Search Results for Titanium

Corrosion failure occurred in a titanium clad tubesheet because of a corrosive tube-side gas-liquid mixture leaking through fatigue cracks in the seal welds at tube-to-tubesheet joints. The tubesheet was a carbon steel plate clad with titanium on the tube side face. The seal weld cracks were initiated by cyclic stress imposed by exchanger tubes. The gas-liquid mixture passed through cracks under tube-side pressure, resulting in severe corrosion of the steel backing plate. The failure started with the loosening of the expanded tube-to-tubesheet joints. Loose joints allowed the exchanger tubes to impose load on seal welds and the shell side cooling water entered the crevice between the tubesheet and the tubes. The cooling water in the crevice caused galvanic reaction and embrittlement of seal welds. Brittle crack opening and crack propagation in seal welds occurred due to the cyclic stress imposed by the tubes. The cyclic stress arised from the thermal cycling of the heat exchanger. The possible effects of material properties on the failure of the tubesheet are discussed.

Failure analysis was performed on threaded Ti-6Al-4V fasteners that had fractured in the threads during installation. Scanning electron microscopy (SEM) and optical metallography revealed that the fractures initiated in circumferential shear bands present at the thread roots. The fractures propagated by microvoid coalescence typical of that observed in notched tensile specimen fractures of the same material. For comparison, Ti-6Al-4V fasteners from various commercial sources were tested to failure in uniaxial tension and examined in the SEM. In all cases, the fracture appearances were similar to that exhibited by the fasteners that failed during installation. In addition, results of optical microscopy indicated that the geometry and extent of the shear bands appeared to depend on the fabrication process employed by the individual manufacturers. Causes of shear band formation are discussed along with potential methods to eliminate these microstructural in homogeneities.

Wear on a titanium screw head with a lip of material that that was transported by fretting at a plate-hole edge was studied. A flat fretting zone was visible on the screw surface over the material lip. A cellular wear structure containing wear debris was found. No morphological signs of corrosion were observed in connection with fretting structures.

Brief overheating of the 89 mm OD 6.4 mm wall thickness titanium heater tubes (ASTM B337, grade 2) was caused by a flow stoppage in a leach heater. Blue-tinted areas and patches of flaky white, yellow, and brown oxide scale was revealed on visual examination. It was disclosed by subjecting the overheated tube to a flattening test that the tube no longer met ASTM B 337 specifications. Large grain size and numerous needlelike hydride particles were disclosed in the microstructure of the overheated tube. Heating to approximately 815 deg C was revealed by the presence of the flaky oxide and increased grain size. Hydrogen and oxygen absorption was revealed by the presence of hydrides and the shallow surface embrittlement and thus susceptibility to cracking at ambient temperatures was observed. It was concluded that the titanium tubes were embrittled due to overheating the tubes and the severe surface embrittlement resulted from oxygen absorption which made the surface layers susceptible to cracking under start up and shutdown. Replacement tubes made of a heat-resistant alloy (e.g., Hastelloy C-276) were recommended.

Two titanium alloy wing attachment bolts from a commercial jetliner failed during the course of a routine service operation. Failure of the bolts occurred during the re-torque process as the wing was being reattached. Metallurgical failure analysis indicated that the fracture mechanism was ductile overload and that the mechanical properties of the bolts were consistent with exemplar bolts that had been supplied. After eliminating other sources of excessive load application, the most probable cause of failure was ascribed to variances between the frictional characteristics of the bolt at the time of re-torque and at the time of initial torque application several years earlier.

