Ryan Cottam

The role of metallurgical solid state phase transformations on the formation of residual stress in laser cladding and heating

There are two major types of solid state phase transformations in metallic materials; the formation of second phase particles during heat treatments, and the transformation of the matrix from one crystalline packing arrangement to another during either heating or cooling. These transformations change the spacing between adjacent atoms and can thus influence the residual stress levels formed. The heating and cooling cycles of materials processing operations using lasers such as cladding and melting/heating, can induce phase transformations depending on the character of the material being processed. This paper compares the effects of the different phase transformations and also the influence of the type of laser processing on the final residual stress formed. The comparisons are made between laser clad AA7075, laser clad Ti-6Al-4V and laser melted nickel-aluminium bronze using neutron diffraction and the contour method of measuring residual stress.

Development of a Processing Window for the Transformation Hardening of Nickel-Aluminium-Bronze

Nickel-Aluminium-Bronzes (NAB) are typically used in marine applications because of their good combination of corrosion resistance and strength. Even though these alloys exhibit good properties they do suffer from wear, corrosion, dealloying, cavitation corrosion-erosion or corrosion fatigue during service. Therefore methods of increasing the resistance of this class of alloy to surface sensitive damage mechanisms are desirable. Transformation hardening through laser processing offers the potential to increase the resistance of these alloys surface sensitive mechanisms of damage and increase their life. A processing window has been developed through the use of an analytical heat transfer model to determine laser processing parameters that are close to the critical temperature for surface melting. The absorption of the laser by NAB has been determined and the processing window calculated taking into account the velocity of the laser, laser spot size and type as well as laser power.

The effect of laser cladding deposition rate on residual stress formation in Ti-6Al-4V clad layers

The effect of deposition rate on the residual stresses formed during the laser cladding of Ti-6Al-4V powder onto a Ti-6Al-4V substrate was investigated. To isolate the deposition rate from the heat input an analytical laser cladding model was employed to control the melt pool depth to 0.1mm. The clad height was also held constant by the model at 1mm. The laser traversing speed was varied between 300 and 1500 mm/min. The residual stresses were measured using the contour method and it was found that the distribution of residual stress was similar for the different deposition rates and that there was a small variation in the tensile stress level reached in the clad and heat affected zone (HAZ) layer. The microstructures for all three clad layers were a’ martensite and the size of the HAZ was consistent from sample to sample. It was concluded that residual stress development is independent of deposition speed for the laser cladding of Ti-6Al-4V.

Potential application and certification of laser cladding technology for repair of ageing aircraft components

In-service damage from corrosion, wear or debris impact is increasingly common with ageing military aircraft fleets. Maintenance of this type of “ageing damage” can be expensive and have significant impact on fleet availability. DSTO (Defence Science and Technology Organisation) and DMTC (Defence Materials Technology Centre) are looking at a number of surface modification/or repair technologies that can be used on an opportunity basis to restore geometry or restore the life of an aircraft component. The basis of the technology selection was that there should be low risk with the technology, and the risk is in developing an approved process for the application to aircraft. With the advent of small high-powered lasers, laser cladding (LC) is one of the surface modification technologies examined. It uses a high-powered laser beam to melt a deposited layer of material onto a substrate. Laser cladding could offer significant through-life cost savings, as a repair alternative to the replacement of damaged components. DSTO and DMTC have demonstrated that laser cladding technology could potentially be used to repair or refurbish a range of different damaged components. This paper briefly summaries the current research work on laser cladding of 7xxx series aluminum alloys and discusses potential applications and a certification path for repair of aircraft components in the near future.In-service damage from corrosion, wear or debris impact is increasingly common with ageing military aircraft fleets. Maintenance of this type of “ageing damage” can be expensive and have significant impact on fleet availability. DSTO (Defence Science and Technology Organisation) and DMTC (Defence Materials Technology Centre) are looking at a number of surface modification/or repair technologies that can be used on an opportunity basis to restore geometry or restore the life of an aircraft component. The basis of the technology selection was that there should be low risk with the technology, and the risk is in developing an approved process for the application to aircraft. With the advent of small high-powered lasers, laser cladding (LC) is one of the surface modification technologies examined. It uses a high-powered laser beam to melt a deposited layer of material onto a substrate. Laser cladding could offer significant through-life cost savings, as a repair alternative to the replacement of damaged compo...