LENSTM Process Research Articles

Additive manufacturing of fine-grained and dislocation-populated CrMnFeCoNi high entropy alloy by laser engineered net shaping

The equiatomic CrMnFeCoNi high entropy alloy is additively manufactured by the laser engineered net shaping (LENSTM) process, and the solidification conditions, phase formation, as-deposited microstructures, and tensile behavior are investigated. The LENSTM-deposited CrMnFeCoNi alloy exhibits a single-phase disordered face centered cubic (FCC) structure, as evidenced by X-ray diffraction (XRD), and rationalized by Scheil's solidification simulation. Furthermore, microstructures at multiple length scales, i.e. columnar grains, solidification substructures, and dislocation substructures, are formed. The tensile deformation process is mainly accommodated by dislocation activities with the assistance of deformation twinning. The tensile yield strength of the LENSTM-deposited CrMnFeCoNi alloy is comparable to that of finer-grained wrought-annealed counterparts, due to the additional initial-dislocation strengthening. However, the uniform tensile elongation, by contrast, is lowered, which is attributed to the increased dynamic dislocation recovery rate and hence the weakened work hardening capability of the LENSTM-deposited CrMnFeCoNi. This study demonstrates the capability of the LENSTM process for manufacturing the CrMnFeCoNi alloy, with high performance, for engineering applications.

Additively manufactured calcium phosphate reinforced CoCrMo alloy: Bio-tribological and biocompatibility evaluation for load-bearing implants

Cobalt-chromium-molybdenum (CoCrMo) alloys are widely used in load-bearing implants; specifically, in hip, knee, and spinal applications due to their excellent wear resistance. However, due to in vivo corrosion and mechanically assisted corrosion, metal ion release occurs and accounts for poor biocompatibility. Therefore, a significant interest to improve upon CoCrMo alloy exists. In the present work we hypothesize that calcium phosphate (CaP) will behave as a solid lubricant in CoCrMo alloy under tribological testing, thereby minimizing wear and metal ion release concerns associated with CoCrMo alloy. CoCrMo-CaP composite coatings were processed using laser engineered net shaping (LENS™) system. After LENS™ processing, CoCrMo alloy was subjected to laser surface melting (LSM) using the same LENS™ set-up. Samples were investigated for microstructural features, phase identification, and biocompatibility. It was found that LSM treated CoCrMo improved wear resistance by 5 times. CoCrMo-CaP composites displayed the formation of a phosphorus-based tribofilm. In vitro cell-material interactions study showed no cytotoxic effect. Sprague-Dawley rat and rabbit in vivo study displayed increased osteoid formation for CoCrMo-CaP composites, up to 2 wt.% CaP. Our results show that careful surface modification treatments can simultaneously improve wear resistance and in vivo biocompatibility of CoCrMo alloy, which can correlate to a reduction of metal ion release in vivo.

The Tribaloy T-800 Coatings Deposited by Laser Engineered Net Shaping (LENSTM).

A Tribaloy family of alloys (CoMoCrSi) are characterized by a substantial resistance to wear and corrosion within a wide range of temperatures. These properties are a direct result of their microstructure including the presence of Laves phase in varying proportions. Tribaloy T-800 exhibits the highest content of Laves phase of all other commercial Tribaloy alloys, which provides high hardness and wear resistance. On the other hand, a large content of the Laves phase brings about a high sensitivity to brittle fracture of this alloy. The main objective of this work was a development of the Tribaloy T-800 coatings on the Ni-based superalloy substrate (RENE 77), which employs a Laser Engineered Net Shaping (LENSTM) technique. Technological limitations in this process are susceptibility of T-800 to brittle fracture as well as significant thermal stresses due to rapid cooling, which is an inherent attribute of laser techniques. Therefore, in this work, a number of steps that optimized the LENSTM process and improved the metallurgical soundness of coatings are presented. Employing volume and local substrate pre-heating resulted in the formation of high quality coatings devoid of cracks and flaws.

