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Local Condition Chemical Descale
Double and triple melting practices are used to help eliminate segregation. Ingots range from 26 to 36 in. in diameter and weigh between 8000 and 15 000 lb.
A relatively new method, called cold hearth melting, is conducted in a water cooled copper hearth. By balancing the heat input from a plasma or electron beam with the heat extracted by water cooling, the melt can be held in the hearth for any period of time with only a solid layer of titanium, called a skull, separating it from the copper hearth. Since this method allows the titanium to remain molten for longer periods of time, it has the advantage of being able to reduce alloy segregation in the final product. Cold hearth melting has proven very successful in reducing the incidence of heavy metal high density inclusions because they sink to the bottom and become trapped in the mushy skull. A final VAR step is still required to improve the chemical homogeneity and the surface condition. Cold hearth melting, in combination with VAR, is now being used for fatigue critical jet engine components. Alloys produced by the VAR in combination with the cold hearth process have proven to be essentially free of melt-related inclusions.9
The majority of ingot defects that cause structural concerns are a result of segregation of alloying elements. Low density inclusions (LDI), hard-a, or a-I inclusions are interstitial defects resulting from high local concentrations of nitrogen in the original sponge that stabilizes the alpha phase and creates local hard spots with low ductility, usually associated with cracks that form during thermomechanical processing. These defects, Fig. 4.10, are hard brittle inclusions containing as much as 10wt% nitrogen, typically in the form of TiN(3). Hard-a is essentially an incipient crack that will propagate under stress, which adversely affects the fatigue strength of titanium alloys and has resulted in in-service failures.10
Another type of defect is high density inclusions (HDI), which can result from contamination introduced during the electrode preparation process. HDIs can be introduced by heavy metal contamination from sources such as tungsten carbide cutting tool edges or tungsten welding electrode tips inadvertently mixed with recycled machining chips. Since HDIs are essentially non-deforming at the stress
levels encountered in titanium alloys, they are sources of strain incompatibility that leads to fatigue cracking. Strict management of chips and turnings, including 100% radiographic inspection, has reduced the incidence of HDIs to relatively low levels.
Beta flecks, caused by solute segregation during solidification of the ingot, are regions of microsegregation that have a lower concentration of alpha than the surrounding matrix and are therefore high in beta stabilized material. They are most prevalent in large diameter ingots and with alloys that contain strong beta stabilizing elements, particularly the beta eutectoid elements chromium, iron, and copper. Beta flecks have different mechanical properties than the surrounding matrix and can be either stronger or weaker than the matrix. During cyclic loading, these regions cause steep local strain gradients and can cause early crack initiation.11 It should be noted that titanium producers go to great lengths to control the entire process to insure high quality ingots, including clean starting materials and a high level of process control throughout the entire process. All as-cast ingots are carefully ultrasonically inspected for the presence of defects.
Titanium alloys are available in most mill product forms: billet, bar, plate, sheet, strip, foil, extrusions, wire, and tubing; however, not all alloys are available in all product forms. Primary fabrication includes the operations performed at the mill to convert ingot into products. Besides producing these shapes, primary fabrication hot working is used to refine the grain size, produce a uniform micro structure, and reduce segregation. It has long been recognized that these initial hot working operations will significantly affect the properties of the final product.
Prior to thermomechanical processing, the as-cast ingot is conditioned by grinding to remove surface defects. The first step in deforming as-cast ingots is a series of slow speed steps including upset forging, side pressing, and press cogging to help homogenize the structure and break-up the transformed beta structure. Cogging is a simple open die forging process between flat dies conducted in slow speed machines such as a hydraulic press. The ingot is fed through the press in a series of short bites that reduces the cross-sectional area.12 Electrically heated air furnaces are used to preheat the ingot to 1300-1400° F and then it is forged at 1700-2150° F in large presses so that the deformation can be applied slowly to avoid cracking. These operations are initially done above the beta transus but significant amounts of deformation are also done below the beta transus, but high in the alpha + beta field, to further refine the microstructure to a fine equiaxed alpha-beta structure while avoiding surface rupturing. Working as the temperature falls through the beta transus is also an effective way of eliminating grain boundary alpha which has an adverse effect on fatigue strength. Final hot working must be carried out in the alpha + beta field to develop a microstructure that has better ductility and fatigue properties than if all of the hot working were conducted above the beta transus. In alpha-beta alloys, slow cooling from above the beta transus must be avoided or alpha will precipitate at the prior beta grain boundaries leading to a decrease in strength and ductility.
To obtain an equiaxed structure in near-alpha and alpha-beta alloys, the structure is sufficiently worked to break-up the lamellar structure and then annealed to cause recrystallization of the deformed structure into an equiaxed structure. The equiaxed structure obtained is a function of the prior microstructure, the temperature of deformation, the type of deformation, the extent of deformation, the rate of deformation, and the annealing temperature and time. The most important variable is to obtain sufficient deformation to cause recrystallization. In general, the finer the initial microstructure and the lower the deformation temperature (i.e., greater percent of cold work), the more efficient is the deformation in causing recrystallization. After forging, billets and bars are straightened, annealed, finished by turning or surface grinding to remove surface defects and alpha case, and ultrasonically inspected.
Slabs from the forging operations are hot rolled into plate and sheet products using two-and three-high mills. For thin sheet, pack rolling is often used to maintain the temperatures required for rolling. Four or five sheets are coated with parting agent and sandwiched together during rolling. Cross-rolling can be used to reduce the texture affect in plate and sheet. Specific hot rolling procedures for bar, plate, and sheet are proprietary to the individual producers; however, typical hot rolling temperatures for Ti-6-4 are 1750-1850° F for bar, 1700-1800° F for plate, and 1650-1700° F for sheet. Typical finishing operations for hot rolled material are annealing, descaling in a hot caustic bath, straightening, grinding, pickling, and ultrasonic inspection.
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