Additive Manufacturing by Direct Metal Deposition - ASM International

Additive Manufacturing by Direct Metal Deposition B. Dutta* S. Palaniswamy POM Group Inc. Auburn Hills, Mich.

J.Choi L.J. Song J. Mazumder,* FASM University of Michigan Ann Arbor

DMD is an enabling technology that allows the addition of the right material to the right location, adding value to products.

*Member of ASM International

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irect metal deposition (DMD) is an advanced additive manufacturing technology used to repair and rebuild worn or damaged components, to manufacture new components, and to apply wear- and corrosionresistant coatings. DMD produces fully dense, functional metal parts directly from CAD data by depositing metal powders pixel-by-pixel using laser melting and a patented closed-loop control system to maintain dimensional accuracy and material integrity. With the feedback system, six-axis deposition, and multiple material delivery capability, DMD can coat, build, and rebuild parts having very complex geometries (Fig. 1). Joint research between POM Group and University of Michigan, through an Advanced Technology Project funded by NIST, led to significant advancements in the monitoring and control of the DMD process. This article reviews DMD technology, recent advancements in process control, and DMD applications.

Fig. 1 — Direct metal deposition in action.

Process overview DMD is a proprietary laser aided manufacturing (LAM) process developed at the University of Michigan and is being further developed and commercialized by POM Group. An industrial laser beam under CNC/robotic control is focused onto a workpiece, producing a melt pool into which a small amount of powder metal is injected, building up the part in a thin layer. The beam is moved to the next location based on CAD geometry, tracing out the part layer by layer. Some features of DMD systems are: • Patented closed-loop feedback control for the process • Coaxial nozzle with local shielding of melt pool • 5-axis DMDCAM software for additive manufacturing • 5-axis moving optics for heavy parts • User-friendly DMD user interface The DMD closed loop feedback system is the key tool for producing a near-netshape product. High speed sensors collect melt pool information, which is directly fed into a dedicated controller that adjusts the process inputs, such as laser power, to maintain part dimensions (Fig. 2). DMDCAM software for additive manufacturing is a comprehensive 6-axis CAM software solution to generate deposition paths for contour, surface, and volume geometry, adopting different deposition path strategies required in additive manufacturing processes. The application derives its strength from the ability to create multiple layers of deposition path or deposition paths for even an entire solid body in a single operation. The generated path is post-processed for required CNC/robotic paths and validated for collision detection by simulation. The DMDVision system offers users a precision part pick up and a quick toolpathing operation for small objects with fine features. The system locates the coordinate position of a part in the machine and al-

ADVANCED MATERIALS & PROCESSES • MAY 2011

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Laser beam

Fig. 2 — Schematic of closed-loop control (left) and intricate shapes built using direct metal deposition.

Final focus optics

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To powder feeder Feedback sensor 2 Feedback sensor 1 Work-holding fixture

Solid free form shape by direct deposition Subtract or die preform

Fig. 3 — DMD IC106 system.

lows easy toolpath generation for accurate deposition. Faster operation and better repeatability improves productivity and yield significantly. DMD systems are equipped with a three- or five-axis head with an additional rotary axis on the work table, which allows deposition of almost any geometry. Standard systems are supplied with fiber coupled diode/disk/fiber lasers. Various models include: • DMD 105D/505D (medium-to-large 5-axis machine on CNC platforms) • DMD 44R/66R (large, flexible systems with 6-axis industrial robot) • DMD IC 106 (small, compact machine with a 6-axis industrial robot inside an inert-gas chamber for processing exotic metals and alloys) The DMD IC106 is targeted at additive manufacturing and materials research and development markets. Dual powder feeder capability lets researchers synthesize new materials through the deposition process, while 6-axis additive programming along with advanced sensor technology allows building of complex geometries. Figure 3 shows a DMD IC106 system. Temperature and cooling-rate control: Recent advances in sensor technology extend closed-loop control system capability from only geometry-based control to temperature-based control. A dual color pyrometer with selected wavelength away from the laser radiation is used to monitor the melt pool temperature. Measured temperature is fed back to a real-time controller to track melt pool temperature to a preset value. During the DMD process, the controller calculates the optimized control value and sends 1.5 Laser driven voltage, V

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Fig. 4 —Variation of melt pool temperature with laser power; red curve shows model predicted temperature; gray curve shows measured temperature. 34


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a control signal to adjust the laser power each time it obtains an updated melt pool temperature. Figure 4 shows the controlled temperatures and the laser actions using a generalized predictive control algorithm that was implemented in a real-time controller during laser cladding of H13 tool steel. The melt pool temperature was successfully tracked to the preset temperature profile. The addition of temperature control is particularly beneficial for dissimilar metal deposition process. Composition monitoring is based on realtime spectroscopic analysis of the plasma plume generated during the deposition process. Figure 5 shows the relationship between the measured Cr/Fe spectral line intensity ratios as a function of Cr/Fe weight ratios in the alloys. A second-order polynomial fitting was used to form the calibration curves for different line pair ratios. By measuring the plasma line intensity ratios and comparing them with the calibration curve, the real-time composition ratio of different elements is obtained during the deposition process. Continued

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Fig. 5 — Plasma Cr/Fe line intensity ratio versus Cr/Fe weight ratio; red circles are measured data; blue lines are second-order polynomial fitting.

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Fig. 6 — DMD-built Inconel gas turbine blade with directionally solidified structure.

Applications As-deposited material is fully dense and its mechanical and physical properties are as good as or better than those of comparable cast or wrought materials. DMD has been applied successfully in a broad range of applications, including remanufacturing of gas turbine components; rebuilding high-value, long lead time components (particularly in defense industry); hardfacing for extended

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life in tools, dies, cutters, etc.; and application of conformal cooling in injection molding tools. Besides near-net-shape part manufacturing capability, controlled heat input during the DMD process allows building parts with desired microstructures. An example is an Inconel turbine blade built with controlled crystal structure (Fig. 6). This mock-up blade demonstrates the potential of DMD technology to build up or repair turbine blades with directionally solidified structures. Summary DMD is an enabling technology that allows the right material to be added to the right place, thus adding value to products. A patented closed-loop control adds to the robustness of the process and helps to produce near-netshape parts. The technology has been applied successfully in areas of remanufacturing, hard coating, and new complex part manufacturing. Recent advancements in process sensor technology open up a new horizon for researchers to explore areas of new material synthesis and additive manufacturing of complex geometries. For more information: Bhaskar Dutta, POM Group Inc., 2350 Pontiac Rd., Auburn Hills, MI 48326; tel: 248/409-7900; email: [email protected]; www.pomgroup.com.