing (AM) technology was first developed in the 1970s. Since that it have been improved, reinnovated and extended tremendously. There are seven major categories of AM technologies. AM technologies are categorised mainly by the mechanism of processing and further classified according to the materials and/or energy source. Metals and polymers (including photopolymers) are widely used materials in AM. AM is a major term given to all technologies that manufacture parts by adding material in layers, as opposed removing material in more traditional subtractive processes. Initially, AM technologies were known as RP and restricted to the manufacture of models and prototypes. RP term was used for many years to refer all layer additive manufacturing processes. Recent researches in advancement in processes, materials, and machines makes possible for producing parts with sufficient mechanical properties to allow required functional applications. This innovation helps layer additive technologies to be used to manufacture finished components without major post processing, which led to the manufacturing method AM. Now, AM is the general term used to represent RP and RM and it describes the respective application of AM technologies. A. Principle of Additive Manufacturing AM technologies produce parts by the polymerization; selective melting, fusing or sintering of materials in predetermined layers without the use of much more tools, as opposed different tools are required in subtractive to a process of material removal to produce the required object. AM can be used for the production of geometries that are almost impossible to produce using other machining or moulding processes with required properties. The process does not require predetermined tools, tool paths and angles or undercuts. The layers of all AM parts are created by slicing CAD data with specialist software. All AM systems work on this principle only; however, different process parameters affect the thickness of the layers depending upon the system. Most machines that are used in commercial applications typically use layers of thickness varying from 16μm up to 200μm. The layers of an AM part are built up one on top of another in the z-axis. The work platform and substrate plate is lowered by one layer thickness in the z-axis as one layer have been processed, and a new layer of material is recoated by wit predetermined thickness of layer using a number of different methods of AM. In resin-based systems the parts submerge in the resin are lowered by one layer thickness and resin is flatten by traversing edge before the material is processed. Similarly in powder based systems, deposition of powder and spread is done using a roller or print cartridge or a nozzle is used, which only deposits the support and part material according to requirement. The time required to recoating of each layer with new material can be take same time or even longer than the time required for the layer to be processed. For this reason, multiple parts are built together so that time required for recoating is minimised. For greater efficiency, nesting software has become available to orientate and position parts which gives virtual representation of parts so that it can be used for optimization for mass production. Currently available examples of nesting software are as follows VisCAMRP (Marcam, Denmark), and SmartSpace used within Magic (Materialise, Belgium). Some parts made using AM technologies require a support structure for holding the part into position on a substrate plate during the build process. This support structure also prevents the part layers from lifting away from their axis as a result of Available Online@ 546

2 material shrinkage that occurs due to the temperature gradient. Different type of support structure is designed for different parts according to geometry to be most effective with a specific material or technique that the system uses to build parts. III. SELECTIVE LASER MELTING (SLM) Selective Layer manufacturing (SLM) has been in use from over 30 years before. It is widely used for the production of three-dimensional parts. This is directly fetched from CAD models and produce by adding material layer by layer, rather than removing material. Nowadays, layer manufacturing process is aiming at producing end user products rather than prototypes due to current improvements. Selective laser melting (SLM) s one of the AM technology which uses metal powder. Full density parts with minimum porosity can be produced by SLM as it fully melts the powder. The SLM holds the advantage over electron beam melting (EBM) and laser metal deposition that the equipment used in SLM is less complex. In EBM high vacuum chamber is needed to guide electron beam as opposed in LBM. Also, proper alignment is not needed between powder feeding mechanism and laser beam in the case of SLM (as opposed to EBM). SLM is widely used in aerospace, Medical, automotive fields. Even though SLM is a very promising technique, but due to the use of intense heat input, problems such as balling effect, rough surface finishing, residual stresses, and part distortion due to temperature gradient are still frequently observed in SLM process. A. Process characteristics Fig. 1 shows the schematic of SLM process. It consists of the building platform system, powder delivery system and laser system. The part is built upon the building plate