Direct Metal Laser Sintering Technology Research Articles

Optimizing surface roughness of workpieces made of stainless steel 316L with DMLS technology: a Taguchi method approach

This study employs Direct Metal Laser Sintering (DMLS) technology to investigate surface roughness in stainless steel 316L 3D printing processes. Utilizing the Taguchi method in experimental design, we examine the influence of independent variables—laser power, scan speed, and hatch spacing - on surface roughness quality. Results indicate that laser power has the greatest impact, followed by scan speed and hatch spacing. Notably, both laser power and hatch spacing positively affect surface roughness, while scan speed adversely affects the top surface quality of printed components. This research enhances comprehension of the intricate relationship between process parameters and surface quality in DMLS-based 3D printing, offering insights for optimizing surface roughness in stainless steel 316L applications. The study holds practical significance for enhancing the quality and performance of 3D-printed components across diverse engineering and manufacturing sectors.

Tensile properties of 3D printed INCONEL 718 cellular specimens

The aim of the presented research by the authors was to compare the behaviour of four types of cellular structures under quasi-static tensile stress, while two samples were formed by mono-structures Gyroid 10 % and Diamond 10 %, and the other two types were bi-structures, which were created by combining two single structures (Gyroid 5 % + Gyroid 5 %) and (Gyroid 5 % + Diamond 5 %). The samples were made of Inconel 718 by Direct Metal Laser Sintering technology on an EOS EOSINT M270 machine, and they were heat treated according to AMS 5664 procedure. Tensile tests were performed on an Instron 8802 servo-hydraulic testing machine with a maximum capacity of 250 kN at ambient temperature. The results showed that the maximum load corresponded to the diamond (D) cellular structure (approximately 48 kN), while the minimum load was observed for the gyroid-gyroid (GG) structure (approximately 32 kN).

Results of cranioplasty using individual titanium implants

Background. Despite its long history, cranioplasty remains a topical problem of neurosurgery, due to the increasing number of traumatic brain injury, the consequences of which are the main driver in preserving the population of patients with postcranioectomy skull defects. As a rule, these defects are extensive, and the use of individual implants in these cases is the method of choice. Over the past 5 years, the use of products created by medical additive manufacturing has increased, in this regard, the assessment of long-term results of surgical interventions with their use is an actual issue of modern medicine.Aim. To evaluate the results of surgical treatment of patients with skull defects of various etiologies using individual titanium implants made by three-dimensional printing.Materials and methods. The study analyzed 94 cases of cranioplasty using individual titanium implants made by three-dimensional printing using DMLS (Direct Metal Laser Sintering) technology. The minimum follow-up period was 12 months from the moment of the intervention. Traumatic brain injury and its consequences was the dominant cause of skull bone defects (n = 56, 59.6 %). The average area of defects in the study group was 99.2 ± 43.4 cm2. For men (n = 53), this parameter corresponded to 106.7 ± 44.7 cm2, for women (n = 41) – 89.5 ± 40.1 cm2.Results. The total number of complications in the study group was 12 (12.7 %) cases, of which 5 (5.3 %) cases were recorded during the in hospital stay of patients, 7 (7.4 %) – during outpatient follow-up. One complication was not related to the performed surgical intervention. Removal of the implant was required in 7 (7.4 %) cases. The terms of implant removal varied from 0 to 14 months from the moment of the performed intervention. The survival rate of individual titanium implants for more than 12 months was 92.6 %.Conclusion. The data on the presence of complications after cranioplasty vary from study to study, while the results of reconstructive interventions performed using individual titanium implants are of significant interest, the use of which has increased significantly over the past five years due to the introduction of additive medical production into clinical practice.