Additive manufacturing of Inconel 718 – Ti6Al4V bimetallic structures

Bimetallic structures belong to a class of multi-material structures, and they potentially offer unique solutions to many engineering problems. In this work, bimetallic structures of Inconel 718 and Ti6Al4V (Ti64) alloys were processed using laser engineered net shaping (LENS™). During LENS™ processing, three build strategies were attempted: direct deposition, compositional gradation and use of an intermediate bond layer. Inconel 718 and Ti64 alloys exhibit thermal properties mismatch along with brittle intermetallic phase formation at the interface resulting in delamination. For a successful build, the use of a compositional bond layer (CBL) was employed, which was a mixture of a third material - Vanadium Carbide - with the parent alloys to form an intermediate layer used in bonding the two immiscible alloys. A crack-free bimetallic structure of Inconel 718 and Ti64 was demonstrated. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction and Vickers hardness were used to characterize these bimetallic structures. XRD analysis indicated presence of Cr3C2 phases. CBL improved the bonding strength by avoiding formation of brittle intermetallic phases such as TiNi3 and Ti2Ni as well as reducing thermal stresses at the interface. Our results successfully demonstrate the formation of Inconel 718 and Ti64 bimetallic structures using a laser-based commercially available additive manufacturing approach.

Direct fabrication of compositionally graded Ti-Al2O3 multi-material structures using Laser Engineered Net Shaping

Laser Engineered Net Shaping (LENS™), which is a laser based additive manufacturing method, was utilized to fabricate Ti-Al2O3 compositionally graded structures. The Ti-Al2O3 graded composites consisted of different sections −Ti6Al4V alloy, Ti6Al4V + Al2O3 composites, and pure Al2O3 ceramic. After LENS™ processing, microstructural characterization, phase analysis, elemental distribution, and microhardness measurements were performed on the cross sections of Ti-Al2O3 graded composites. Each section had their unique microstructures and phases. Moreover, hardness measurements demonstrated that the pure Al2O3 section had the highest hardness of 2365.5 ± 64.7 HV0.3. Conventional ceramic processing requires extensive post-processing including high temperature sintering, which makes it difficult for direct fabrication of metal-ceramic multi-layer structures. The results demonstrate that LENS™ can be utilized to process multi-material metal ceramic composites in a single step while maintaining the size, shape and compositional variations based on computer aided design files. Since this is a first-generation work, and limited research results are available in published literature related to LENS™ processing of both metals and ceramics in one operation, the demonstration of this work is expected to inspire future studies on manufacturing of multi-material composites using AM.

A study of process parameters on workpiece anisotropy in the laser engineered net shaping (LENSTM) process

The process of laser engineered net shaping (LENSTM) is an additive manufacturing technique that employs the coaxial flow of metallic powders with a high-power laser to form a melt pool and the subsequent deposition of the specimen on a substrate. Although research done over the past decade on the LENSTM processing of alloys of steel, titanium, nickel and other metallic materials typically reports superior mechanical properties in as-deposited specimens, when compared to the bulk material, there is anisotropy in the mechanical properties of the melt deposit. The current study involves the development of a numerical model of the LENSTM process, using the principles of computational fluid dynamics (CFD), and the subsequent prediction of the volume fraction of equiaxed grains to predict process parameters required for the deposition of workpieces with isotropy in their properties. The numerical simulation is carried out on ANSYS-Fluent, whose data on thermal gradient are used to determine the volume fraction of the equiaxed grains present in the deposited specimen. This study has been validated against earlier efforts on the experimental studies of LENSTM for alloys of nickel. Besides being applicable to the wider family of metals and alloys, the results of this study will also facilitate effective process design to improve both product quality and productivity.

Additive Manufacturing of Reactive In Situ Zr Based Ultra-High Temperature Ceramic Composites

Reactive in situ multi-material additive manufacturing of ZrB2-based ultra-high-temperature ceramics in a Zr metal matrix was demonstrated using LENS™. Sound metallurgical bonding was achieved between the Zr metal and Zr-BN composites with Ti6Al4V substrate. Though the feedstock Zr power had α phase, LENS™ processing of the Zr powder and Zr-BN premix powder mixture led to the formation of some β phase of Zr. Microstructure of the Zr-BN composite showed primary grains of zirconium diboride phase in zirconium metal matrix. The presence of ZrB2 ceramic phase was confirmed by X-ray diffraction (XRD) analysis. Hardness of pure Zr was measured as 280 ± 12 HV and, by increasing the BN content in the feedstock, the hardness was found to increase. In Zr-5%BN composite, the hardness was 421 ± 10 HV and the same for Zr-10%BN composite was 562 ± 10 HV. It is envisioned that such multi-materials additive manufacturing will enable products in the future that cannot be manufactured using traditional approaches particularly in the areas of high-temperature metal–ceramic composites with compositional and functional gradation.

