Microstructural Study of Laser Formed Ti-6Al-4V
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The AcroMet Laser Additive Manufacturing Process .44-.'
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Figure 1: The AeroMet LasformingsM process.
Figure 2: A "rib-on-web" Lasformed part where linear Ti-6A1-4V ribs (7.6 cm high, 1.3 cm wide) have been deposited on a Ti-6AI-4V plate. The part shown is3 33 cm long, 15.2 cm high, and 15.2 cm wide.
3 Mat. Res. Soc. Symp. Proc. Vol. 625 © 2000 Materials Research Society
using the LasFormsM process. Understanding of the microstructure formed as a result of the complex thermal history experienced during the building of a part is the primary step in using LAM as a tool to build functionally gradient materials. Work has been completed at Sandia National Labs on understanding the microstructural development using the Laser Engineered Net Shaping (LENSTM) process to build H13 tool steel parts.a' 5 In the Sandia study, a thermocouple was used to measure the in-situ temperature excursions as a function of time resulting from layers being deposited to build the H13 tool steel part. Currently, experimental thermal history data from the manufacturing of Ti-6A1-4V parts is not available to the authors of this paper; therefore, one possible thermal excursion "scenario" will be presented in order to explain the formation of particular microstructures and to show the importance of such data. EXPERIMENT LasformedsM Ti-6A1-4V test coupons, similar to the one shown in Figure 3, were received from AeroMet to undergo microstructural evaluation. Each deposited layer was about 6 mm thick; however, each ripple seen in the photograph below is about 3 mm thick. The substrate is 6 mm thick Ti-6A1-4V plate, mill annealed at 700-730 'C for 2 hours. Sample Preparation Micrographic specimens were excised from the asdeposited coupons using a non-intrusive sectioning saw. A height-width profile (y-z plane in Figure 1) was polished and etched to reveal the microstructure. The etchant was a solution containing 100 ml of deionized water and 5 ml of hydrofluoric acid and was applied for 10 seconds. Hardness Measurements Vickers macrohardness and microhardness measurements were taken from the bottom to the top of the as-deposited part in order to determine if there was a hardness profile as a result of the temperature excursions experienced during the part build. Each indentation was measured 3 times in order to minimize error. Vickers macrohardness measurements were made using a load of 20 kgf applied for 10 seconds. For the microhardness measurements, a load of 300 gf was applied for 10 seconds.
Figure 3: As-deposited Ti-6AI-4V LasformedsM coupons.
RESULTS The as-deposited macrostructure along the height and width of the part is shown in Figure 4. There are several distinct features shown in this photograph, including the heat-affected zone (HAZ) in the substrate (Region A), large columnar prior-3 grains, and the presence of equally spaced horizontal bands near the cusps ofjoining layers (layer bands, Region B). The layer bands appear as a thin strip of darker material running along the width of the part. Each of these areas i
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