Thermal Stresses In Components Manufactured Using Direct Metal Laser Sintering Process.
Section 1:
Introduction
Direct metal laser sintering process involves the use of computer aided design to achieve the desired geometry of a part after which the geometry is transferred on a printing plate on a multiple axis where the component material is fed in grain sizes each layer at a time in varying thicknesses. The deposited material is heated via a high energy laser where it melts partially and sintering occurs to form a solid component (Fang).
Metal components can be manufactured using direct metal laser sintering process which is one of additive layer manufacturing technologies. However thermal stresses in these components can lead to cracks. However the manufacturing process can be optimized; using parameters including laser power, components size, material properties, building speed and layer thickness.
Laser sintering involves the process of producing parts by lying layer by layer of a working powder which solidifies. While it’s notable that fatigue in metal part production is a common phenomenon, designers utilize annealing methods and heating processes to relief stress in metals. Fatigue samples are always built in double conic and cylindrical shapes after which they are observed under electron micrograph to identify possible locations of initiation of cracks and fatigue points (Bártolo) (Huamin).
Additive manufacturing has been responsible in the production of complex niche parts as those used in medical and aero parts and industrial fields to impart fracture and wear resistance, reduce corrosion and to achieve biocompatibility as with use inside biological tissues. In selective laser melting, titanium alloys find wide range of uses following research in fractography that has proven to give good results (Kang).
While additive manufacturing hold the key to complexity, its success can be attributed to its capabilities to utilize the processing powers of a computer, the graphics capability that ensure precision and versatility. The use of high capability graphics user interface has not only been of great importance in gaming but in design too. Computer aided designs are able to be designed and reproduced in a 3D form with ease enabling complexity to be achievable. The use of computer numerical control machines does give precision to parts enabling high caliber machine control with the use of high frequency sensors for immediate feedback. Computation that is done real-time enables machine to use logic instantly by use of precision motors, lenses and mirrors to place laser in exact positions (Kang) (Xu).
Lasers for heating requirement carry high energy in the ultraviolet range to cut through metal and cause metal fusion and melting in localized positions without heat buildup and thus the part is formed with minimal distortion due to thermal stresses. Regarding the setting of thermal stresses, key is the step by step procedure involved in selecting the metal powder to be used (Publishing(Bingley)). After this metal analyzers record the particle sizes to establish size distribution between D10 and D90 values, usually the particles follow a normal distribution.
Between testing phases most sintered materials will relieve stress through cracks in the fatigue relief regions that may occur on the surface or inside the core. By use of ultrasound inside core defects can be easily detectable (Joshi and Dixit). These are sampled to get the mean defect characteristics as with relevance to shape and locations giving an easy data to analyze. The testing may usually involve compressive forces of 500 MPa to determine the repeatability of defects in the produced parts
The process of metal failure usually starts with the initiation of cracks within the matrix after which the crack propagates to other regions and what follows is ultimate failure. Gromov notes that it is a common phenomenon for some defects to arise due to the effectiveness of the metal powder sintering itself, thermal stresses are seen as a major propagator of defects as thermal stresses usually relieve in points of weakness where the sintering process may not have fully occurred (Gromov and Teipel).Though internal and external pores can be observed to reduce defects in the method use to produce the part. As a result high wattage lasers are applied that lead to fast tooling and lesser defects (Kang).
History of Additive Manufacturing
The history of additive layer manufacturing takes its roots from developments in 1980’s in computer aided design and laser technology to come up with a product that is complete having been produced a layer at a time. Additive manufacturing entails the creation of 3d objects by adding layer to top of another layer stepwise by use of computer digital designs.
In 1950s and 1960 show the revolutionize laser of computing technology and the rise of computers, lasers and logic controllers sparking research in the applicability of technology in fabrication processes. In the year 1984 there was the rise of patents in United States of America and japan elaborating the concepts of 3D systems in additive manufacturing and stereo lithography technologies. Further developments in the industry led to laminated object manufacturing (LOM), cubital and others like the selective sintering process by the DTM Company (Gu, Laer Additive Manufacturing of High Performance Materials).
