Michelle L. Griffith

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.

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.

Femtosecond laser machining of steel.

Femtosecond laser processing is a promising new technology for the fabrication of micro-scale components from engineering materials, such as metals. In the femtosecond time regime, the ablation process is nearly a solid to vapor transition, thereby providing access to cut smaller features. Sandia National Laboratories has constructed a femtosecond laser microfabrication system to study the ability to produce microscale components in metals and glasses. In this paper, we will report on our initial studies to understand the metal ablation process with respect to manufacturing process parameters. With this understanding, we will show that femtosecond laser processing can fabricate complex components with fine feature detail and clean surfaces. A key finding in this work is the substantial effect of layer decrement on resulting recast material deposition when processing in air.

The effects of varying substrate angle on feature quality in femtosecond laser ablation of ferrous alloys

This paper presents the results of an experimental study to establish process parameters for repeatable, high quality ablated features in ferrous substrates using a Ti:sapphire femtosecond laser system. Initial trials with stainless steel substrates were conducted in ambient atmospheric conditions. Laser power and exposure parameters were varied, in addition to the angle of the substrate relative to the beam. Ablated holes were sectioned, and examined. Data was reduced according to the Taguchi/ANOVA method. The optimal process parameter set minimized the figures of merit for quality or accuracy of the ablated hole. In trials using pulsed ablations, high accuracy holes were associated with laser power greater than 600 mW, substrate angles of 30-45 degrees, and 1000 pulses. In the dwell experiments, high accuracy holes were achieved with a similar power level, and a 1-second dwell time. In contrast to the pulse results, a shallow substrate angle (30 degrees or less) yielded favorable results. In subsequent trials, kovar substrates were processed in a vacuum at constant fluence with a 1-second dwell time. A localized flow of nitrogen removed ablation products. Results were compared to those of the initial trial, leading to significant observations regarding the use of vacuum and secondary process gas.

Laser Wire Deposition (WireFeed) for Fully Dense Shapes LDRD

Direct metal deposition technologies produce complex, near net shape components from Computer Aided Design (CAD) solid models. Most of these techniques fabricate a component by melting powder in a laser weld pool, rastering the weld bead to form a layer, and additively constructing subsequent layers. This report will describe anew direct metal deposition process, known as WireFeed, whereby a small diameter wire is used instead of powder as the feed material to fabricate components. Currently, parts are being fabricated from stainless steel alloys. Microscopy studies show the WireFeed parts to be filly dense with fine microstructural features. Mechanical tests show stainless steel parts to have high strength values with retained ductility. A model was developed to simulate the microstructural evolution and coarsening during the WireFeed process. Simulations demonstrate the importance of knowing the temperature distribution during fabrication of a WireFeed part. The temperature distribution influences microstructural evolution and, therefore, must be controlled to tailor the microstructure for optimal performance.