Experimental Laser Powder Bed Fusion System for Difficult to Process Metallic Materials

The focus of this research was twofold, such as development of defect free fabrication parameters for laser powder bed fusion processing of crack prone or difficult to process metallic materials, and study of the temperature dependence of emissivity for some commonly used metal alloy powders in LPBF process. The later objective extends to the implementation of multiwavelength pyrometer technology for in situ true surface temperature measurement in LPBF process. LPBF is an additive manufacturing process capable of layer-by-layer manufacturing by successive laser melting of each layer according to CAD data. AM manufacturing has the inherent advantages of fabricating optimally designed geometry with least required material and the shortest lead time that would not be possible all at once by more conventional manufacturing techniques such as machining, casting, welding, etc. Tendency to crack during LPBF fabrication of some of the widely used aluminum alloys with large solidification range, such as AA6061, AA7075, etc. created a bottleneck where AM specific advantages of these materials important for aerospace and automotive industry could not be realized. An open architecture LPBF machine AcontyONE enabled with powder bed heating up to 1000 °C was used for fabrication of aluminum 6061 alloy test coupons, such as cubes and cylindrical tensile specimens in the XY plane. Variation of processing parameters such as laser power, scanning speed, hatch spacing, layer thickness, and powder bed heating temperature was carried out to find a combination of parameters that allowed crack free fabrication of AA6061. The following parameters such as 450 W laser power, 1400 mm/s scan speed, 100 μm layer thickness, 80 to 100 μm hatch spacing, and 500 °C powder bed heating resulted in crack free specimens as confirmed by visual inspection, optical microscopy, and tensile testing. In scanning electron microscopy, coarsening of grains and precipitates was observed due to high temperature preheating, and a preferential growth in [100] direction was also evident from X-ray diffraction analysis. The heat treated, and as-fabricated tensile tested specimens showed equivalent ultimate tensile strength and yield strengths when compared to their wrought counterpart; however, loss in elongation at breakpoint was observed in the LPBF fabricated specimens. This part of the research showcased the potential of high temperature powder bed heating for LPBF processing of crack prone metallic materials without the need for additional preparation of the precursor metal powders. Unlike the commercial LPBF systems currently in use, the open architecture experimental LPBF system used in this study had the capability of achieving controlled high temperature at the powder bed. Whether this high set temperature that proved to deter visible crack formation in AA6061 fabrication, would be carried throughout the height of a build- required true in situ surface temperature measurement of the powder bed. Therefore, an important aspect of the current research was to evaluate the thermal environment of the build area in LPBF process. Changing emissivity of metallic materials is a major difficulty in evaluation of true temperature during LPBF processes. The existing in situ monitoring systems relied on single or dual wavelength pyrometers, raw sensor data, high-speed visual cameras, and infrared thermography for monitoring and surface temperature measurements; but, without the emissivity information of the target materials, these systems output a qualitative measurement of temperature instead of an accurate surface temperature. The changing emissivity resulting from the non-grey nature of radiation from metallic materials upsets the assumptions of single and dual-wavelength pyrometry, and thereby, renders the measurements inaccurate. On the other hand, Multiwavelength pyrometer technology would not require any prior information of the target emissivity for accurate temperature measurement and provided a real-time value of emissivity of the target material by analyzing the IR spectra it would receive from the target. During the LPBF process, on a given layer the precursor powder material in solid state would turn into molten metal tracks after interaction with laser at selected spots, and finally, consolidate to bulk solid state when the rest of the powder on the bed would not go through phase change. At least three different metal states with potentially different emissivity were identified, such as metal powder in solid state, molten metal tracks, and solidified metal tracks. To measure this emissivity for different materials and study their variation with temperature, a Whipmix Pro 200 furnace was modified for optical access to a SpectroPyrometer MW pyrometer system. These emissivity measurements could serve as input for IR thermography for extending accurate surface temperature measurements capability from a pointwise measurement using pyrometer to the accurate temperature mapping of an area using the IR thermography. (Abstract shortened by ProQuest.