Additive Manufacturing

JUL 2018

ADDITIVE MANUFACTURING is the magazine devoted to industrial applications of 3D printing and digital layering technology. We cover the promise and the challenges of this technology for making functional tooling and end-use production parts.

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JULY 2018 Additive Manufacturing FEATURE / Hybrid Manufacturing 40 that," Bernhard says. "The process is working really well and we are faced with more demand for hybrid parts than what we can deliver. But so far the process is so complicated and runs within fixed parameter settings for all the different materials, geome- tries, etc. which we have not been able to automate yet." Metal powders (including hot/cold-working steels, stainless, Invar, pure iron, copper and bronze) are the base material for additive manufacturing with Hermle MPA technology. The physical properties of the applied metal layers and the transi- tions between materials must meet the high mechanical and thermal requirements defined by the operational environment of the manufactured component. Precise adjustment of the process parameters is therefore essential for each metal powder used. The properties of the resulting material microstructure are determined in extensive test series. In addition to tension and compression tests, information about particle and layer adhesion, porosity, and material inclusions is derived from the examination of grindings in an optical microscope. "Our experts are able to use the machine with all necessary parameter settings, but we would have to sell them along- side the machine if we wanted to sell it at this point of time," Berhnard says. "It is similar to EDM machines in the old days, where the operator was faced with hundreds of buttons to set up and adjust the process parameters." Micro-Forging to Build Completely Sealed Materials MPA technology is a thermal spray process that applies ma- terial by kinetic compacting—or micro-forging—using metal powder, water, electricity and oxygen-reduced compressed air to build completely sealed structures. Powder particles with grain sizes of 25 to 75 microns are accelerated to very high speeds (as much as three times the speed of sound) by means of a carrier gas and applied to the substrate via a "de Laval" nozzle at temperatures ranging to 1,000°C. The heat, speeds and pressure lead to strong deformation of individual particles and results in a binding contact surface between them. The material is deposited in layers until the contours of the component are accessible for milling. After machining the con- tours, the deposition process is repeated. Alternate deposition and removal is carried out on five axes using an in-house CAM development, Hermle's MPA Studio. Up to six powder convey- ors can be controlled simultaneously, enabling functionally graded materials and material mixtures layering as many as six dissimilar metals. According to Hermle, the strength of the MPA technol- ogy lies in the deposition of large volumes on semi-finished products with free-form surfaces in combination with cavities such as cooling channels, internal cavities, undercut contours or integral heating wires, applications which cannot be covered by any other generative manufacturing process. A special filling material allows for the creation of inner hollow areas, channels and undercut contours. This material is water-soluble and flushed out at the end of the manufacturing process to expose the inner geometry. Subsequent heat treatment optimizes the component's microstructure and ensures the component has the desired hardness. Dissimilar materials like a heat-conducting copper core inside a tool steel exterior can also be created. Workpiece dimensions of more than 0.5 × 0.5 m with total weights ranging to 600 kg can be created on the machine. Moreover, in contrast with deposition welding, in the MPA process the deposited powder is not fusion-bonded, which means that the resulting stresses in the component are very small. This mold core contains a cooling channel following the surface contours. For their production, Hermle used massive blanks in which the cooling channels were milled in. In comparison to layer-by-layer buildup of the whole parts, using blanks is more time- and cost-efficient. One advantage to the hybrid MPA process is the ability to add conformal cooling channels. First the channel is milled into the prepared blank, then filled with a water-soluble filling material. Here, a layer of 1.2344 hot-working steel was applied. Pure copper was applied to the surface and milled, and then a second layer of 1.2344 steel was added before the copper inlay openings were milled. Finally, the filling material in the channel was dissolved.

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