The U-0.8wt%Ti alloy is often used in weapon applications where high strength and fairly good ductility are necessary. Components are immersion quenched in water from the gamma phase to produce a martensitic structure that is amenable to aging. Undesirable conditions occur when a component occasionally cracks during the quenching process, and when tensile specimens fail prematurely during mechanical testing. These two failures prompted an investigative analysis and a series of studies to determine the causes of the cracking and erratic behavior observed in this alloy. Quench-related failures whereby components that cracked either during or immediately after the heat treatment/quenching operation were sectioned for metallographic examination of the microstructure to examine the degree of phase transformation. Examination of premature tensile specimen failures by scanning electron microscopy and X-ray imaging of fracture surfaces revealed pockets of inclusions at the crack origins. In addition, tests were conducted to evaluate the detrimental effects of internal hydrogen on ductility and crack initiation in this alloy.

Total knee prostheses were retrieved from patients after radiographs revealed fracture of the Ti-6A1-4 VELI metal backing of the polyethylene tibial component. The components were analyzed using scanning electron microscopy. Porous coated and uncoated tibial trays were found to have failed by fatigue. Implants with porous coatings showed significant loss of the bead coating and subsequent migration of the beads to the articulating surface between the polyethylene tibial component and the femoral component, resulting in significant third-body wear and degradation of the polyethylene. The sintered porous coating exhibited multiple regions where fatigue fracture of the neck region occurred, as well as indications that the sintering process did not fully incorporate the beads onto the substrate. Better process control during sintering and use of subsequent heat treatments to ensure a bimodal microstructure were recommended.

Recurring, premature failures occurred in TiN-coated M2 gear hobs used to produce carbon steel ring gears. Fractographic and metallographic examination, microhardness testing, and chemical analysis by means of EDS revealed that the primary cause of failure was a coarse cellular carbide network, which created a brittle path for fracture to occur longitudinally. As the cellular carbide network must be dispersed and refined during hot working of the original bar of material, the hobs were not salvageable. Minor factors contributing to the hob failures were premature wear resulting from lower matrix hardness and high sulfur content of the material, which contributed to lower ductility through increased nucleation sites. It was recommended that the hob manufacturer specify a minimum amount of required reduction for the original bar of tool steel material, to provide for sufficient homogenization of the carbides in the resultant hob, and lower sulfur content.

The cause of low fatigue life measurements obtained during routine fatigue testing of IMI 550 titanium alloy compressor blades used in the first stage of the high-pressure compressor of an aeroengine was investigated. The origin of the fatigue cracks was associated with a spherical bead of metal sticking to the blade surface in each case. Scanning electron microscope revealed that the cracks initiated at the point of contact of the bead with the blade surface. Energy-dispersive X-ray analysis indicated that the bead composition was the same as that of the blade. Detailed investigation revealed that fused material from the blade had been thrown onto the cold blade surface during a grinding operation to remove the targeting bosses from the forgings, thereby causing local embrittlement. It was recommended that extreme care be taken during grinding operations to prevent the hot, fused particles from striking the blade surface.

A fluorescent liquid-penetrant inspection of an experimental stator vane of a first-stage axial compressor revealed the presence of a longitudinal crack over 50 mm (2 in.) long at the edge of a resistance seam weld. The vane was made of titanium alloy Ti-6Al-4V (AMS 4911). The crack was opened by fracturing the vane. The crack surface displayed fatigue beach marks emanating from the seam-weld interface. Both the leading-edge and trailing-edge seam welds exhibited weld-metal expulsions up to 3.6 mm (0.14 in.) in length. Metallographic examination confirmed that metal expulsion from the resistance welds was generally present. The stator vane failed by a fatigue crack that initiated at internal surface discontinuities caused by metal expulsion from the resistance seam weld used in fabricating the vane. Expulsion of metal from seam welds should be eliminated by a slight reduction in welding current to reduce the temperature, by an increase in the electrode force, or both.

The head on a golf club driver developed multiple cracks during normal use. The head was a hollow shell construction made from a titanium alloy. Analysis and additional investigation revealed a progressive failure that initiated on the interior surface of the face plate along a deep, concentric groove created during a press forming operation. It was also determined that atmospheric contamination occurred during the welding of the head, causing embrittlement, which may have also contributed to the failure. Recommendations were made addressing the problems that were observed.