Titanium and titanium based alloys Superplasticity

1 titanium , titanium based alloys

1.1 ti-al-mn (ot4-1) alloy
1.2 bulging process
1.3 case study

1.3.1 results , discussions
1.3.2 conclusion

titanium , titanium based alloys

in aerospace industry, titanium alloys such ti—6 al—4v find extensive use in aerospace applications, not because of specific high temperature strength, because of fact large number of these alloys exhibit superplastic behavior. superplastic sheet thermoforming has been identified standard processing route production of complex shapes, , amenable superplastic forming (spf). however, in these alloys additions of vanadium make them considerably expensive , so, there need developing superplastic titanium alloys cheaper alloying additions. ti-al-mn alloy such candidate material. alloy shows significant post-uniform deformation @ ambient , near-ambient temperatures.

ti-al-mn (ot4-1) alloy

ti-al-mn (ot4-1) alloy being used aero engine components other aerospace applications forming through conventional route typically cost, labour , equipment intensive. ti-al-mn alloy candidate material aerospace applications. however, there virtually little or no information available on superplastic forming behaviour. in study, high temperature superplastic bulge forming of alloy studied , superplastic forming capabilities demonstrated.

the bulging process

the gas pressure bulging of metal sheets has become important forming method. bulging process progresses, significant thinning in sheet material becomes obvious. many studies made obtain dome height respect forming time useful process designer selection of initial blank thickness non-uniform thinning in dome after forming.

case study

the ti-al-mn (ot4-1) alloy available in form of 1mm thick cold-rolled sheet. chemical composition of alloy. 35-ton hydraulic press used superplastic bulge forming of hemisphere. die set-up fabricated , assembled piping system enabling not inert gas flushing of die- assembly prior forming, forming of components under reverse pressure, if needed. schematic diagram of superplastic forming set-up used bulge forming necessary attachments , photograph of top (left) , bottom (right) die spf.

a circular sheet (blank) of 118 mm diameter cut alloy sheet , cut surfaces polished remove burrs. blank placed on die , top chamber brought in contact. furnace switched on set temperature. once set temperature reached top chamber brought down further effect required blank holder pressure. 10 minutes allowed thermal equilibration. argon gas cylinder opened set pressure gradually. simultaneously, linear variable differential transformer (lvdt), fitted @ bottom of die, set recording sheet bulge. once lvdt reached 45 mm (radius of bottom die), gas pressure stopped , furnace switched off. formed components taken out when temperature of die set had dropped 600 °c. easy removal of component possible @ stage. superplastic bulge forming of hemispheres carried out @ temperatures of 1098, 1123, 1148, 1173, 1198 , 1223 k (825, 850, 875, 900, 925 , 950 °c) @ forming pressures of 0.2, 0.4, 0.6 , 0.87 mpa. bulge forming process progresses, significant thinning in sheet material becomes obvious. ultrasonic technique used measure thickness distribution on profile of formed component. components analyzed in terms of thickness distribution, thickness strain , thinning factor. post deformation micro-structural studies conducted on formed components in order analyze microstructure in terms of grain growth, grain elongation, cavitations, etc.

results , discussions

the microstructure of as-received material two-dimensional grain size of 14 µm shown in fig. 8. grain size determined using linear intercept method in both longitudinal , transverse directions of rolled sheet.

successful superplastic forming of hemispheres carried out @ temperatures of 1098, 1123, 1148, 1173, 1198 , 1223 k , argon gas forming pressures of 0.2, 0.4, 0.6 , 0.8 mpa. maximum time limit of 250 minutes given complete forming of hemispheres. cut-off time of 250 minutes given practical reasons. fig. 9 shows photo-graph of blank (specimen) , bulge formed component (temperature of 1123 k , forming gas pressure of 0.6 mpa).

the forming times of formed components @ different forming temperatures , pressures. travel of lvdt fitted @ bottom of die (which measured bulge height/depth) estimate of rate of forming obtained. seen rate of forming rapid , decreased gradually temperature , pressure ranges reported in table 2. @ particular temperature, forming time reduced forming pressure increased. @ given forming pressure, forming time decreased increase in temperature.

the thickness of bulge profile measured @ 7 points including periphery (base) , pole. these points selected taking line between centre of hemisphere , base point reference , offsetting 15° until pole point reached. hence points 1, 2, 3, 4 , 5 subtend angle of 15°, 30°, 45°, 60° , 75° respectively base of hemisphere shown in fig. 10. thickness measured @ each of these points on bulge profile using ultrasonic technique. thickness values each of formed hemispherical components.

fig. 11 shows pole thickness of formed hemispheres function of forming pressure @ different temperatures. @ particular temperature pole thickness reduced forming pressure increased. cases studied pole thickness lay in range of 0.3 0.4 mm original blank thickness of 1 mm.

the thickness strain, ln(s/s0), s local thickness , s0 initial thickness, calculated @ different locations formed components. particular pressure thickness strain reduced forming temperature increased. fig. 12 shows thickness strain, ln(s/s0) function of position along dome cross section in case of component formed @ 1123 k @ forming pressure of 0.6 mpa.

the post-formed microstructure revealed there no significant change in grain size. fig. 13 shows microstructure of bulge formed component @ base , pole component formed @ temperature of 1148 k , forming pressure of 0.6 mpa. these microstructures show no significant change in grain size.

conclusion

the high temperature deformation behaviour , superplastic forming capability of ti-al-mn alloy studied. successful forming of 90 mm diameter hemispheres using superplastic route carried out @ temperature range of 1098 1223 k , forming pressure range of 0.2 0.8 mpa. following conclusions drawn: