Metal additive manufacturing (AM), being a transformational technology with unique capabilities, has received considerable attention from both industrial and academic sectors. To date, quality and repeatability are still regarded as the critical technological barrier that hinders their widespread applications, especially for high-value components with stringent requirements. One significant approach to overcome this formidable challenge is in-process monitoring combined with real-time closed-loop control, which has been explored by great research efforts. This paper describes ongoing work on the in-situ monitoring and characterization during processes of powder-fed laser-aided direct metal deposition (DMD), a form of metal AM. Much emphasis has been placed on the optical techniques, such as the temperature and morphology measurement of the melt pool by two-wavelengths pyrometers, and high-speed CMOS cameras, respectively. In particular, the state-of-the-art with respect to an emerging field-the full-field deformation monitoring of part and/or substrate by digital image correlation (DIC), is addressed.

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 (LENSTM) 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.

The direct additive manufacturing of metallic components can present several process challenges. At present, there are several techniques for the accomplishment of this goal l ,2, each withits own set ofstrong points and limitations. At Sandia,LaserEngineered Net Shaping, or LENS, is a process.which has been. developed for the direct additive·. manufacturing i of fully dense three dimensional parts. In LENS, a Nd-YAGlaser is focused onto a metallic substrate or onto previously deposited material. Thelaser melts the metal and a small pool of metalforms. Powder is injected into the pool and a bead forms. Byrastering an x-y table to which the partis affixed in a controlled fashion, the bead is pulled and afully dense metal partis formed. As currently configured, LENS isa 2VzD process.

Laser Engineered Net-Shaping, otherwise known as LENS{trademark}, is an advanced manufacturing technique used to fabricate complex near net shaped components directly from engineered solid models without the use of dies or machining. The ultimate objective of this project is to develop predictive simulation capability which will allow the LENS{trademark} processors to determine fabrication conditions given the material, shape, and application of the final part. In this paper, the authors will present an incremental achievement to meeting the ultimate goal, a model capable of simulating the coarsening of microstructural features under the unique thermal history to which a LENS{trademark} part is subjected during processing. The simulation results show how grains of very different shapes and sizes form within the same deposition line. They also show that relatively minor changes in the dynamic temperature profile results in microstructures with vastly different characteristics. The implications of this work for LENS{trademark} fabrication is that controlling the temperature profile is essential to tailoring the microstructure of a component to its application.

During the past few years, solid freeform fabrication has evolved into direct fabrication of metallic components using computer aided design (CAD) solid models. Laser Engineered Net Shaping (LENS{trademark}) is one such technique being developed at Sandia to fabricate high strength, near net shape metallic components. In the past two years a variety of components have been fabricated using LENS{trademark} for applications ranging from prototype parts to injection mold tooling. To advance direct fabrication capabilities, a process must be able to accommodate a wide range of materials, including alloys and composites. This is important for tailoring certain physical properties critical to component performance. Examples include graded deposition for matching coefficient of thermal expansion between dissimilar materials, layered fabrication for novel mechanical properties, and new alloy design where elemental constituents and/or alloys are blended to create new materials. In this paper, the authors will discuss the development of precise powder feeding capabilities for the LENS{trademark} process to fabricate graded or layered material parts. They also present preliminary results from chemical and microstructural analysis.

Rapid Prototyping and Near Net Shape manufacturing technologies are the subject of considerable attention and development efforts. At Sandia National Laboratories, one such effort is LENS (Laser Engineered Net Shaping). The LENS process utilizes a stream of powder and a focused Nd YAG laser to build near net shape fully dense metal parts. In this process, a 3-D solid model is sliced, then an X-Y table is rastered under the beam to build each slice. The laser 1 powder head is incremented upward with each slice and the deposition process is controlled via shuttering of the laser. At present, this process is capable of producing fully dense metal parts of iron, nickel and titanium alloys including tool steels and aluminides. Tungsten components have also been produced. A unique aspect of this process is the ability to produce components wherein the composition varies at differing locations in the part. Such compositional variations may be accomplished in either a stepped or graded fashion. In this paper, the details of the process will be described. The deposition mechanism will be characterized and microstructures and their associated properties will be discussed. Examples of parts which have been produced will be shown and issues regarding dimensional control and surface finish will be addressed.