In the contrary, additive manufacturing has not been without hitches, the LOM machine by Heilys failed with the solido process laminates polymer sheets failing in their manufacturability. It’s notable that 5-axis mechanisms have also failed most times in the past.
The process commonly known as stereo lithography or photo polymerization has been recently of great concern in fine work piece manufacturing owing to its versatility and able to optimize designs. Laser sintering started as a delicate process where thin sheets of thermoplastic were formed from their powders by use of carbon dioxide lasers to heat to a point below melting point that which cad forms the geometry. The technology having started with use of materials that were easily workable, today the technology applies the use of metal powders to undertake laser sintering which results in durable parts, the new additive layer manufacturing was then born, the year 2000. As a result highly dense parts were now reproducible with this technology enabling the workability with ferrous and nonferrous materials and enabling engineers to work out alloys through wire extrusion and blown powders. Today, the use of laser sintering has enabled the forming of high temperature thermoplastics due to high forming temperatures achievable (Kang) (Laboratory).
With laser technologies, processes like selective laser sintering have been produced enabling designers to come up with complex products through sintering localized portions of powder in cross sections to achieve solid parts. The sheet lamination process to is another method of producing complex shapes by use of cheap technology, invented by Michael Feygin, he proved that by placing sheets of plastic or metal on top of another sandwiched in a bond was a cheap and easy way of producing parts (Gu, Laer Additive Manufacturing of High Performance Materials).
Background/History of DMLS
Direct metal laser sintering has been developed to selectively melt powder in a localized location thus enabling parts to be produced by use of a high power laser beam that easily fuses the metal powders together. DMLS patented in 1995 by Fraunhofer institute in Germany saw the method tested successfully with results matching those of electron beam melting. The process starts with digitization of thin slices of the 3D object to be produced of thicknesses between 20 to 100 micrometers thick, an industrial standard. What follows is the deposition of these fine slices on the printing plate after which high energy laser is focused on an x and y axis parameters by use of precise high frequency scanning mirrors. Once each layer is focused by the high intense laser, usually with hundreds of watts of energy, the metal powder melts partially causing welding among the particles. The process is repeated resulting in a 3d part. Materials that can be produced include titanium, cobalt chrome, tool steel, stainless steel and aluminum. Since this process leads to rapid prototyping, it holds great potential in revolutionizing the manufacturing world (Fang) (Boljanovic).
In DMLS applications carbon dioxide lasers have been used quite for many years. These lasers have been proved to seamlessly generate massive energy required for sintering processes. Today YAG lasers and other types that offer better beam quality, most common being the laser fibers and disc lasers have found great application.
While DMLS technologies produce high performance tool steel, it’s able to fine tune density details while achieving the required mechanical properties (Huamin). These technologies have played a key role in improving speed in production of components since components like dies and molds are eliminated. This adds to cost reduction too since the design changes don’t require coming up with molds that take time to fabricate hence giving the designer a freedom of choice. It thus enables on demand production of components thus relieving the consumer a time taking process to wait for fabrication and also relieves the producer the cost of producing items that may not be sold.
Types of Lasers and Metal Powders (Ti-6Al-4v Must Be Mentioned and Yb Laser)
While today DMLS technologies utilize a wide variety of lasers, the EOSINT M 270 laser commonly using the 200 watts ytterbium fiber laser has been commonly used. With alternatives like Trumaform LF 250 that use disc laser, they find it hard to compete with EOSINT 270 with beam qualities of magnitude 1.0 m^2 quality. Since this lasers have beam diameters as short as 100micrometers they are able to be focused on smaller build areas giving precision as opposed to carbon dioxide lasers. As a result this is attributed to shorter wavelengths thus enabling more power to be transferred to working area within short time intervals hence their 200 watt power usually corresponds to reliable power intensities in the magnitude of 25kW/mm^2. The shorter wavelength results in higher absorption rates in metal. It therefore enables effectiveness in sintering process and hastening buildup speeds (Gromov and Teipel) (Huamin).