which is fastened to the build platform and adjusted to be horizontal. Then the metal powder is poured over substrate plate and spread over the plate by the using roller. The thickness of powder can be set by the user, however, each powder layer has, at least, the thickness of the diameter of the powder. It uses a fine powder according to requirement system to distribute a μm layer onto the substrate plate. Laser scans this metal pool in the predefined path after the powder has been spread over the plate. This causes the fusion of layer by selectively scanning laser beam on the powder surface. The intensive laser energy fully melts the metal powders to form a solid metal. After scanning of one layer is finished, the build platform is lowered in the Z direction by the depth of a layer and new powder is spread over the previous layer. This process is repeated continuously and the new layer is built over and over again on the previous layer until the part has been finished. The process principle of SLM is shown in Fig. 2, when the laser beam strikes on the powder surface, energy transformation takes place from the top surface to subsurface through various phenomena, some of those are absorption and scattering of the laser, heat transfer via conduction, flow of fluid within the molten pool, evaporation, and some chemical reactions. Powder melting takes place when the temperature reaches the melting temperature of metal due to impingement of laser. Average applied energy per unit volume effects on the dimension of the metal pool, which is mainly controlled by process parameters including laser power and scanning speed. The principle of laser heating of a powder layer is completely different from heating of a solid body because of following reasons. Metal powder layer has higher laser absorption power than that of the bulk body due to their different granular form. Fig. 1: Schematic of SLM process Fig. 2: Process steps IV. NEED OF STUDY The laser energy input in SLM is highly concentrated on the metal powder which results in large thermal gradients, this leads to thermal stress in the part. Deformation, warpage cracking, internal porosity, rough surface and dimensional errors are most common defects. A. SLM Post-processing As shown in fig. 3 the major work carried out in SLM as a post-processing is the removal of the support structure and finishing used to avoid warpage defect. There are two types of support materials used: (a) material which covers the product as a built-in process (natural supports), and (b) Artificial rigid structures which are designed separately and placed in metal pool, this are restrained or attach the part being built to a build platform (synthetic support). In SLM, in the first case, the surrounding powder itself behaves as a natural support and gives support for the part of the build process. The part could be easily removed from the loose powder in SLM. In most cases where geometry is complex, extra synthetic support structures need to be designed and fabricated to reduce deformation caused by thermal stress. In SLM posts processing such type of synthetic support, structures are needed to be removed. Thus, LSM does not give surface finish up to requirement due to oxidisation and also after removal of structures it gives rough surface. Thus surface finishing is to be carried out as a post processing. Fig 3: Warpage of overhanging parts [4] Available Online@ 547

3 V. LITERATURE REVIEW SLM is emerging process mostly used for manufacturing of aluminium alloys. But due to some inherent properties of material and process, it becomes very difficult to produce workpiece without defects. Pore holes, keyholes, oxide films, poor surface finish, warpage, etc. defects occur while SLM processing. Porosity is the major problem which occurs while processing Aluminium. Thus, it needs to be studied and find out the solution on this process. Lots of scientists have conducted the study on the similar basis. Overview of all this study has been presented in this paper. A. Material The material used in this study is aluminium alloys. Different authors have used different alloying elements and proportions but the parent element is aluminium. The AlSi10Mg powder is used by Noriko Read & Nesma T. Aboulkhair with different compositions [1], [2]. One of these material compositions is shown in Table 1. The size range was microns, as measured using Coulter LS230 laser diffraction particle size analyser. It is obvious that the powder particles are not spherical. The particles used in SLM shows a very irregular morphology, having small irregular, elongated particles along with the big particles. Powder flowability and melting behaviour and other properties are mostly dependent on the particle size and their distribution. Despite the irregular morphology, still the powder has reasonable flowability and Hausner's ratio for SLM [1]. Al-12Si (in wt. %) powder was used in the study by X.P. Li. For some builds, air at 100 C was used for powder drying for around 60 min just before processing [3]. For other builds, as-received powder which was stored at ambient temperature (T0) in air for a period of more than 30 days was used without drying [3]. Table 1: Composition of AlSi10Mg [2] X.P. Li has made Al-12Si cubes (10mm 10mm 10mm) were produced from either as-received or dried powder on a ReaLizer SLM-100 machine (ReaLizer GmbH, Germany) with a