Chemical analysis, structure and mechanical properties of discrete and compositionally graded SS316–IN625 dual materials

Discrete and compositionally graded dual materials based on SS316 and IN625 alloys were made using LENS™ process by layer wise deposition, controlling feed rates of individual alloy powders. The two-alloy interface regions were characterized with respect to microstructure, elemental distribution, micro-hardness, and tensile properties of the deposits. For discrete as well as graded deposits, change in composition near the interface is gradual. This observation is explained on the basis of dilution by the substrate. Irrespective of whether the variation in powder feed rate is changed gradually or discretely, the yield strength of the materials is always comparable to that of the lower strength material of the two, viz., SS316 and failure occurred within SS316 indicating good bonding in the two alloy interface region.

Laser Engineered Net Shaping Process for 316L/15% Nickel Coated Titanium Carbide Metal Matrix Composite

This paper presents the investigation of the macrostructure, microstructure, and solidification structure of a 316L/15% nickel coated TiC metal matrix composite produced by the laser engineered net shaping (LENS™) process. The focus of this work was to (1) identify the solidification structure and to estimate growth/cooling rates at the solid/liquid interface, (2) identify and quantify discontinuities in the build structure, and (3) examine the effect of solidification and thermal history on the sample microstructure to further the understanding of the LENS process. A Numerical method was also developed to examine the influence of material type and LENS™ process parameters on the forming of the specific microstructures from thermodynamics and fluid dynamics point of view. Samples of 316L stainless steel were examined, microstructures of samples were used to estimate the corresponding cooling rate, and the cooling rate was compared with the results of numerical modeling. The computational results show reasonable agreement with experimentally determined cooling rate.

Laser engineered net shaping process for SAE 4140 low alloy steel

This paper presents the investigation of the macrostructure, microstructure, and solidification structure of two ferrous based materials produced by the LENS™ process. The material deposited and examined in this work was a SAE 4140 low alloy steel. The focus of this work is to (1) identify the solidification structure and measure the Dendritic Arm Spacing (DAS) to determine growth/cooling rates at the solid/liquid interface, (2) identify and quantify discontinuities in the build structure, and (3) examine the affect of solidification and thermal history on the sample microstructure to further the understanding of the LENS™ process.

Experimental and Numerical Study of the LENS Rapid Fabrication Process

Several aspects of the thermal behavior of deposited stainless steel 410 (SS410) during the laser engineered net shaping (LENS™) process were investigated experimentally and numerically. Thermal images in the molten pool and surrounding area were recorded using a two-wavelength imaging pyrometer system, and analyzed using THERMAVIZ™ software to obtain the temperature distribution. The molten pool size, temperature gradient, and cooling rate were obtained from the recorded history of temperature profiles. The dynamic shape of the molten pool, including the pool size in both travel direction and depth direction was investigated, and the effect of different process parameters was illustrated. The thermal experiments were performed in a LENS™ 850 machine with a 3 kW IPG Photonics laser for different process parameters. A three-dimensional finite element model was developed to calculate the temperature distribution in the LENS™ process as a function of time and process parameters. The modeling results showed good agreement with the experimental data.

