The yield strength-to-modulus ratio in titanium results in a significant amount of springback, as much as 15-25° after cold forming. To compensate for springback, titanium is normally overformed or hot sized after cold forming. Besides springback, cold forming takes more force, requires stress relieving between forming operations, and must be done at slow forming speeds to prevent cracking. Strain hardening also increases the yield and tensile strengths while causing a slight drop in ductility. Cold formed titanium alloys also experience a decrease in the compressive yield strength in one direction when a tensile strain is applied in the other direction, a phenomena known as the Bauschinger effect. For example, a 2% tensile strain applied to solution treated Ti-6-4 causes the compressive yield strength to drop to less than half; however, a full stress relief will restore the properties as shown in Fig. 4.17.
Except for thin gages, near-alpha and alpha-beta alloys are usually hot formed. Beta alloys, which contain the BCC structure, are much amendable to cold forming and one of them (Ti-15-3), along with the CP grades, can be successfully formed at room temperature in sheet form. In general, the bends must be of a larger radii than in hot forming. Titanium can also be stretched formed at room temperature at slow speeds with dies heated to around 300° F. Cold forming is usually followed by hot sizing and stress relieving to reduce residual stresses, restore the compressive yield strength, improve dimensional accuracy, and make the part more resistant to delayed cracking. During hot sizing, the part is held in fixtures or dies to prevent distortion. All sheet products that are going to
Fig. 4.16. Titanium Laser Deposited Rib Section Source: The Boeing Company
Fig. 4.16. Titanium Laser Deposited Rib Section Source: The Boeing Company be formed should be free of scratches, and gouges and burrs and sharp edges should be filed smooth to prevent edge cracking.
Hot forming, conducted at 1100-1500° F, greatly improves formability, reduces springback, and eliminates the need for stress relieving; however, the requirement to hot form increases the cost of titanium structures by increasing the cost by having to heat the material; the need for more expensive tools capable of withstanding the temperature; and the requirement to remove surface contamination (i.e., alpha case) after forming. In addition, the material needs to be free of all grease and any residue that could cause stress corrosion cracking. Severe forming operations are done in hot dies with preheated stock. Due to the tendency of titanium to gall, forming lubricants containing graphite or molybdenum sulfide will reduce both part and tool damage, especially for severe forming operations such as drawing. Since forming is usually done above 1200° F, alpha case will form on the surface of unprotected titanium and must be removed by either machining or chemical milling. Although forming can be done in a
Fig. 4.17. Bauschinger Effect on Ti-6Al-4V
vacuum, under a protective atmosphere, or with oxidation resistant coatings, the normal practice is to conduct the forming in air and then remove the alpha case by chemical milling.
Vacuum, or creep, forming can also be used to form sheet and plate to mild contours. In this process, the plate is placed on a ceramic tool that contains integral heaters. Insulation is then placed over the sheet followed by a silicone rubber vacuum bag. The part is slowly heated to temperature while a vacuum is applied to the bag. The part slowly creeps to shape under the combined influence of heat and pressure.
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