fibre laser having the maximum power of 200 W the part bed and generating a laser beam with a wavelength of 1.06 micron and. Atmosphere was maintained with high purity rich argon having minimum oxygen content during processing to avoid oxidation. Four laser scan speeds 500, 1000, 1500 and 2000 mm/s were used, while the laser power, hatch spacing, powder layer thickness, and substrate temperature was fixed at 200 W and 200 C, respectively. Archimedes method is used for calculation of relative density of cubes made in this study [3]. C. Experimental work and study All specimens were built using a Z-increment (vertical) of 30 lm. All processing was surrounded by inert gas like Argon with an oxygen-content < 0.1%. Specimens produced in this study by an island scanning strategy in which the filled layer is divided into several square (islands) with each island being built randomly and continuously. Inside this strategy, the laser is raster-scanned individually. After selective melting the islands, perimeter of layer is scanned by laser to improve the surface finish. For each subsequent layer, these islands are moved in the X and Y- directions by 1 mm, as illustrated in Fig. 2. The aim of the island deposition is just to balance out the residual stresses in the build pool [1]. The testing cubes produced in this study having the size of 5 mm 5 mm 5 mm. The platform on which the cubes were built was kept at 200 C to maintain the part at high temperature so that warping can be avoided due to non-uniform thermal expansion. Fig. 4 shows the different parameters that affect the characteristics of the process. In order to study the effect of the process parameters on process of pore generation, three different phases are studied those are listed below [2]. B. Experimental setup In SLM process Yb-fibre laser is used along with other needed instruments to carry out further study of specimens. Noriko Read has used specimens which were fabricated using a Concept Laser M2 Cusing SLM (laser powder-bed) system. Yb-Fibre laser having laser track width of 150 microns and the laser power up to 200 W, with laser scan speed up to 7000 mm/s was used in M2 system [1]. Nesma T. Aboulkhairhas produced test cubes using a Realizer GmbH SLM-50, Germany, with a 100 W yttrium fibre laser (YLM-100-AC) [2]. Malvern UK Mastersizer 3000 is used to determine particle size distribution for the powder. Which uses laser diffraction as a principle to measure the particle size through measuring the light intensity that is scattered when laser beam passes through a dispersed particulate sample. Powder morphology was evaluated by using a Philips XL30 scanning electron microscope (SEM) with a 20 kv accelerating voltage. In addition, the SEM was equipped with an energy dispersive X-ray (EDX) detector that was used for chemical composition analysis. The SLM process was carried out under an Argon atmosphere with an oxygen level below 0.5% [2]. Fig. 4: Controlling Parameters in SLM [2] Hatch spacing study In this study five sets of samples, each contains three samples were produced using different hatch spacing values. The other parameters were kept constant for all samples in this batch [2]. Scanning speed study This is carried out by keeping constant thickness of layer and input laser power. Scanning speed (v) was varied between Available Online@ 548

4 250 and 1000 mm/s. Two different hatch spacing values are used along with 250mm/s scanning speed [2]. Scan orientation study Three different sample sets were built with different scanning speed accordingly (500, 750, and 1000 mm/s). Different scanning strategies like X, 2X, etc. X. P. Li has used Al-12Si cubes of size 10mm 10mm 10 mm were produced from either as received or dried powder and X-ray photoelectron spectroscopy (XPS, Kratos AXIS Ultra DLD) was carried out using monochromatic Al K-alpha X-rays. For the as-received powder samples, drying was carried out using a resistive heater in thermal contacts with sample holders made up of copper in XPS and a hot electron filament source was used to compensate for charging and calibration was done by the adventitious carbon main peak alignment to the equivalent for the solid sintered samples. A total of three cube samples were measured for both as-received and dried powder [3]. D. Results and discussion According to the study made in Noriko Read research paper, different process parameters are optimised using a genetic algorithm which gives predicted degree of density. Fig. 5 shows the interaction effect between the scan speed and hatch spacing on the formation of porosity. point approximately 60 J/m 3 was achieved. At this value minimum pore fraction for this alloy. Tensile or creep strength of AlSi10Mg does not change with the direction of build. The study conducted by Nesma T. Aboulkhair is summarised in the schematic diagram in Fig. 6. Heat accumulation in the melt pool occurs when smaller hatch spacing is used. It allows slow cooling of the layer giving sufficient time for formation of homogeneous and continuous. Lower speed leads to the formation of metallic pores whereas keyhole pores are created with increasing scanning speed, along with a decreasing metallurgical pores. Irregularities on the surface caused by balling, that occurs with increasing scan speed. This promotes the capture of powder that is not fully melted by the laser beam; hence, a keyhole pore is formed. Alternating