Gary K. Lewis

Practical considerations and capabilities for laser assisted direct metal deposition

Abstract Laser assisted direct metal deposition refers to the additive layered manufacturing technology for building components from a computer-aided design (CAD) model. A motion control program, developed from the CAD model of a desired metal component, is used to control the motion of a laser focal spot to trace all areas of the part, typically a planar layer at a time. Metal powders, injected into the laser focal zone, are melted and then re-solidify into fully dense metal in the wake of the moving molten pool created by the laser beam. Successive layers are then stacked to produce the entire component volume of fused metal representing the desired CAD model. Development of this technology has been pursued at both Los Alamos and Sandia National Laboratories and has resulted in the Directed Light Fabrication (DLF) and Laser Engineered Net Shaping (LENS TM ) processes. These processes have been proven feasible for fabricating components from nearly any metal system to near-net shape accuracy with mechanical properties approaching and in some cases exceeding the properties found in conventionally processed wrought structures. Single step processing by LENS and DLF produce cost savings realized by elimination of conventional multi-step thermo-mechanical processing. Design features such as internal cavities or over-hanging features can be made without joined assemblies. Hard to process materials such as intermetallics, refractory metals, and high temperature alloys can be processed in a single step. Functionally graded compositions can be created within three-dimensional components to vary the properties to match localized requirements due to the service environment. The technology offers the designer a rapid prototyping capability at the push of a button, without the need to fabricate dyes or use forming equipment or extensive machining and joining processes to produce a part. Future development is still required for these processes to be commercially accepted and used in industry. Parts are deposited with a surface roughness of 10 μm, arithmetic average, making a secondary finishing operation necessary for some applications to achieve high accuracy and polished surface texture. Residual stress measurement and control is also required to avoid distortion of deposited components. Motion path and control code needs to be optimized to reduce overall process time from the CAD model to the finished part.

Directed light fabrication of a solid metal hemisphere using 5-axis powder deposition

Abstract Directed light fabrication (DLF) is a direct metal deposition process that fuses metal powders, delivered by gas into the focal zone of a high-powered laser beam, to form a fully-dense metal deposit. Computer-based design and numerical controls are used in conjunction with the metal deposition process to guide the formation of 3D parts. This study demonstrates the ability to directly fabricate complex shapes using a 5-axis DLF machine. As an example, the production of a hemispherical shape is described, with the associated fabrication case study, metallographic examination and part characterization. The deposition of fully-dense stainless-steel components is achieved in all orientations, from horizontal to vertical, and dimensional comparisons between the DLF-deposited shape and the original part definition, illustrates that near-net shape tolerance levels are attainable within a 0.1 mm envelope. The single-step production of fully-dense, near-net shaped, 3D metal parts directly from a computer model is achieved without the use of forming dies, tooling or machining. As a result, significant process flexibility over conventional processing capabilities are recognized, with potentially lower productions costs and higher quality components.

Directed light fabrication

Directed Light Fabrication (DLF) is a rapid prototyping process being developed at Los Alamos National Laboratory to fabricate metal components. This is done by fusing gas delivered metal powder particles in the focal zone of a laser beam that is programmed to move along or across the part cross section. Fully dense metal is built up a layer at a time to form the desired part represented by a 3 dimensional solid model from CAD software. Machine “tool paths” are created from the solid model that command the movement and processing parameters specific to the DLF process so that the part can be built one layer at a time. The result is a fully dense, near net shape metal part that solidifies under rapid solidification conditions.Directed Light Fabrication (DLF) is a rapid prototyping process being developed at Los Alamos National Laboratory to fabricate metal components. This is done by fusing gas delivered metal powder particles in the focal zone of a laser beam that is programmed to move along or across the part cross section. Fully dense metal is built up a layer at a time to form the desired part represented by a 3 dimensional solid model from CAD software. Machine “tool paths” are created from the solid model that command the movement and processing parameters specific to the DLF process so that the part can be built one layer at a time. The result is a fully dense, near net shape metal part that solidifies under rapid solidification conditions.

DEVELOPMENT OF A NEAR NET SHAPE PROCESSING METHOD FOR RHENIUM USING DIRECTED LIGHT FABRICATION

Abstract Directed Light Fabrication (DLF) is a direct metal deposition process that fuses gas delivered powder, in the focal zone of a high powered laser beam to form fully fused near net shaped components. The near net shape processing of rhenium and refractory metals is currently in development and may offer significant cost savings compared with conventional processing. A full y associative 3D design through manufacturing model is presented for the application of DLF for the fabrication of a near net shaped nozzle part. This study describes the ability to move from a parametric 3D model integrated into a manufacturing model, creating a control file which can run on the DLF machine to produce a near net shaped metal component. Examples of DLF deposited rheniurn and iridium show a continuously solidified microstructure in rod and tube shapes. Entrapped porosity indicates the required direction for continued process development. These results demonstrate a new methodology for fabricating complex near net ...