A Review of Conventional versus Additive Manufacturing for Metals: Life-Cycle Environmental and Economic Analysis

Additive manufacturing (AM) is a recent emerging technology that is being adopted in various industry sectors and supply chains. Life cycle assessment (LCA) and life cycle costing (LCC) are powerful methods that can be used for assessing the environmental and economic performance of emerging manufacturing technologies. This study aims to evaluate the life cycle environmental impacts and cost of computerized numerical control-based (CNC) manufacturing and direct metal laser sintering technology (DMLS) through a cradle-to-gate life cycle analysis. This research has four main novel elements: (i) conducting a recent comprehensive review of metal AM and conventional manufacturing (CM) processes using a systematic method and meta-analysis (ii) comparing the conventional process “CNC machining” and the additive technology “direct metal laser sintering” from the environmental (LCA) and economic (LCC) perspectives, (iii) investigating the influence of geometry complexity and shape size factors on the environmental and cost performance of both manufacturing methods, and (iv) conducting a Monte Carlo simulation-based sensitivity analysis to tackle uncertainty in LCC input parameters. The midpoints and endpoints impact for CNC and AM processes were estimated using the Ecoinvent v3.8 database and ReCiPe (E) impact assessment method. The review revealed that global warming potential is one of the most widely studied environmental indicators; however, only 6% of the studies have investigated the life cycle economic impacts of AM technologies using sensitivity and uncertainty analysis. The results have shown that in terms of ReCiPe endpoints, DMLS has the highest environmental impact on human health while CM has more impact on the eco-system quality. Electricity consumption is the main contributor to environmental impact categories in both manufacturing technologies. This is due to the high electricity utilization for casting and milling conventionally manufactured parts and printing the AM parts. LCC net present values revealed that manufacturing all parts with AM costs 91% more compared to CNC. The LCC analysis has shown that AM is more suitable and cost-effective for parts with highly complex geometries. Whereas CNC machining was found to be economically feasible for large-sized and low-complexity parts. The Monte Carlo sensitivity analysis demonstrated that for the case of AM, the most significant parameter is the processing cost followed by material cost, which highlighted the importance of energy-efficient AM and dematerialization through design for circularity.

Microstructure and mechanical characterization of direct metal laser sintered Ti-6Al-4V alloy for orthopedic implant applications

Additive manufacturing (AM) technologies like direct metal Laser sintering (DMLS) or powder bed fusion technologies enable the fabrication of complex-shaped biomedical implants. Since process parameters influence the microstructure and thus, mechanical properties of a built structure, optimizing the parameters to produce implants that can withstand the imposed biomechanical forces is necessary. In this study, biocompatible Ti-6Al-4 V samples were fabricated using nine different combinations of process parameters for DMLS technology. The optimal printing parameters were determined using mechanical and microstructural analysis of printed samples. The mechanical and microstructure analysis revealed that parts built with volumetric energy density input of 37–93 J/mm3 are nearly defect-free and suitable for biomedical implant applications.

Effect of Hardening Temperature on Maraging Steel Samples Prepared by Direct Metal Laser Sintering Process

This paper deals with the application of the direct metal laser sintering (DMLS) process, which already has a dominant position in the area of additive manufacturing (AM). This DMLS technology is used in many branches of industry and medicine, especially in piece production, small series, and prototypes. The portfolio of used metal powder materials includes aluminum alloys, austenitic steels, maraging steels, special alloys of nickel and titanium. The properties of these products are very often improved by further heat treatment after printing, such as a hardening process, by which microstructure and hardness can be increased. Heat treatment processes of metal AM components are already described, but experiments focused on optimization of these processes are still missing. In the article, the maraging steel samples printed by the DMLS method are subjected to testing after hardening processes, which differ by reducing the maintaining time at a defined temperature, recommended by the manufacturer. The result of the evaluation will be the reaching of similar results, which are set by the powder manufacturer, however, with shorter time of samples treatment. Therefore, the elevated temperature is selected, with the purpose of monitoring the shortest possible time of a temperature impact. The experimental temperature was set 590 °C with different durations at this temperature, for 1, 2, 3, 4, 5 and 6 h. The cooling process runs controlled in the furnace or in the still air. The maintaining time proved to be the most ideal already at 1 h exposure and cooled in the still air, where a higher hardness value of around 50 HRC was reached. During the resulting microstructure evaluations, fine carbids and martensitic lamellae were observed. More uniform and finer lamellar microstructure occurred at 5 and 6 h temperature intervals.