Three-dimensional heat transfer analysis of LENSTM process using finite element method

Laser-engineered net shaping, referred to as LENSTM process, is an additive manufacturing technique for building metallic parts, layer by layer, by direct deposition of metal powders in a melt pool created by a focused laser beam. The process involves rapid melting and solidification of a controlled amount of injected metal powders as a laser beam scans over each layer building the structure from the bottom to the top. Due to its unique capability to deposit precise amounts of powder material at a desired location, the LENSTM process finds potential application in rapid tooling, prototyping, precision repair work, and manufacture of complex, intricate components with varying compositions. The peak temperature and thermal cycle experienced by each layer influence the final mechanical properties and dimensional accuracy of the part. An understanding and quantitative knowledge of the peak temperature, melt pool dimensions, and thermal cycles experienced in the deposited layers are essential for a priori selection of the process parameters in LENSTM technique. It is important to ensure that the deposited layers have the desired dimensions, good interlayer bonding, and requisite mechanical properties. In an attempt to understand the process parameters to be used in achieving the desired nature of deposition, a three-dimensional model is developed based on finite element method to numerically simulate heat transfer phenomenon in LENSTM process considering deposition of SS316 powders on a substrate of the same material. The computed temperature profiles are first validated with experimental results reported in the literature. The influence of process parameters on peak temperature, thermal cycles, and melt pool dimensions are studied subsequently. The continuous movement of laser and synchronized activation of elements depicting addition of powder particles are incorporated through an externally written user subroutine and using the element deactivation and activation features in the commercial finite element software ABAQUS 6.7. A unique non-dimensional parameter specific to LENSTM process is defined considering the combined influence of process parameters and material properties. The non-dimensional parameter is further used to serve as a guideline for the selection of appropriate process parameters that can result in a steady melt pool dimension, thereby ensuring a target layer width with good interlayer bonding.

Functionally graded Co–Cr–Mo coating on Ti–6Al–4V alloy structures

Functionally graded, hard and wear-resistant Co–Cr–Mo alloy was coated on Ti–6Al–4V alloy with a metallurgically sound interface using Laser Engineering Net Shaping (LENS™). The addition of the Co–Cr–Mo alloy onto the surface of Ti–6Al–4V alloy significantly increased the surface hardness without any intermetallic phases in the transition region. A 100% Co–Cr–Mo transition from Ti–6Al–4V was difficult to produce due to cracking. However, using optimized LENS™ processing parameters, crack-free coatings containing up to 86% Co–Cr–Mo were deposited on Ti–6Al–4V alloy with excellent reproducibility. Human osteoblast cells were cultured to test in vitro biocompatibility of the coatings. Based on in vitro biocompatibility, increasing the Co–Cr–Mo concentration in the coating reduced the live cell numbers after 14 days of culture on the coating compared with base Ti–6Al–4V alloy. However, coated samples always showed better bone cell proliferation than 100% Co–Cr–Mo alloy. Producing near net shape components with graded compositions using LENS™ could potentially be a viable route for manufacturing unitized structures for metal-on-metal prosthetic devices to minimize the wear-induced osteolysis and aseptic loosening that are significant problems in current implant design.

Wear resistance of laser-deposited boride reinforced Ti-Nb–Zr–Ta alloy composites for orthopedic implants

The inherently poor wear resistance of titanium alloys limits their application as femoral heads in femoral (hip) implants. Reinforcing the soft matrix of titanium alloys (including new generation β-Ti alloys) with hard ceramic precipitates such as borides offers the possibility of substantially enhancing the wear resistance of these composites. The present study discusses the microstructure and wear resistance of laser-deposited boride reinforced composites based on Ti–Nb–Zr–Ta alloys. These composites have been deposited using the LENS™ process from a blend of elemental Ti, Nb, Zr, Ta, and boron powders and consist of complex borides dispersed in a matrix of β-Ti. The wear resistance of these composites has been compared with that of Ti–6Al–4V ELI, the current material of choice for orthopedic femoral implants, against two types of counterfaces, hard Si 3N 4 and softer SS440C stainless steel. Results suggest a substantial improvement in the wear resistance of the boride reinforced Ti–Nb–Zr–Ta alloys as compared with Ti–6Al–4V ELI against the softer counterface of SS440. The presence of an oxide layer on the surface of these alloys and composites also appears to have a substantial effect in terms of enhanced wear resistance.