Yttrium aluminum garnet lasers commonly known as YAG lasers are made from doped neodymium and erbium. This laser developed in the 1960’s, is a solid state has been used in metal sintering processes due to its ability to deliver high energy quantities in the magnitude of kilowatts (Huamin).
Initial lasers used 2D technologies with micro mirror devices being able to focus laser on a whole plane of the working plate. This process was known to fuse laminates, liquid polymer, molten material and mostly discrete particles of various building materials. It’s notable that most authors have argued that the future holds the possibilities to fabricate complete objects with this technology applying a single pass of laser by combining vector and raster-base methods of scanning the prototype (Joshi and Dixit).
The use of discrete metal particles usually metal powders is a common method in direct metal laser sintering where the constituents may be graded according to specifications or in a normal distribution. For polymer fabrication to be successful they must process thermoplastic properties to enable melting to solidifying and the same process repeated again to enable the sheets to be placed on top of the new surface (Huamin) (Laboratory).
The application of 3D printer technology in metal sintering was first developed by MIT le at the point. Advancements to molten material systems called the fused deposition modeling. This later led to the proliferation of direct metal systems. The presence of machines like the EOSINT-M20 have enabled laser sintering to use laser engineered lenses for precision, however alternatives by use of sheet laminates that can be sintered by laser has been an alternative in some component specific requirements optimized products. A company from Sweden however uses electron beam melting processes (EBM), a relatively similar operation to DMLS.
Other existing methods include the LENS powder delivery methods where the lens is located in a tipped arm and the power is delivered within the lens to cause the sintering process. The powder happens to melt causing sintering at the point where the laser jet hits the grains of the metal enabling the repair of high end metal products such as blades of gas turbines (Gromov and Teipel).
With recent technologies aiming at precision and optimization of desired properties, manufacturers have come up with hybrid systems able to perform better. The common being the planar milling where each surface is milled before a new layer is applied for sintering. This is a process that is involved in the sanders and objects machines thus avoiding accumulation of errors due to errors that occur as a result of different heights of the particles deposited on each new surface, it thus achieves a smooth planar surface on every new deposition. This method has however been noted as not optimal despite the quality of the components produced due to wastage of material that may be hard to recycle with the same method (Lefteri). However engineers have come up with methods that can merge subtractive and additive element manufacturing to come with optimized systems as is the case with Stratoconception approach. In this method the computer aided models are separated to thick layers that can be easily machined. Later after these parts are machined, they can be joined together to form a complete part which could not be machined in the normal multi axis centers due to tool accessibility issues. Since the strength of bonding between the parts in this method is of great consideration, highly effective methods such as diffusion bonding are mostly used. Similarly lower cost solutions such as the subtractive RP methods can be used where Roland desktop milling machine produce slices configured to the required shape of each slice (Xu).
The shape deposition method (SDM) is another method that has been used to undertake additive manufacturing. Although it has not been commercialized to date, it’s an easier version of its precursors by using easily manufacturable parts which are later combined together (Gu, Laer Additive Manufacturing of High Performance Materials). This has enabled the parts to be made from different materials and later combined together and as such materials like ceramics, plastics, metals and other materials may be modeled into one component. It’s notable however that the commercialization of this method is hindered by the fact that the material sheets are not necessarily planar hence making it hard to mass produce components of varying designs. The use of titanium alloys especially the Ti 6Al-4v has offered lightweight and a high strength material that is highly formable. It’s less corrosive and its lightweight properties have seen its application in aircraft industry in building turbine blades and other parts. With sintering processes the use of this titanium compound has enabled precise repairs in parts and offered efficiency. In today’s industry the manufacture of turbine blades that used to take 44 weeks to produce a blade takes 8 weeks to produce a better blade with direct metal laser sintering process (Gromov and Teipel).
