John E. Smugeresky

UNDERSTANDING THERMAL BEHAVIOR IN THE LENS PROCESS

Abstract In direct laser metal deposition technologies, such as the laser engineered net shaping (LENS) process, it is important to understand and control the thermal behavior during fabrication. With this control, components can be reliably fabricated with desired material properties. This paper will describe the use of contact and imaging techniques to monitor the thermal signature during LENS processing. Development of an understanding of solidification behavior, residual stress, and microstructural evolution with respect to thermal behavior will be discussed.

The laser forming of metallic components using particulate materials

Direct fabrication technology, which utilizes computer-aided design solid models to automatically control the manufacture of functional piece parts, is rapidly gaining popularity as a means to significantly reduce the time to market of new concepts. Since the introduction of stereolithography in 1982, several different rapid prototyping technologies have evolved using surrogate rather than actual materials of construction. Most recently, researchers have begun to develop laser-based methods to obtain fully dense metallic components directly from a computer-aided design solid model. Each of these methods is unique, but possesses aspects that are similar to the others. Many of these methods hold a great deal of promise for applications; however, none have been developed into a commercial product.

Laser engineered net shaping (LENS™): A tool for direct fabrication of metal parts

For many years, Sandia National Laboratories has been involved in the development and application of rapid prototyping and dmect fabrication technologies to build prototype parts and patterns for investment casting. Sandia is currently developing a process called Laser Engineered Net Shaping (LENS~) to fabricate filly dense metal parts dwectly from computer-aided design (CAD) solid models. The process is similar to traditional laser-initiated rapid prototyping technologies such as stereolithography and selective laser sintering in that layer additive techniques are used to fabricate physical parts directly from CAD data. By using the coordinated delivery of metal particles into a focused laser beam apart is generated. The laser beam creates a molten pool of metal on a substrate into which powder is injected. Concurrently, the substrate on which the deposition is occurring is moved under the beam/powder interaction zone to fabricate the desired cross-sectiwal geometry. Consecutive layers are additively deposited, thereby producing a three-dmensional part. This process exhibits enormous potential to revolutionize the way in which metal parts, such as complex prototypes, tooling, and small-lot production parts, are produced. The result is a comple~ filly dense, near-net-shape part. Parts have been fabricated from 316 stainless steel, nickel-based alloys, H13 tool steel, and titanium. This talk will provide a general overview of the LENS~ process, discuss potential applications, and display as-processed examples of parts.

Using the laser engineered net shaping (LENS) process to produce complex components from a CAD solid model

The laser engineered net shaping (LENSTM) process, currently under development, has demonstrated the capability to produce near-net shape, fully dense metallic parts with reasonably complex geometrical features directly from a CAD solid model. Results to date show that excellent mechanical properties can be achieved in alloys such as 316 stainless steel and Inconel 625. In fact, due to the highly localized nature of the laser heating, a fine grain structure will occur resulting in a significant increase in yield strength at no expense of ductility. The current approach lends itself to produce components with a dimensional accuracy of plus or minus .002 inches in the deposition plane and plus or minus .0.015 inches in the growth direction. These results suggest that this process will provide a viable mens for direct fabrication of metallic hardware directly from the CAD solid model.

Low-temperature solubility of copper in beryllium, in beryllium-aluminum, and in beryllium-silicon using ion beams

Ion implantation and ion backscattering analysis have been used to measure the solubility of copper in beryllium over the temperature range 593 to 1023 K, and to determine the effect on the copper solubility of aluminum and silicon impurities. The binary data extend 280 K lower in temperature than previous results, while the ternary measurements are unique. This information is pertinent to the use of copper for solution strengthening of beryllium. Diffusion couples were formed by ion implantation of copper into singlecrystal beryllium at room temperature, followed where appropriate by implantation of aluminum or silicon. The samples were then annealed isothermally, and the time-evolution of the composition-vs-depth profile, determined by ion backscattering analysis, yielded the solubility of copper. Measurements at exceptionally low temperatures were facilitated by the short diffusion distances, ≅0.1 μm, and the use of neon irriadiation to accelerate diffusion. The resulting binary data for the solubilityC0 of copper in beryllium merge smoothly into previous results at higher temperatures. The combined data, covering the temperature range 593 to 1373 K, are well described byC0=(12.6 at. pct)·exp(−842 K/T). In the ternary regime, the effects of aluminum and silicon on the solubility of copper were found to be small.