option to prevent the formation of keyhole the scan strategy of a double scan at high speeds was implemented instead of alteration in scan speed, as the latter introduces metallurgical pores [2]. Fig. 5 Effect of the scan speed and hatch spacing on the porosity [1] Scan speed doesn t produce any effect on the porosity when low hatch spacing is used; whereas high hatch spacing tends to increase the influence of scan speed on porosity. Likewise, lack of sufficient overlapping between the two laser scan tracks due to greater hatch spacing causes incomplete consolidation. Thus, the increase in hatch spacing leaves a large impact on the formation of porosity. Heat input can be controlled by controlling parameters like laser power, scan speed, and hatch spacing. Thus, heat input can be adjusted so that porosity should get minimised for example by using low laser intensity and slow scanning speed. By considering the results shown in graphs, it can be seen that in order to eliminate or minimise the porosity of the product, a high laser power input and low scan speed along with small hatch spacing gives better results. This study has also shown the following outcomes: A statistical method known as the genetic algorithm has been used by the author to identify the influence of process parameters on the formation porosity of AlSi10Mg. The result shows the trends of porosity in the SLM fabricated samples. From this result, an optimised value of a critical energy density Fig. 6 Interrelation between parameters [2] The relative density of the Al-12Si cubes fabricated by SLM in the study by X.P. Li at different scan speeds using dried powder and as-received is shown in Fig. 7. Given that all other processing parameters were kept constant while processing, the input energy density of laser is inversely proportional to the scan speed [3]. Similar to other alloys processed by SLM, an increase in energy causes the relative density of the cubes to increase for both the as-received and dried powder. As a result of the higher energy density delivered to the powder bed at a lower scan speed. The cubes Manufactured using dried powder shows a relative density having 1 3% higher than the cubes fabricated with as-received powder. Thus by using dried powder, the relative density can be reached up to 99%. Fig. 7 Relative density at different scan speed for died and as received powder [3] Available Online@ 549

5 VI. FUTURE SCOPE The demands from industries will be continuously rising for quality products. As an emerging process, SLM has lots of chances to grasp this opportunity. Though a lot of studies has been made by different authors which are reviewed here still there is some scope for further development and studies in following fields. 1. Study on the different combination of metal compositions to widen the approach to the process. 2. Environmental effects on process and control over it. 3. Use of multi-laser system to reduce processing time. VII. SUMMARY The Additive Manufacturing (AM) is the solution for current industrial demand. SLM which is one of the processes from AM used for producing objects from metal powder is growing in the current phase. Very soon SLM will replace most of the conventional method used for complex geometry and light weight application. Though it has some drawbacks like porosity, surface roughness which limits the application of LSM, latest research work carried on this problem make it more useful. After studying different works carried by authors we can conclude that 1. Process optimization and control over different process parameters leads to control over porosity and good final product [1], [2]. 2. Density can be achieved up to 99% by drying the powder and preheating the metal pool [3]. 3. This also reduces temperature gradient which avoids warpage without using anchors due to sudden cooling [4], [6]. Acknowledgements I would like to thank Mr A. P. Pandhare for giving me an opportunity to do the project work on Additive Manufacturing by Selective Laser Melting of Aluminium Alloys. It was his support and guidance which made me complete the project on time. I am extremely grateful to him for providing his support and guidance in spite of his busy schedule. References [1] N. Read, W. Wang, K. Essa, M. Attallah, Selective laser melting of AlSi10Mg alloy: Process optimization and mechanical properties development, Materials and Design 65 (2015) ,September 2014 [2] N. Aboulkhaira, N. Everitta, I. Ashcroftb, C. Tuckb, Reducing porosity in AlSi10Mg parts processed by selective laser melting, Additive Manufacturing 1 4 (2014) 77 86,August 2014 [3] X. Lia, K. O Donnellb, T. Sercombe, Selective laser melting of Al-12Si alloy: Enhanced densification via powder drying, Additive Manufacturing 10 (2016) 10 14, February 2016 [4] P. Vora, K. Mumtaz, I. Todd, N. Hopkinson, AlSi12 in-situ alloy formation and residual stress reduction using anchorless selective laser melting, Additive Manufacturing 7 (2015) 12 19, June 2015 [5] N. Harrison, I. Todd, K. Mumtaz, Reduction of micro-cracking in nickel super alloys processed by Selective Laser Melting: A fundamental alloy design approach, Acta Materialia 94 (2015) 59 68, May 2015 [6] K. Froyen, J. Vaerenbergh, P. Mercelis, M. Romboutsb, B. Lauwers, Selective laser melting of iron-based powder, J.P. Journal of Materials Processing Technology 149 (2004) , November 2003 Available Online@ 550