The mechanical and microstructural characteristics of laser-deposited IN718

The paper is concerned with the application of laser deposition to the nickel-based superalloy INCONEL™ 718 (IN718). Several blocks of material were manufactured using the LENS™ process and subsequently heat treated and tensile tested. Following this, further sections of the original blocks were hot isostatically pressed (HIPed) and then reassessed for mechanical properties and microstructure. It was found that prior to HIPing the deposit exhibited anisotropic properties that appeared to be associated with non-optimised processing conditions. HIPing led to a reduction in anisotropy within the deposit, but generated considerable grain growth within the (IN718) substrate.

Microstructures of LENS™ Deposited Nb-Si Alloys

ABSTRACTNb-Si “in-situ” metal matrix composites consist of Nb3Si and Nb5Si3 intermetallic phases in a body centered cubic Nb solid solution, and show promising potential for elevated temperature structural applications. Cr and Ti have been shown to increase the oxidation resistance and metal loss rate at elevated temperatures compared to the binary Nb-Si system. In this study, the LENS™ (Laser Engineered Net Shaping) process is being implemented to construct the Nb-Ti-Cr-Si alloy system from elemental powder blends. Fast cooling rates associated with LENS™ processing yield a reduction in microstructural scale over conventional alloy processes such as directional solidification. Other advantages of LENS™ processing include the ability to produce near net shaped components with graded compositions as well as a more uniform microstructure resulting from the negative enthalpy of mixing associated with the silicide phases. Processing parameters can also be varied, resulting in distinct microstructural differences. Deposits were made with varying compositions of Nb, Ti, Cr and Si. The as-deposited as well as heat treated microstructures were examined using SEM and TEM techniques. The influence of composition and subsequent heat treatment on microstructure will be discussed.

Laser Deposition of In Situ Ti – TiB Composites

Due to their enhanced mechanical properties and potentially wide applicability, there is considerable interest in the development of metal-matrix composites consisting of titanium borides in a titanium alloy matrix. Despite the development of a variety of different processing routes for these composites, there are relatively few ones capable of processing a fully dense, near-net shape component with a relatively fine dispersion of boride precipitates. This paper will discuss the in situ laser deposition of Ti-TiB composites using the laser engineered net-shaping (LENS™) process from a blend of elemental titanium (or titanium alloy) and boron powders. The microstructure of the LENS™ deposited Ti-TiB composite has been compared with that of a conventionally cast in situ composite of the same composition. The conventionally cast composite exhibits a significantly coarser scale microstructure. Thus, the ability to produce a fine dispersion of TiB precipitates in dense Ti-TiB composites of near-net shape using LENS™ processing can be attributed to the rapid solidification effects during such processing.

Microstructural Evaluation of LENS™ Deposited Nb-Ti-Si-Cr Alloys

ABSTRACTNb-Ti-Si “in-situ” metal ceramic composites consist of Nb3Si and Nb5Si3 intermetallic phases in a body centered cubic Nb solid solution, and show promising potential for elevated temperature structural applications. The addition of Cr has also been shown to increase the oxidation resistance at high temperatures. In this study, the LENS™ (Laser Engineered Net Shaping) process is being implemented to construct the Nb-Ti-Cr alloy system from elemental powder blends. Advantages of the LENS™ process include the ability to produce near net shaped components with graded compositions as well as a more uniform microstructure resulting from the negative enthalpy of mixing associated with the silicide phases. This study focuses on characterization of the microstructure of the Nb-27Ti-5Si-10Cr (at%) system using SEM and TEM analysis.

Computer simulations of the evolution of solidification microstructure in the LENS™ rapid fabrication process

A modified cellular automaton-based method is developed to study the evolution of the solidification grain microstructure during the LENS™ rapid fabrication process. The method is coupled with a finite difference-based procedure for computation of the temperature field during both the substrate re-melting/feed-powder injection stage and the solidification stage of the LENS™ process. A distribution of the potency of nucleation sites is used to model heterogeneous nucleation both within the melt pool and at the substrate/melt interface. The dendrites are enabled to grow preferentially in the cubic crystallographic directions and the kinetics of dendrite-tips motion is assumed to be controlled by the local undercooling. The method developed enables the establishment of the relationships between the LENS™ process parameters (e.g. laser power, laser velocity), and the resulting solidification microstructure in any binary metallic material. The application of the model to Al–7.0 wt.% Si alloys yielded results which are in a generally good agreement with their experimental counterparts.