Investigation of the Performance of Ti6Al4V Lattice Structures Designed for Biomedical Implants Using the Finite Element Method

The development of medical implants is an ongoing process pursued by many studies in the biomedical field. The focus is on enhancing the structure of the implants to improve their biomechanical properties, thus reducing the imperfections for the patient and increasing the lifespan of the prosthesis. The purpose of this study was to investigate the effects of different lattice structures under laboratory conditions and in a numerical manner to choose the best unit cell design, able to generate a structure as close to that of human bone as possible. Four types of unit cell were designed using the ANSYS software and investigated through comparison between the results of laboratory compression tests and those of the finite element simulation. Three samples of each unit cell type were 3D printed, using direct metal laser sintering technology, and tested according to the ISO standards. Ti6Al4V was selected as the material for the samples. Stress–strain characteristics were determined, and the effective Young’s modulus was calculated. Detailed comparative analysis was conducted between the laboratory and the numerical results. The average Young’s modulus values were 11 GPa, 9 GPa, and 8 GPa for the Octahedral lattice type, both the 3D lattice infill type and the double-pyramid lattice and face diagonals type, and the double-pyramid lattice with cross type, respectively. The deviation between the lab results and the simulated ones was up to 10%. Our results show how each type of unit cell structure is suitable for each specific type of human bone.

Efficiency Optimization of the Electroerosive Process in µ-WEDM of Steel MS1 Sintered Using DMLS Technology.

Although the application of mathematical optimization methods for controlling machining processes has been the subject of much research, the situation is different for µ-WEDM. This fact has prompted us to fill the gap in this field in conjunction with investigating µ-WEDM’s very low productivity and overall process efficiency, since the current trend is oriented towards achieving high quality of the machined area at a high manufacturing productivity. This paper discusses in detail the application of non-linear programming (NLP) methods using MATLAB to maximize the process performance of µ-WEDM maraging steel MS1 sintered using direct metal laser sintering (DMLS) technology. The novelty of the solution lies mainly in the selection of efficient approaches to determine the optimization maximum on the basis of a solution strategy based on multi-factor analysis. The main contribution of this paper is the obtained mathematical-statistical computational (MSC) model for predicting high productivity and quality of the machined area with respect to the the optimal efficiency of the electrical discharge process in the µ-WEDM of maraging steel MS1 material. During the experimental research and subsequent statistical processing of the measured data, a local maximum of 0.159 mm3·min−1 for the MRR parameter and a local minimum of 1.051 µm for the Rz parameter were identified simultaneously during µ-WEDM maraging steel MS1, which was in the range of the predicted optimal settings of the main technological parameters (MTP).

An Overview Of Direct Metal Laser Sintering (DMLS) Technology For Metal 3D Printing

<p>Additive manufacturing is the process of building a component or a product layer-by-layer, as opposed to casting the component and then performing various subtractive machining processes like turning, drilling, milling which are the approach of subtractive manufacturing. The term 3D printing refers to the family of additive manufacturing processes, which utilize different mechanisms in order to build the product from a sliced computer aided design (CAD) model fed to the machine. direct metal laser sintering (DMLS) is the one method of 3D printing functional metal parts are suitable for engineering applications and has the potential to provide a viable alternative to conventional methods of manufacturing and produce superior quality components with great flexibility in design using a wide range of materials. This paper presents the overview of DMLS technology, process parameters, design considerations, case studies of parts manufactured by DMLS and its applications in metal casting and rapid tooling.</p>

Direct Metal Laser Sintering Conformal Waveguide Array Antenna for Millimeter-Wave 5G Communications

An omnidirectional conformal array antenna has been implemented in air-filled waveguide technology for the 37–40 GHz band. The input port of the antenna is a standard WR28 rectangular waveguide. The TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> mode excited in the rectangular waveguide is used to generate the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode inside a circular waveguide by means of a TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> –TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode converter. The circular waveguide section is progressively widened, forming a radial waveguide structure, where the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode propagates radially through several radiating apertures. Due to the rotational symmetry of the structure, an omnidirectional radiating performance with low ripple can be achieved. This structure has been three-dimentional (3-D) manufactured using the direct metal laser sintering technology. A good agreement between simulations and measurements has been found. A reflection coefficient of below −15 dB, a low-ripple omnidirectional performance in a conical cut plane, a low cross-polar radiation, and a total efficiency higher than 83% are obtained. This conformal antenna design is proposed for fifth generation (5G) New Radio indoor applications.

