Guest editorial

Full text: With the rapid development of the aerospace industry, manufacturing technologies have to continuously develop and adjust themselves to ever-growing demands coming from more complex component designs and the use of highly engineered materials. Today there is an increased number of manufacturers contributing to the realization of final products, i.e. avionics, so it is easy to perceive the truly globalized dimension of the aerospace manufacturing business. With this comes the demand for further engineering developments on which the academic/industrial research institutes need to deliver solutions to real aerospace manufacturing problems. This is a challenging task since aerospace manufacturing technologies have to cover a wide range of materials (from composites to advanced Ni/Ti alloys), processes (from forging to non-traditional machining and assembly), and parts’ dimensions/batch sizes (from airframes to turbine blades). In this wide context, this Special Issue includes high quality theoretical and experimental scientific contributions on the following topics related to the aerospace manufacturing technology: (a) machining of advance aerospace alloys; (b) abrasive processes applied to aerospace components; (c) surface treatments to enhance fatigue performance of aerospace components; (d) joining and assembly of aerospace components; (e) laser machining of aerospace alloys; (f) automated/supervised manufacture of aerospace components; (g) quality supervision of aerospace manufacturing routes. The breadth of topics in this Special Issue is perhaps indicative of the complexity and challenges that the research related to aerospace manufacturing technology can offer. We hope that this issue will act as a catalyst for the development of further research, academic and industrial interactions, and publications related to aerospace manufacturing technologies for the benefit of the academic and industrial research communities.

Cracking in the welding of cupro-nickel alloys

The problem of variation in weld crack susceptibility caused by small variations in alloy and impurity elements for the 70-30 cupro-nickel alloy has been investigated. Both wrought and cast versions of the alloy have been studied, the main techniques employed being the Varestraint test and weld thermal simulation. In the wrought alloys, cracking has been found to occur mainly in the weld metal, whilst in the cast alloys cracking is extensive in both weld metal and heat affected zone. The previously reported effects of certain impurities (P,S,Si) in increasing cracking have been confirmed, and it has also been shown that Ti and Zr may both have a crack promoting effect at levels commonly found in cupro-nickels, whilst C can interact with several of the other elements investigated to produce a beneficial effect. The testing carried out using the weld thermal simulator has shown that a relationship does exist between hot ductility and weld cracking. In particular, the absence of the peak in ductility in the range 1100°C-900°C on cooling from a temperature near to the solidus is indicative of a highly crack susceptible alloy. Principal practical implications of the investigation concern the relationship of weld metal cracking to alloy composition, especially the level of certain impurities. It would appear that the upper limits permitted by the alloy specifications are unrealistically high. The introduction of lower impurity limits would alleviate the current problems of variability in resistance to cracking during welding.

The relationship between fracture toughness and microstructure in titanium alloys

Linear Elastic Fracture Mechanics has been used to study the microstructural factors controlling the strength and toughness of two alpha-beta, titanium alloys. Fracture toughness was found to be independent of orientation for alloy Ti/6A1/4-V, but orientation dependent for IMI 700, bend and tension specimens giving similar toughness values. Increasing the solution temperature led to the usual inverse relationship between strength and toughness, with toughness becoming a minimum as the beta transus was approached. The production of a double heat treated microstructure led to a 100% increase in toughness in the high strength alloy and a 20% increase in alloy Ti/6A1/4V, with little decrease in strength. The double heat treated microstruoture was produced by cooling from the beta field into the alpha beta field, followed. by conventional solution treatment and ageing. Forging above the beta transus led to an increase in toughness over alpha beta forging in the high strength alloy, but had little effect on the toughness of Ti/6A1/4V. Light and electron microscopy showed that the increased toughness resulted from the alpha phase being changed from mainly continuous to a discontinuous platelet form in a transformed beta matrix. Void formation occurred at the alpha-beta interface and crack propagation was via the interface or across the platelet depending on which process required the least energy. Varying the solution treatment temperature produced a varying interplatelet spacing and platelet thickness. The finest interplatelet spacing was associated with the highest toughness, since a higher applied stress was required to give the necessary stress concentration to initiate void formation. The thickest alpha platelet size gave the highest toughness which could be interpreted in terms of Krafftt's "process zone size" and the critical crack tip displacement criterion by Hahn and Rosenfield from an analysis by Goodier and Field.