Literature Review
With powder generation methods varying across different laser type applications, precision is a great concern and as much as the production costs are involved. Despite powder spraying remaining relatively unpopular among manufacturers, EOS has patented the recoater blade system method that has proved to be effective and less complex. In this method a metal powder in excess is applied at the forming table with the rotating blade leveling across the table for a flatter sheet as the extrusion die sinks giving each layer chance to be sintered. With EOSINT M 270 lasers thin layers as thin as 20 micrometers are produced with little to no deviations with EOS’ 20 micrometer technologies. As a result some technologies with bigger errors have seemed unfit for high precision jobs as they add machining costs to fine tune dimensions to near precisions. Post machining is known to add more relative errors and making the producing hardware more complicated to the disadvantage of the manufacturer (Fang) (Kang).
The use of direct metal laser sintering process is critical with relevance to the metal powders used, since the optimality of the job produced is the main goal of a manufacturer, it’s critical for most parameters to be adapted for the specific metal element or alloy (Gu, Laser additive manufacturing of high-performance materials).
When in practical applications, tooling by DLMS process is able to achieve high speeds since the inner core of tooling is of less importance relative to mechanical properties of the outer skin. This technology enables the product core to be produced 8 times faster than the outer core where finish properties such as smoothness, hardness, porosity and general skin parameters are of great importance. The use of EOSINT M laser technologies has enabled the working of gold, titanium alloys, stainless steels, copper and alloys, aluminum, metal composites and silver which can also be easily produced. With EOSINT M 250 Xtended tooling higher efficiency is achievable with results showing considerable results both in porosity and tensile strength, tolerances of as low as 20micrometers, porosity of less than 5%, tensile strengths of greater than 1,100 MPa are achievable. It is also well noting that this technology can achieve hardness of up to 42 Rockwell C (Joshi and Dixit).
The DMLS technology has been widely used in production of inserts used in injection molds for their capabilities to resist high pressures and resisting wearing so as to maintain tight tolerances. However these attributes become more of a reality with each development, the sintered parts often fail to achieve polish finishes of greater quality as opposed to other parts produced by alternative methods. The remaining porosity after polishing has however been eradicated in various materials such as the Direct steel H20 which has shown the least surface defects with this method (Huamin).
In the thermal testing processes heat treated alloy or normal element pieces are immersed in 1900degrees fahrenheit for varying periods usually one hour or 1550 degrees Fahrenheit for four hours to achieve various mechanical properties. It however common for direct metal laser sintered components not to be annealed as some of their properties near those of annealed metals a process called LH1150 or LH900 (Lefteri).
The process will usually follow examination under optical microscope after the sample is etched or finely polished. The etching and polishing process is aimed at revealing the microstructure within the metal matrix. The polishing being carried by use of diamond paste on polishing pads is able to achieve polishing of 0.25 micrometers range for finer details, however Villella’s etching reagent consisting of hydrochloric acid, methanol and picric acid is usually used in exposing microstructure in steel metals. The prepared specimens then are coated with gold palladium, this serves to increase conductivity and so aiding in visibility under electron microscope. It’s however notable that backscatter electron imaging is an alternative to electron micrograph (Gromov and Teipel) (Lefteri).
Lattice distortions are then determined by use of x-ray diffraction to reveal the crystalline structure and packing of atoms within the metal structure. This is usually used to note alloy properties when heating occurs during sintering at different time lapses. By use of JADE software phase quantification method the volumetric percentages of austenite phase are determined and an x-ray diffraction curve plotted.