Additively manufactured porous titanium 3D–scaffolds with antibacterial Zn-, Ag- calcium phosphate biocoatings

The porous 3D scaffolds with three types of pore structure A, B and C were created by the direct metal laser sintering (DMLS) technology from the powders of titanium Ti-6Al-4V alloy. Calcium phosphate (CPC) and Zn‑calcium phosphate (Zn-CPC) coatings were formed on the 3D scaffolds by the micro-arc oxidation (MAO) method in the acidic electrolyte. As well as another calcium phosphate (CPL) and Ag‑calcium phosphate (Ag-CPL) coatings, that were synthesized in the alkaline electrolyte. Preliminary studies of the surface morphology, elemental and phase composition, and strength properties of the coatings were carried out on the plate samples from titanium Ti-6Al-4V alloy using scanning electron microscopy (SEM), X-ray diffraction, and scratch testing. The surface morphology of the coatings on the 3D scaffolds was also investigated via SEM method. The internal pore structure of both the coated and uncoated samples was examined by the Micro-CT analysis. It was found that the coatings were evenly distributed over the surface and in the inner pore structure of the all types of the 3D scaffolds. The biological studies carried out on both the plate samples and the 3D samples have shown that the coatings demonstrated no cytotoxic effect on cell culture of human fibroblast pFb. All the coatings, excluding CPL, exhibited high antibacterial activity against bacteria MRSA ATCC 43300 and Escherichia coli ATCC 25922. • The porous 3D scaffolds with various types of pore structure were created by DMLS technology from the Ti-6Al-4V alloy. • Ag- and Zn‑calcium phosphate coatings were evenly distributed in the inner pore structure of the 3D scaffolds. • The coatings demonstrated no cytotoxic effect on cell culture of human fibroblast and high antibacterial activity.

Assessing the influence of DMLS production process factors on fatigue resistance of Maraging steel MS1 in the finite life domain using ANN prediction abilities

Analysis-of-variance (ANOVA) is a standard statistic method for assessment of the influence of various factors on fatigue resistance in the finite life domain. However, the previous research has shown that this method was not capable to determine with sufficient confidence if the build orientation, the thickness of allowance for machining, and the position in the production chamber affect fatigue resistance of Maraging steel MS1 products made by direct metal laser sintering (DMLS) technology. To contribute to a better understanding of the subject, the results of fatigue test experiments were used for training of four types of artificial neural networks (ANN) for assessment of fatigue resistance in the finite life domain. Each ANN had different structure of inputs, which corresponded to a different combination of the factors of DMLS production process. The differences between the predictive abilities of the ANN were attributed to influences of the respective factors on the fatigue resistance of the material in the finite life domain. The approach was verified by the agreement with the conclusive results of ANOVA analyses. Furthermore, in the cases when ANOVA does not lead to a clear result, the analyses of the predictive ability of the ANN strongly suggest that build orientation and thickness of allowance do not influence, while the position of a part in production chamber influences, the fatigue resistance in the finite life domain of Maraging steel MS1 produced by DMLS technology.

Design and development of a novel additively manufactured geothermal heat exchanger

The existing geothermal heat exchangers pose a complex installation process and occupy large spaces which can potentially be cost-prohibitive to many potential users. In the present study, an additively manufactured heat exchanger was designed and developed to address this issue. The designed heat exchanger can be integrated with conventional geothermal heating and cooling systems for improved efficiency and easy installation. The additively manufactured heat exchanger developed in the present study eliminates the current need to excavate the soil and install the long piping as conventionally required for the existing geothermal systems. The prototype of the proposed geothermal heat exchanger was designed using CREO software and fabricated using the direct metal laser sintering technique (DMLS) to be integrated into a heat pump to exchange heat between a constant temperature of a water bath circulator and a water heat sink. Furthermore, the support structure model generated for metal printing of the proposed geothermal heat exchanger was exposed and discussed. Two customized geothermal heating and cooling modes were separately developed to evaluate the heat transfer performance of the heat exchanger analytically and numerically. Using the analytical approach, the system sizing was designated based on the metal additive manufacturing and thermal considerations, and then average heat transfer coefficients and pressure drops were estimated for a range of mass fluxes lying in the transitional and turbulent regimes. The distributions of wall temperature, fluid's bulk temperature, and fluid's velocity along the helically-coiled channel of the heat exchanger was also investigated numerically using the COMSOL-Multiphysics. Lastly, a detailed cost analysis was conducted to estimate the total cost per part for the prototype fabricated using the DMLS technology. Novelty Statement A novel ground-source heat exchanger was additively fabricated using the direct metal laser sintering (DMLS) technique. The unique compact screw-type design eliminates the need of excavating soil and features easy installation with less maintenance costs. Heat transfer characteristics of the proposed design were evaluated analytically and numerically. The proposed geothermal heat exchanger contributes effectively to the deployment of sustainable energy systems.

