Material Properties
A number of parts were built to investigate the microstructure and mechanical properties of SMD parts. An initial concern from end-users was that SMD parts are always slightly discoloured. In traditional welding, this indicates O2 contamination. Analysis of SMD parts shows that the material has the same composition as the wire used and meets the standard for the Grade V Titanium alloy.
Oxygen and Nitrogen Levels in Typical SMD Parts |
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As-built part: |
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O %wt |
N %wt |
O %wt |
N %wt |
O %wt |
N %wt |
O %wt |
N %wt |
Wire Used |
0.142 |
0.003 |
0.142 |
0.005 |
0.145 |
0.008 |
0.145 |
0.008 |
SMD Build |
0.160 |
0.004 |
0.150 |
0.002 |
0.150 |
0.004 |
0.150 |
0.002 |
Grade 5 (MAX) |
0.200 |
0.050 |
0.200 |
0.050 |
0.200 |
0.050 |
0.200 |
0.050 |
Generally, the deposited material is fully dense, but it does contain a few spherical pores, all smaller than 200µm in diameter; these are within the limits allowed by AMS4999A. The material morphology shows a layered structure due to the deposition technique used. This also encourages the growth of large epitaxial, elongated grains which are inclined towards the weld torch. A cross section of the material shows that the final 10mm of the build has a different microstructure where the parallel bands visible throughout the part are absent. The bulk of the SMD material has a coarse Widmanstätten structure with acicular α phase lamellae in a retained β phase, whereas the final section of the build contains bundles of fine lamellae needles growing perpendicular to the surface.
Thermal and metallurgical models of the SMD process were developed, based on finite elements limited to the solid domain. The model was solved with hexahedral measurements, tri-linear interpolation of displacements and interpolation of stresses. A double-ellipsoidal heat-source was used to represent the TIG-torch. These models were validated using temperature evolution measurements during depositions and cut-ups of the parts produced. The model can correctly predict the size of the top region of the build with the finer microstructure as microstructural evolution is considered during the build. The sequential heating and cooling cycles are also modelled and this allows the percentage of martensite, grain-boundary a and Widmanstätten microstructure in the part to be estimated. Observations of micrographs of SMD material made using different parameter matched model predictions well
SMD parts are anisotropic, with the mechanical properties varying with the orientation of the deposited layers. Young’s Modulus is similar to low-oxygen Titanium while the High Cycle Fatigue (HCF) is superior to wrought Titanium. The SMD material meets AMS4999A for all criteria except for ductility horizontal to the build direction.
Mechanical properties of Ti-6Al-4V SMD material |
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Along the layers |
Perpendicular to the layers |
UTS |
1014 MPa |
936 MPa |
Ductility |
6% - 11% |
14% - 21% |
Young’s Modulus |
117 GPa |
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HCF |
770 MPa |
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