Laser Powder Deposition of a Near Net Shape Injection Mold Core — A Case Study

Abstract Directed Light Fabrication (DLF) is a direct deposition process that fuses metal powder, in the focal zone of a laser beam, to form fully fused near net shaped components. A fully associative 3D design-through-manufacturing model is presented as a case study for the fabrication of a near net shaped injection mold core part. This study evaluates the ability to move from a parametric 3D model, integrated into a manufacturing model, to creating a control file used to produce a component using a nickel base alloy. Evaluation of the efficiency, quality, attributes and limitations of the process is reported. A high degree of accuracy for large feature location was obtained but improvements in surface finish and the dimensional accuracy of small features are required to fully realize benefits of this technology. The competing benefits of deposition accuracy and manufacturing time are discussed in terms of computerized numerical control code generation.

Microstructure and Properties of Laser Deposited and Wrought Alloy K-500 (UNS N05500)

Feasibility tests were performed by manufacturing transition joints between Nickel-200 (Ni-200) and Alloy K-500 (K-500) with direct laser deposition. Both sharp and functionally graded interfaces were manufactured with no apparent issues. In addition, Alloy K-500 specimens were manufactured with direct laser deposition to analyze the isothermal hardening response of Alloy K-500. The laser deposited and wrought materials were precipitation hardened at three different temperatures (600 °C, 650 °C, and 700 °C) and various times. A relationship between the hardness and the volume fraction of Ni3(Ti,Al) was developed, and the subsequent analysis showed that the corresponding Johnson-Mehl-Avrami time constants (or n-values) ranged from 1.1 to 1.3 at an aging temperature 600 °C, and decreased to values ranging from 0.6 to 0.66 at an aging temperature of 700 °C. For site saturation and spherical precipitates, the expected n-value is 1.5. This analysis showed that the mechanism for precipitation changed as a function of temperature. The results of the analysis at higher temperatures were rationalized by the possibility of the decay of quenched-in vacancies, precipitation along the grain boundaries, and precipitation along the dislocations. It was, however, difficult to confirm these possibilities with transmission electron microscopy (TEM), since imaging the precipites is difficult at very early aging times. Finally, it was determined that the heat treatment schedule of laser welded or laser clad Alloy K-500 should be similar to that of wrought Alloy K-500.

The role of impurities in bubble formation during directed light processing of tantalum

The utility of directed-light fabrication (DLF) as a method for the manufacture of refractory metal parts has been hampered by the formation of bubbles in the finished product. This study examines the connection between these bubbles and impurities found in several Ta feedstock powders. Bulk and surface impurities associated with the powders were determined using glow-discharge mass spectroscopy (GDMS), thermal desorption and X-ray photoelectron spectroscopy (XPS). A cylindrical part with a high bubble density was fabricated from the Ta powder using DLF and was subsequently fractured in vacuum. The exposed bubble surfaces were examined with Auger electron spectroscopy (AES) and secondary electron microscopy (SEM). Unlike the surrounding region, the bubble surfaces were coated with a K-rich layer. Potassium was an impurity found in the feedstock powder by GDMS. Due to incompletely understood process dynamics, a simple equilibrium model was used to examine the likelihood that gaseous K was trapped in the molten Ta to produce the bubble growth. The results suggest that another impurity, such as hydrogen, may have a primary role in the bubble formation. Analysis of recycled powder that had not been fused during DLF processing showed a decreased concentration of K, Na, F, H and water impurities, implying that some high-temperature purification of the feedstock powder might improve the quality of parts fabricated in this manner.

Directed Light Fabrication of Refractory Metals and Alloys

This report covers deposition of refractory pure metals and alloys using the Directed Light Fabrication (DLF) process and represents progress in depositing these materials through September 1998. In extending the DLF process technology to refractory metals for producing fully dense, structurally sound deposits, several problems have become evident. (1) Control of porosity in DLF-deposited refractory metal is difficult because of gases, apparently present in commercially purchased refractory metal powder starting materials. (2) The radiant heat from the molten pool during deposition melts the DLF powder feed nozzle. (3) The high reflectivity of molten refractory metals, at the Nd-YAG laser wavelength (1.06{micro}m), produces damaging back reflections to the optical train and fiber optic delivery system that can terminate DLF processing. (4) The current limits on the maximum available laser power to prevent back reflection damage limit the parameter range available for densification of refractory metals. The work to date concentrated on niobium, W-25Re, and spherodized tungsten. Niobium samples, made from hydride-dehydride powder, had minimal gas porosity and the deposition parameters were optimized; however, test plates were not made at this time. W-25Re samples, containing sodium and potassium from a precipitation process, were made and porosity was a problem for all samples although minimized with some process parameters. Deposits made from potassium reduced tungsten that was plasma spherodized were made with minimized porosity. Results of this work indicate that further gas analysis of starting powders and de-gassing of starting powders and/or gas removal during deposition of refractory metals is required.