A multi-objective optimization approach for exploring the cost and makespan trade-off in additive manufacturing

Additive manufacturing (AM) suggests promising manufacturing technologies, which complement traditional manufacturing in multiple areas, such as biomedical, aerospace, defense, and automotive industries. This paper addresses the production planning problem in multi-machine AM systems. We consider all relevant physical and technological parameters of the machines and the produced parts, for using direct metal laser sintering (DMLS) technology. In DMLS technology, each machine produces jobs, where each job consists of several parts arranged horizontally on the build tray. Starting a new job requires a setup operation. We address the simultaneous assignment of parts to jobs and jobs to the machines, while considering the cost and makespan objectives. A unified mixed-integer linear-programming (MILP) formulation that can minimize the above objectives separately and simultaneously is suggested, along with analytical bounds and valid inequalities. Experimentation demonstrates the effectiveness of the proposed formulation with single objectives versus similar formulations from the literature. An efficient frontier approach is applied to the multi-objective problem while generating a diverse set of exact non-dominated solutions. The trade-off between the objectives is analyzed via experimentation. Results show that when identical machines are used, the trade-off is relatively small, and hence the decision-maker can use any of the single objectives. However, when non-identical machines are used, it is important to consider both objectives simultaneously. Moreover, the trade-off increases with the number of machines and heterogeneity of the system, with respect to the size and settings of the machines.

Study of the influence of laser energy density on the surface roughness of Scalmalloy® samples manufactured by DMLS technology

Direct Metal Laser Sintering (DMLS) is an additive metal manufacturing technology that allows complex parts to be built without the design constraints of conventional technologies. In addition, the possibility of reusing powder reduces raw material consumption and the amount of waste generated. However, one of the disadvantages of this technology is that, in most cases, it requires post-processing tasks that increase manufacturing costs and delivery times. Some of these additional tasks are aimed at reducing the surface roughness of the parts obtained, using technologies such as grinding, mechanical polishing, shot blasting or electropolishing. This work proposes to improve the surface quality of parts manufactured by DMLS by controlling the processing parameters. In this way, the costs associated with these post-processing tasks could be reduced. Specifically, it is proposed to study the effect of the laser energy density applied to the surface of the parts during the manufacturing process on their surface quality. To carry out this study, powders of the Al-Mg-Sc alloy known as Scalmalloy®, developed to manufacture parts by means of DMLS with application in the aeronautical sector, have been used. In this sector, surface quality is one of the factors limiting the implementation of DMLS technology for the manufacture of parts considered critical.

A Novel Guided Zygomatic and Pterygoid Implant Surgery System: A Human Cadaver Study on Accuracy

The aim of this human cadaver study was to assess the accuracy of zygomatic/pterygoid implant placement using custom-made bone-supported laser sintered titanium templates. For this purpose, pre-surgical planning was done on computed tomography scans of each cadaver. Surgical guides were printed using direct metal laser sintering technology. Four zygomatic and two pterygoid implants were inserted in each case using the guided protocol and related tools. Post-operative computed tomography (CT) scans were obtained to evaluate deviations between the planned and inserted implants. Accuracy was measured by overlaying the real position in the post-operative CT on the virtual presurgical placement of the implant in a CT image. Descriptive and bivariate analyses of the data were performed. As a result, a total of 40 zygomatic and 20 pterygoid implants were inserted in 10 cadavers. The mean deviations between the planned and the placed zygomatic and pterygoid implants were respectively (mean ± SD): 1.69° ± 1.12° and 4.15° ± 3.53° for angular deviation. Linear distance deviations: 0.93 mm ± 1.23 mm and 1.35 mm ± 1.45 mm at platform depth, 1.35 mm ± 0.78 mm and 1.81 mm ± 1.47 mm at apical plane, 1.07 mm ± 1.47 mm and 1.22 mm ± 1.44 mm for apical depth. In conclusion, the surgical guide system showed accuracy for all the variables studied and allowed acceptable and accurate implant placement regardless of the case complexity.

