Protective Coatings for Titanium During Forging
Titanium and titanium alloys must be protected from contamination by oxygen, nitrogen, hydrogen and carbon during heating for forging. This is most effectively done by coating the forging slugs in a liquid glaze or glass which is allowed to dry before heating for forging. The coating fuses at between 500 and 600°C forming a viscous surface which protects the material from contamination. The proprietary liquid glass coatings have lubricating properties which assist metal flow during forging. Some forgers have found that the addition of a liquid based graphite to the die surface further improves die life and metal flow.
The Effect of Forging Temperatures on Microstructure
Since virtually all titanium forging alloys are double melted, they rarely contain segregation of other materials likely to cause variations in forgeability. The initial breakdown of titanium alloys is usually done above the beta transformation temperature because the body centred cubic structure is more ductile and forging pressure requirements are lower. However forging in the beta temperature range leads to excessive grain growth and attendant low ductility. Consequently final forging is usually done at temperatures just below the beta transformation temperatures.
Strain Rates and Forgeability
Variations in strain rate have little influence on forgeability of alpha and alpha plus beta alloys. Both alloy types are readily forgeable in hammers or presses. The beta alloy Ti13V11Cr3Al also exhibits good forgeability in both presses and hammers when forged above 760°C. However when forged just below 760°C the alpha phase begins to precipitate and the alloy is more susceptible to cracking, particularly in drop-hammer-forging.
The Effect of Forging Temperatures on Forging Loads
The forging temperature of titanium alloys is absolutely critical to the process and die heating is essential, as excessive heat losses through the tooling will produce defective forgings. As an example of the effect of temperatures titanium alloy Ti6Al4V when forged at 940°C requires the same forging load as SAE4340 steel forged at 1260°C. At 870°C forging data indicates that Ti6Al4V requires twice the forging load of SAE4340 steel.
Most forging companies advise that Ti6Al4V requires 1.5 to 2 times the equipment capacity needed for forging alloy steels in comparable shapes.
The marked effect of forging temperature on the forging load required for Ti6Al4V is characteristic of titanium alloys in general. Thus in ordinary die forging operations, cooling of the workpiece has a more detrimental effect on forging load for titanium than for steel.
Contamination During Forging
Despite the protection offered by the glass coating, a small amount of contamination does occur and must be removed by grinding or chemical etching.
When forging titanium, care should be taken to prevent contact with steel scale. A thermal type reaction can occur and seriously damage a forging die. Apparently the titanium reduces iron oxide in an exothermic reaction set off by pressure and high temperature.
Straightening of Titanium Forgings
Because of the low elastic modulus and relatively high strength of titanium alloys, forgings are difficult to cold straighten either by coining or reverse bending. Such operations are usually done at temperatures between 380-530°C. At times it is necessary to maintain a straightening load on a forging for several seconds. This technique is especially useful for removing large warpages.
The production of close tolerance, precision titanium alloy forgings has been successfully practised. However, such factors as excessive die wear, the need for expensive tooling, problems of microstructural control and contamination make the cost of close forgings excessive. Metallurgical quality is sometimes compromised if several dies are needed to produce a forging. Successful precision forging, therefore, is confined to small forgings such as blades and fittings that do not have complex flow patterns.