Powder-bed beam-based metal additive manufacturing (AM) such as electron beam additive manufacturing (EBAM) has a potential to offer innovative solutions to many challenges and difficulties faced in the manufacturing industry. However, the complex process physics of EBAM has not been fully understood, nor has process metrology such as temperatures been thoroughly studied, hindering part quality consistency, efficient process development and process optimizations, etc., for effective EBAM usage. In this study, numerical and experimental approaches were combined to research the process temperatures and other thermal characteristics in EBAM using Ti–6Al–4V powder. The objective of this study was to develop a comprehensive thermal model, using a finite element (FE) method, to predict temperature distributions and history in the EBAM process. On the other hand, a near infrared (NIR) thermal imager, with a spectral range of 0.78 μm–1.08 μm, was employed to acquire build surface temperatures in EBAM, with subsequent data processing for temperature profile and melt pool size analysis. The major results are summarized as follows. The thermal conductivity of Ti–6Al–4V powder is porosity dependent and is one of critical factors for temperature predictions. The measured thermal conductivity of preheated powder (of 50% porosity) is 2.44 W/m K versus 10.17 W/m K for solid Ti–6Al–4V at 750 °C. For temperature measurements in EBAM by NIR thermography, a method was developed to compensate temperature profiles due to transmission loss and unknown emissivity of liquid Ti–6Al–4V. At a beam speed of about 680 mm/s, a beam current of about 7.0 mA and a diameter of 0.55 mm, the peak process temperature is on the order around 2700 °C, and the melt pools have dimensions of about 2.94 mm, 1.09 mm, and 0.12 mm, in length, width, and depth, respectively. In general, the simulations are in reasonable agreement with the experimental results with an average error of 32% for the melt pool sizes. From the simulations, the powder porosity is found critical to the thermal characteristics in EBAM. Increasing the powder porosity will elevate the peak process temperature and increase the melt pool size.

Thermal characteristics such as process temperatures and melt pool sizes offer important information in metal additive manufacturing (AM) technologies such as powder-bed electron beam additive manufacturing (EBAM). In this study, a near infrared (NIR) thermal imager was employed to acquire build surface process temperatures during EBAM fabrications using Ti-6Al-4V powder. Challenges in NIR temperature measurements for EBAM were tackled including compensating temperatures due to the transmission loss and estimating the emissivity of Ti-6Al-4V in its molten state. At a beam speed of about 728 mm/s, a beam current of about 7.2 mA and a diameter of 0.55 mm, the maximum process temperature is on the order of around 2700 °C, and the melt pools have dimensions of about 2.72 mm and 0.72 mm in length and width, respectively.

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Abstract

On Process T em p eratu re in Pow der-B ed Electron Beam A dditive M anufacturing : Process P a ra m e te r EffectsBuild part certification has been one of the primary roadblocks for effective usage and broader applications of metal additive manufacturing (AM) technologies including powder-bed electron beam additive manufacturing (EBAM). Process sensitivity to oper ating parameters, among others such as powder stock variations, is one major source of property scattering in EBAM parts. Thus, it is important to establish quantitative rela tions between the process parameters and process thermal characteristics that are closely correlated with the AM part properties. In this study, the experimental techniques, fabri cations, and temperature measurements, developed in recent work (Cheng et al., 2014, "On Process Temperature in Powder-Bed Electron Beam Additive Manufacturing: Model Development and Experimental Validation," ASME J. Manuf. Sci. Eng., (in press)) were applied to investigate the process parameter effects on the thermal characteristics in EBAM with Ti-6Al-4V powder, using the system-specific setting called "speed function (SF)" index that controls the beam speed and the beam current during a build. EBAM parts were fabricated using different levels ofSF index (20-65) and examined in the part surface morphology and microstructures. In addition, process temperatures were meas ured by near infrared (NIR) thermography with further analysis of the temperature pro files and the melt pool size. The thermal model, also developed in recent work, was further employed for EBAM temperature predictions, and then compared with the experi mental results. The major results are summarized as follows. SF index noticeably affects the thermal characteristics in EBAM, e.g., a melt pool length o f 1.72 mm and 1.26 mm for SF20 and SF65, respectively, at 24.43 mm build height. SF setting also strongly affects the EBAM part quality including the surface morphology, surface roughness and part microstructures. In general, a higher SF index tends to produce parts of rougher surfaces with more pore features and large /? grain columnar widths. Increasing the beam speed will reduce the peak temperatures, also reduce the melt pool sizes. Simulations conducted to evaluate the beam speed effects are in reasonable agreement compared to the experi mental measurements in temperatures and melt pools sizes. However, the results of a lower SF case, SF20, show larger differences between the simulations and the experi ments, about 58% for the melt pool size. Moreover, the higher the beam current, the higher the peak process temperatures, also the larger the melt pool. On the other hand, increasing the beam diameter monotonically decreases the peak temperature and the melt pool length. 3 Results and Discussion 3.1 Beam Speed Effects. The average translational electron beam speed was calculated, using a method developed [22], for T a b le 1 P a r a m e te r s u s e d in s im u la t io n Parameters Values Scan speed, V (mm/s) Experimentally obtained...