Comparison of Effect of Orientation on SS304 and SS316 Material Samples Fabricated with DMLS Technology

SS316 and SS304 are most widely used materials in mechanical engineering. Many times the products are subjected to tensile load during work. The knowledge of tensile properties is essential for the mechanical engineer working with materials. SS304 and SS316 samples were fabricated using Direct Metal Laser Sintering (DMLS) as per JIS standards in different orientations. The samples were then tested for their tensile strength. The same has been simulated on the NX platform for prediction of variations in the tensile strength with change in orientation. The results obtained from the simulation are observed to be in good agreement with the experiment results when the process parameters are same in both the cases. Further, the simulation has been used to predict the tensile strength of the samples that were not experimented.

Experimental Optimization for Fatigue Life Maximization of Additively Manufactured Ti-6Al-4V Alloy Employing Ultrasonic Impact Treatment

Ultrasonic Impact Treatment (UIT) is an effective surface modification method used to increase fatigue life of welding application by inducing compressive residual stresses. In this paper, we extend the application of UIT to include additively manufactured metallic parts with particular focus on Ti-6Al-4V manufactured with Direct Metal Laser Sintering technology. Design optimization for the application parameters of the UIT was conducted with the objective of fatigue life maximization. Application parameters of the UIT included the static pre-stressing force of UIT impact needle against the treatment surface and its scanning speed during application while the scanning pattern is maintained constant. Results show that a static load of 30N and a scanning speed of 2000mm/min can provide an optimal fatigue life increase up to 250% compared to untreated parts. It is shown that the increase in fatigue performance correlates strongly with an increase in the micro-hardness and compressive residual stress and less strongly with surface roughness. Two Stress-Life (S-N) curves are fully characterized experimentally based on a set of samples treated with the determined optimal UIT parameters and another set of untreated versions. Endurance limit of the optimally treated specimens is increased by 25% from about 200-250MPa.

Dimensional Characterizations Using Scanning Electron Microscope and Surface Improvement With Electrochemical Polishing of Additively Manufactured Microchannels

Abstract Microchannel manufacturing is one of the fastest growing areas in advanced manufacturing with numerous applications, including turbine blade cooling structures, compact microchannel heat exchangers, and electronic cooling devices. Recent development of metallic additive manufacturing (AM) based on direct metal laser sintering technology is capable of fabricating microscale structures with high complexity and design flexibility. However, powder bed laser sintering process produces rough surface characteristics caused by hatch overlaps and particle attachments, leading to channel size reductions and rough surfaces. In this paper, dimensional metrology of cross-sectional views of multirow microchannels made by AM was conducted by a scanning electron microscope (SEM) at different locations along the printing direction. Channel size reduction, surface roughness, and circularity tolerance of the as-printed channels were analyzed based on micrographs captured by SEM. Results showed that both channel sizes and hole pitches affected the printing qualities of microchannels. The as-printed channel sizes reduced by more than 15% compared to the designed values. Two approaches were made in this paper to improve printing qualities. The first one was to redesign channel size in computer-aided design (CAD) model to make the as-printed channel sizes closer to the objective values. Electrochemical polishing (ECP) was then applied as a second way using sulfuric acid solutions. Surface roughness value was reduced by more than 40% after the ECP process.

Evaluation and Comparison of the Internal Fit and Marginal Accuracy of Cobalt-Cromium Alloy Copings Fabricated Through Cad-Cam Milling and Direct Metal Laser Sintering Technology- An In Vitro Study

Evaluation and Comparison of the Internal Fit and Marginal Accuracy of Cobalt-Cromium Alloy Copings Fabricated Through Cad-Cam Milling and Direct Metal Laser Sintering Technology- An In Vitro Study