Phase equilibria and diffusion in the Be-Al-Fe

Phase boundaries enclosing the α-Be region of the Be-Al-Fe system have been investigated by a new approach which utilizes high-energy ion beams. Ion implantation of aluminum and/or iron into single-crystal beryllium was used to create a desired second phase in the first ≃0.1 μm of the sample. Under continued isothermal annealing the second phase dissolved into the underlying bulk α-Be, and this process was monitored nondestructively by ion backscattering. Analysis then gave the solid solubility and the diffusivity of the species under consideration. The solubility of iron in beryllium was measured between 773 (170 appm) and 1123 (1600 appm); above 973 the present data merge smoothly into previous results from the literature. A new upper bound was established for the solubility of aluminum below the Al-Be eutectic: ≲ 70 appm at 873 K. In the ternary regime, the solubility of iron in the presence of excess aluminum was measured for the first time; below 1123 it is ≲ 100 appm.

Low-pressure spray forming of 2024 aluminum alloy

Abstract In this paper, a newly developed low-pressure spray forming (LPSF) technique is described. The experimental results obtained with an as-deposited 2024 aluminum alloy are reported. It is shown that the application of reduced pressure significantly decreases porosity as compared to conventionally spray-formed 2024 aluminum alloy. Moreover, the resultant microstructures are similar to those achieved with conventional spray forming. The mechanisms of porosity formation in deposited materials, obtained using both low pressure and conventional spray forming, are discussed. Gas entrapment and interstitial porosity are proposed to be the two major sources of the porosity present in the as-deposited materials. On the basis of the present study, the controlled low-pressure environment during LPSF appears to influence the droplet trajectories and the gas flow field, leading to flow-straightening effects which result in significant reduction of porosity in the deposited materials.

Characterization of a Rapidly Solidified Iron-Based Superalloy

Rapidly-solidified powders of an iron-based superalloy were characterized before and after consolidation by hot isostatic pressing. Powders made by inert gas atomization were compared to powders made by centrifugal atomization. Although many of the powder characteristics were similar, the microstructures were not. The inert gas atomized powder structure is cellular while the centrifugally atomized powder structure is dendritic. In general the finer powder particles have the finer micro-structure with the effect more noticeable in centrifugally atomized powders. After consolidation, the differences in microstructure are more dependent on the consolidation temperature and post-consolidation heat treatment than in the powder type or size. Higher consolidation temperatures and/or post-consolidation heat treatment will result in transformation of the as-solidified microstructures. The transformed microstructure and the mechanical properties can in some cases be related to the as-solidified structure. Heat treatment is needed to obtain mechanical properties equivalent to those of ingot metallurgy processed material.

Hybrid Al + Al3Ni metallic foams synthesizedin situvia laser engineered net shaping

A hybrid, Al + Al3Ni metallic foam was synthesized in situ via laser engineered net shaping (LENS®) of Ni-coated 6061 Al powder in the absence of a foaming agent. During LENS® processing, the Ni coating reacted with the Al matrix, resulting in the simultaneous formation of a fine dispersion of Al3Ni, and a high volume fraction of porosity. As a reinforcement phase, the intermetallic compound formed particles with a size range of 1–5 µm and a volume fraction of 63%, with accompanying 35–300 µm pores with a 60% volume fraction. The microstructure of the as-deposited Al + Al3Ni composite foams was characterized using SEM, EDS, XRD and TEM/HRTEM techniques. The evolution of the microstructure was analyzed on the basis of the thermal field present during deposition, paying particular attention to the thermodynamics of the Al3Ni intermetallic compound formation as well as discussing the mechanisms that may be responsible for the observed porosity. The mechanical behavior of the as-deposited material was characterized using compression and microhardness testing, indicating that the yield strength and hardness are 190 MPa and 320 HV, respectively, which represents an increase of over three times higher than that of annealed Al6061, or similar to heat-treated Al6061 fully dense matrix, and much higher than those of traditional Al alloy foams, and with a low density of 1.64 g/m3.

Effect of hydrogen on the mechanical properties of iron-base superalloys

In this study the effect of Ti, Al, Ni, and Mn variations on the strengths of eight laboratory scale heats of an iron-base superalloy (Fe + 15 pct Cr + 25 pct Ni + Ti + Al) was investigated. The aging response at 873 K, after either a 1173 or 1323 K solution treatment temperature, was monitored to determine which alloys (with respective heat treatment) had the highest yield strengths. Those having yield strengths of 700 to 1055 MPa and ductilities of 22 to 56 pct (RA) were thermally charged in hydrogen and tested in air, 69 MPa hydrogen and 69 MPa helium. After charging, the yield strengths were unchanged, but RA losses ranged from 40 to 90 pct. Microstructural observations are consistent with hydrogen transport by dislocations and trapping at the matrix-precipitate interface where the hexagonal Ni3Ti(ώ) is the precipitate.