Characterization of Laser-Deposited TiAl Alloys
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ceptible to oxygen contamination. In addition to the processing advantages in net shape, containerless melting (solid freeform) and dramatically shortened development cycle times for engineering systems, LENS processing can also offer fine, rapidly solidified microstructures (cooling rates up to 106 Ks'). Materials processed in this way experience small heat-affected zones, which will have significant potential for further improvements in properties relative to cast and wrought materials. In this paper, recent results are presented involving the microstructural characterization of TiAl intermetallics produced by LENS, with the aim of better understanding the effect of LENS processing on the microstructure, and ultimately on the mechanical properties, of the deposited materials. Comparison of microstructures produced by LENS and conventional processing techniques will also be given in the paper. EXPERIMENTAL The parts characterized in this study were produced with a CW Nd:YAG laser operated at 130190 W and focused with an six inch focal length lens to a spot size of approximately 500-600 p.m. KK5.2.1 Mat. Res. Soc. Symp. Proc. Vol. 552 0 1999 Materials Research Society
Figure 1. Some examples of the components deposited by LENS.
During deposition, the powder flowed continuously through the four focusing nozzles. The substrate was traversed at a rate of 8.5 mm.s-1 . Layers were built-up with a nominal layer thickness of 0.25 mm. A schematic diagram of the deposition experiment arrangement is shown in FIG 2. A two-phase TiAl alloy with composition of Ti-48A1-2Nb-0.4Ta has been deposited on the different substrates, which include stainless steel, a Nb alloy, a Ti alloy and TiAl.
(b)
(a) Z-axis Positioning of Focusing Lens and Powder Delivery Nozzle
Powder Delivery Nozzles
Laser Beam
Positio nli Powder Delivery Nozzle
Substrat e
Powder Delivery Nozzles
Beam/Powder . Interaction Region
Position II SX-Y Positioning Stages
-1
S s.
Substrate
Figure 2. (a) Schematic diagram of the LENS system configuration and (b) two Z-axis starting positions used in the present work.
Metallographic samples were prepared from the deposited materials using standard practices. The etched samples were examined with optical microscopy (OM) and SEM/EDS. For TEM, 3mm discs were taken perpendicular to the growth direction, from three different locations, i.e., the substrate, deposit, and the interface between these two regions. The 3mm discs were ground to -100mm thickness, then mechanically dimpled to -30mm and thinned to perforation using a Gatan DuoMillTM Model 600 ion-mill. A Philips XL-30 FEG SEM, a Philips CP-30 SEM, a Philips CM200 TEM equipped with a LaB6 filament, and a Philips CM300 FEGTEM were used for microstructural characterization. KK5.2.2
Typical TMP-21
a-processing + Hl"'
LENS + HT
1000-2000
30-50
5-20
2000-5000
200-500
2-10
250-400
500-700
to be determined
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