Results from findings establish that between the grain matrixes, the combination of shorter and longer grains is responsible for affecting the response of a part to thermal stresses. The layering effect is found to occur at its best with a mix-up of varied metal grain sizes resulting in a part that usually has higher resistance to thermal stresses. Its notable that during rapid cooling, elongated metal grains will usually disintegrate to shorter grains, this effect is common owing to rapid cooling that occurs with direct metal laser sintering process, however a higher percentage of elongated grains will usually occur in the direction of heat flow growing as they exit the melt pool at the localized point (Gu, Laer Additive Manufacturing of High Performance Materials) (Publishing(Bingley)). It is this attribute that there is less considerable grain growth in a sintering process, this is due to lower temperatures resulting in better mechanical properties of the part.
Cross sections of parts produced by this method usually reveal gas pockets and voids that set as the metal cools instantly relieving through contraction processes. Moreover there is the entrapment of nitrogen gas during the gas atomization of the powder and since the process of cooling is usually rapid, the part solidifies in a defect. While most manufacturers advocate for a post heat treatment air cool of the parts to room temperatures, there has not been highly effective methods to avoid gassing that occurs rapidly during the heating process (Kang). With most ferrite metals, an annealing process is considered as a last step towards achieving output samples that are similar to those produced with a cast process. While this process is known to increase the strength of the part, it however imparts negatively on a part that was designed for high ductility property due to retained austenite. Producers have replaced nitrogen environments to produce parts under DMLS in inert gases like argon to reduce gas atomization and absorption and arguably reducing austenite retention (Bártolo).
References
Applications, National Research council(US).Committe on a scientific assesment of free-electron laser technology for naval. Scientific Assesment of High Power free-electron laser technology. Washingtom DC: National Academies Press, 2009.
Bártolo, Paulo. Virtual and rapid manufacturing : advanced research in virtual and rapid prototyping. New York: Taylor & Francis, 2008.
Boljanovic, Vukota. Metal Shaping Processes: casting and molding, particulate processing, deformation processes, and metal removal. New York: I ndustrial press, 2010.
Chen, Tiebing. "Analysis and Modelling of Direct Selective Laser Sintering of Two-component Metal Powders." Direct Metal Manufacturing (2014): 76.
Fang, Zhigang Zak. Sintering of advanced materials : fundamentals and processes. Oxford ; Philadelphia, PA: Woodhead Publications, 2010.
Gromov, Alexander and Ulrich Teipel. Metal nanopowders : production, characterization, and energetic applications. Weinheim, Germany: Wiley-VCH Verlag, 2014.
Gu, Dongdong. Laer Additive Manufacturing of High Performance Materials. Berlin: Springer publishers, 2015.
—. Laser additive manufacturing of high-performance materials. Berlin: Springer publishers, 2015.
Huamin, Cao. Laser Technology. Boca Raton, Fla: CRC Press/ Taylor & Francis, 2010.
Joshi, Shrikrishna N and Uday S Dixit. Lasers based manufacturing : 5th International and 26th All India Manufacturing Technology, Design and Research Conference, AIMTDR 2014. New Delhi: Springer India, 2015.
Kang, S -J L. Sintering : densification, grain growth, and microstructure. Amsterdam ; Boston ; London : Elsevier Butterworth-Heinemann, 2005.
laboratories, sandia national, et al. Laser Engineered Net Shaping; a tool for direct fabrication of metal parts. washington DC: National Government Publication, 1998.
Laboratory, Knolls Power Atomic. Direct Laser Powder Deposition-'State of the Art'. Washington DC: US govt publishers, 1999.
Lefteri, Chris. Making it : manufacturing techniques for product design. London: Laurence King Pub, 2012.
Publishing(Bingley), Emerald Group. A focus on SLS and printingSLM methods in 3D. Wagon Lane: Emerald Publisher, 2015.
Xu, Dayun. Engineering solutions for intensification of production : selected, peer reviewed papers from the 2014 2nd International Conference on Manufacturing Engineering and Technology for Manufacturing Growth (METMG 2014), January 20-21, 2014, Miami, State of Fl. Zurich, Switzerland: Trans Trap Publications, 2014.
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