Objectives
- Optimisation of the use of buffer layers and functional grading of materials to minimise residual stresses and deleterious cracking (often problematic between substrate and coating when working with dissimilar materials)
- Expansion of cladding parameter envelope towards higher laser powers speeds and powder feeds, to achieve higher deposition rates (production capability) via increased cladding speeds and/or increased layer thickness. The aim is to achieve a 50% reduction in laser cladding process time
- Develop repeatable control of thermal management via pre-heating and controlled cooling rates along a clad surface using induction heating systems. Confirmation of success via minimisation of cracking, avoidance of deleterious martensite in heat affected zones, etc
- Demonstration of at least two different material combinations on two industrial relevant applications
- Continuation of a feasibility development of a reliable in-line LUT monitoring system for crack formation in deposited coatings, to facilitate earliest possible in situ adjustment of cladding parameters
Project Outline
This CRP project will develop and demonstrate the Laser Metal Deposition (LMD) process for the deposition of suitable materials (for example: Triballoys, Colmonoys and WC particulate reinforced Ni-Cr alloys) onto industrially relevant components that are subjected to severe in-service wear and corrosion. Appropriate use of buffer layers and functional graded coatings (FGM) will be investigated to aid dissimilar materials joining (between substrate and deposit), along with the use of controlled induction pre-heating and post-clad cooling profiles in order to optimise the application of materials and minimise cracking in the clad layers. This work will be supported by LUT developments looking at the feasibility for online quality monitoring, supported by computed tomography and ultrasonic and 2D x-ray testing for integrity validation of coatings (samples for inspection are difficult to prepare by mechanical means). The outcomes of this programme of work will offer a knowledge driven coatings process with potentially significant production benefits for TWI and its members, including repair and remanufacture of worn in-service components, increased service lifetimes, reduced material usage and waste, inspection and validation.
There are many commercial surface coating technologies that use different heat sources to melt powders for surface cladding applications (Laser, Plasma, Arc, Flame, hot gasses). In the case of large surface area claddings, Gas Metal Arc Welding (GMAW also known as MIG), Submerged Arc Welding (SAW) or thermal spray techniques are often chosen based on their high material deposition rates. However, the heat input involved for GMAW and MIG results in large heat penetration leading to significant dilution of the clad material into the underlying substrate, high residual stresses and component distortion. Conversely, thermal spray coatings are only mechanically bonded to the substrate and can spall when subjected to impact, bending stresses or high loadings.
The laser is excellent for applying fully welded protective coatings, but with low dilution and with a highly controllable heat input to minimise distortion and the heat affected zone below the clad layer. Laser cladding is also characterised by its high flexibility in terms of gradient structure (transition function, layer thickness, phase content and composition) and component geometry in combination with high quality material.
However additional work is required to optimise and reliably deposit some of the more advanced protective coating alloys, particularly onto some of the more highly alloyed steels/nickel base substrates.
Industry Need
The application of advanced coatings to improve the service life of components is widespread in industry. Laser deposition offers significant advantages over conventional cladding techniques such as Tungsten Inert Gas (TIG), Plasma Transferred Arc (PTA) and thermal spray (HVOF), including more controlled and localised heat input resulting in excellent metallurgical bonding with low dilution and low distortion of substrate material and low thermal stresses in material. TWI is currently engaged in several SCP projects looking at the LMD application of coatings and repair on linear and complex surface profiles. However, there is a great deal of further significant enquiries from members and potentially new members about different material choices (carbides, Cermets), material/substrate combinations (through hardened substrates) and material property characteristics (wear rate / corrosion resistance / hardness) for a range of industrial applications:
- Hard facing surfaces on down-hole drill string tooling used in oil and gas industry
- Wear and corrosion protection on external and internal surfaces of valves, bearings, shafts and related components used in industrial pumps in sub-sea and industrial applications
- Elevated temperature wear and corrosion protection of rotating shafts used in industrial turbo machinery applications
- Hard facing surfaces on ground engaging tooling (protection around carbide/diamond cutting inserts) used in mining and minerals extraction industry
As a consequence of the significant interest, this activity has recently been identified as high priority by both LAM and VAL section technology road maps and SWOT analyses. There is a significant industry pull for this technology that requires the combined efforts of materials knowledge, surface deposition of graded layers and more efficient inspection techniques.