Materials Science Forum Vols. 539-543

Abstract: Titanium alloys are subject to high expectation of damage tolerance in terms of high fracture ductility and low rate of fatigue crack propagation to suit the preference in material consideration for aircraft manufacture. This paper reviews the researches in damage-tolerant medium- and high-strength titanium alloys.

Abstract: Microstructures of alkali- and/or heat-treated films on a Ti-15Zr-4Nb-4Ta alloy were analyzed by means of scanning electron microscopy, thin film X-ray diffraction and Auger electron spectroscopy. The cohesiveness of films was also evaluated by scratch tests. The films were formed by immersion in 5M aqueous NaOH solution at 60 °C for 86.4 ks (alkali treatment) followed by heating at 400–600 °C for 3.6 ks. The film on alloy formed by alkali treatment exhibits the same strucutre as that formed on an alkali-treated titanium. Compositional gradient of alloying elements, Zr, Nb and Ta, is detected in the film. The cohesion of alkali-treated film is considerably increased by the heat treatment, and the maximum cohesion is obtained by heating at 600 °C. The increase in cohesion of alkali-treated film by heat treatment is due to both the diffusion of Zr into film and the formation of sodium titanate on substrate.

Abstract: Friction stir processing (FSP) of cast NiAl bronze has resulted in significant increases in properties including more than doubling the yield greater that 40 percent increase in the threshold fatigue life; all achieved while increasing ductility. These and other strength, greater than a 60 percent increase in tensile strength, and property improvements were realized following studies of FSP procedures specifically for NiAl bronze. Within this manuscript, FSP procedures and other “lessons learned” are presented. Details of property improvements are documented elsewhere within this conference proceedings (see Fuller et al.). Presented herein are tool designs for efficient material flow, tool materials capable of long life at 1000°C, rastering procedures covering large surface areas, and other results pertinent to achieve improved properties in cast NiAl bronze following friction stir processing.

Abstract: Friction Stir Welding (FSW) has been applied to ultra fine grained (UFG) steel which is the plane carbon steel with the average grain size of 1mm. The plates of 5mm thickness was successfully welded using pcBN tool (polycrystalline cubic boron nitride) and the metallurgical and mechanical properties of the joint were evaluated. In the stir zone, Bainite has been formed which shows the highest harness in the joint. On the other hand, grain growth is observed in the heat affected zone. The joint effect of 88% has been obtained for UFG steel by FSW process.

Abstract: Friction stir welding (FSW) has been generating interest in association with friction stir processing (FSP), a new technique that employs FSW tooling. FSP is being investigated as a thermo-mechanical processing tool to transform a heterogeneous microstructure into a more homogenous microstructure.
However, very little data is available on the use of FSW for processing composite materials. In this study, a novel method of local manufacture of metal matrix composites (MMCs) using FSW is proposed and its application to friction stir spot welding (FSSW) is described. Trials investigating local manufacture of aluminum oxide particulate reinforced 6063 Al by friction stirring were carried out on a modified milling machine. The results are discussed in terms of weldability and residual microstructure.

Abstract: A thixomoulded magnesium alloy containing thermally stable Al2Ca phase was friction stir processed, and its microstructure and mechanical property were investigated. In friction stir zone, the cast structure of the base metal is replaced by fine magnesium grains containing fine dispersoides of Al2Ca. Due to the refined grain size of 0.8-1 μm, hardness of friction stir zone is higher than that of base metal. Texture was developed at top and bottom surfaces, (0002) being parallel to the plate surface, whereas (0001) plane is randomly oriented in the center of friction stir zone.

Abstract: Friction stir processing (FSP) has been employed for localized modification and control of microstructures in NiAl bronze materials, which are widely utilized for marine components. The thermomechanical cycle of FSP results in homogenization and refinement and the conversion of microstructures from a cast to a wrought condition within stir zones in the material. However, the direct measurement of stir zone temperatures, strains, strain rates and cooling rates is difficult due to steep gradients and transients in these quantities, and this is an impediment in the assessment of FSP-induced microstructures and properties. Quantitative microstructure analyses following FSP of cast NiAl bronze materials have been used to develop estimates of stir zone thermomechanical cycles. The estimation procedures will be reviewed and the microstructure-based estimates will be compared to results from computational models and embedded thermocouples measurements. Stir zone microstructures comprise a mixture of primary α grains and transformation products of the β that formed during processing. Recrystallization in the primary α occurred due to particle-stimulated nucleation in this low stacking fault energy material. Factors that influence the distribution of strength and ductility in the stir zone appear to include the mixture of microstructure constituents and gradients in microstructure due to gradients in processing conditions.

Abstract: Friction stir processing (FSP) produced local microstructural refinement in cast Ni Al Bronze. The refined microstructure quality was evaluated with mechanical property characterization using monotonic tension and fatigue testing as a function of FSP raster patterns. Modifying the cast NiAl bronze with FSP resulted in a 140 - 172 % increase in yield strength, and a 40 - 57% increase in tensile strength. Changing the raster pattern from a linear to a rectangular spiral raster increased the tensile elongations by 40 - 134%. This increase in elongation was attributed to increased microstructural uniformity through the depth of the FSP raster. The ability to transfer FSP technology was demonstrated with consistent tensile property data produced by three different laboratories. Fatigue characterization (both uniaxial and rotating-bending fatigue) showed that FSP improved the cast NiAl bronze fatigue resistance. Both types of fatigue testing showed differences in fatigue resistance as a function of processing parameters.

Abstract: Friction stir welding (FSW) was applied to a 0.53% nitrogen stainless steel. The nitrogen content change and the microstructural evolution in the weld were investigated. The nitrogen content analysis revealed that the stir zone (SZ) showed roughly the same nitrogen content as the base material (BM). This result suggests that FSW is an effective welding process for keeping up nitrogen content of high-nitrogen steel weld. The microstructural observation showed that the weld had the BM, the SZ, the partially recrystallized zone (PRZ) and the heat affected zone (HAZ). The coarse grain structure of the BM changed to relatively fine microstructure of equiaxially recrystallized austenite grain structure in the SZ during FSW. The PRZ contained both the fine and coarse grain structures. The detailed TEM observations showed that the particles with about 11m in size were present in the advancing side of the SZ both along grain boundaries and grain interiors. The Cr nitride less than 100 nm was formed in the HAZ along grain boundaries, which indicates that a slight sensitization occurred in the HAZ during FSW.