Volume 51, Issue 5

This article reports on the development of the rotational friction welding process for the welding of γ-TiAl- casting alloy Ti-47Al-3.5(Mn+Cr+Nb)-0.8(B+Si) to Ti6Al4V base metal and on the characterisation of the mechanical and microstructural properties of dissimilar metal friction welding. Pre- and post welding heat treatment is necessary in order to reduce the occurrence of bonding defects. The mechanical behaviour of friction welded joints under tensile loading at room temperature is primarily determined by the properties of the inter-metallic γTiAl- casting alloy base metal and not by the bonding zone itself. At the relevant service temperature of 700 °C, failure by fracture does not take place on the Ti6Al4V side of the dissimilar metal friction welded joint due to the super-plastic deformation of this alloy.

Carbon Fiber Reinforced Plastic composites were fabricated through vacuum resin infusion technology by adopting two different processing conditions, viz., vacuum only in the first and vacuum plus external pressure in the next, in order to generate two levels of void-bearing samples. They were relatively graded as higher and lower void-bearing ones, respectively. Microscopy and C-scan techniques were utilized to describe the presence of voids arising from the two different processing parameters. Further, to determine the influence of voids on impact behavior, the fabricated +45°/90°/−45° composite samples were subjected to low velocity impacts. The tests show impact properties like peak load and energy to peak load registering higher values for the lower void-bearing case where as the total energy, energy for propagation and ductility indexes were higher for the higher void-bearing ones. Fractographic analysis showed that higher void-bearing samples display lower number of separation of layers in the laminate. These and other results are described and discussed in this report.

Cold working is well known to change the properties of metals and alloys. Deformation increases the strength of metals but usually reduces its toughness and leads to anisotropy of properties, that is, directionality. Hot working also produces similar affects; the microstructural results after hot work with low finishing temperatures may appear to be the same as from cold working. Hot rolling of shapes, sheet, plate or bar, for example, elongates the nonmetallic inclusions in the deformation direction, which will reduce the isotropy of mechanical properties. Hot working can also lead to segregation being elongated parallel to the deformation direction, which also reduces isotropy. Reducing the finishing temperature, i. e., the temperature of the steel at the final deformation pass, will promote “banding” – parallel alignment of the constituents into layers, such as alternate bands of ferrite and pearlite. This also promotes anisotropy of mechanical properties, chiefly toughness and ductility. Strength is not usually affected to a significant degree by banding, compared to toughness and ductility. In single phase metals and alloys, the grains will become elongated due to cold working. In hot working, the grain shape may not be equiaxed upon cooling if the finishing temperature is not high enough to promote complete recrystallization upon cooling to ambient temperature.

A small fuel oil return line of a stationary gas turbine engine failed by high cycle fatigue cracking. In the metallurgical root cause investigation, performed by the original equipment manufacturer's laboratory, it was determined that insufficient support of the fuel line and the ensuing vibration led to dynamic alternating bending stresses, that had their maximum immediately adjacent to the connector weld of the flange. The piping system was made of metastable austenitic stainless steels. Constraint in the system at the flange, geometric notches in the weld caused by root reinforcement and excess weld metal at the weld toe, were all determined to have been contributing factors. What is more, the unavoidable metallurgical notch represented by the heat affected zone (HAZ) of the weld joint with its coarse-grained microstructure, may also have contributed to the cracking. The root cause of the failure was a design problem, because sufficient pipe support was not specified. The primary cause of failure was assembly, where the missing supports were not realised to be a potential problem. The organisational cause of failure was insufficient training of design and assembly personnel. Finally, the metallurgical or physical cause of failure was fatigue. It was recommended to provide for additional support brackets in the affected pipe length, in order to prevent the fuel oil return line from vibrating excessively.