Corrosion remains a significant threat to the reliability and sustainability of materials, with costs around 4% GNP in the UK alone. However, with adequate corrosion control, the cost of corrosion damages could be effectively reduced. Since corrosion is a surface phenomenon, surface engineering can be a solution to minimizing or eliminating such surface initiated failures by modifying the microstructure and/or chemical composition of the near surface region of a component without affecting the bulk material.
Surface engineering embraces a wide range of techniques from conventional galvanising, electroplating, thermal spray, to more emerging technologies based on application of electron, ion and laser beams. Among these techniques, a laser, as a chemically clean source of heating with unique optical properties, offers a wide range of surface engineering methods from heating to synthesis of finished components (Figure 1). In general, the laser beam is focused to a small spot with sufficient power density for a surface to absorb and convert into heat, to achieve surface heating or melting. Such process is characterized by an extremely fast heating/cooling rate (104–1011 K/s), that allows the development of an exotic microstructure and composition in the near surface region with large extension of solid solubility and formation of metastable even amorphous phases. These alloys with non-equilibrium phases offer the possibility of new material properties that cannot be achieved by conventional processing techniques.
Figure 1: Typical Laser surface engineering techniques for corrosion protection
Laser surface melting (LSM): In LSM, a thin surface layer, with a typical thickness from a few mm to a few hundred mm, is rapidly melted by a laser beam followed by rapid solidification to produce a microstructure different from that of the bulk material. The rapid cooling rate results in homogenization/refinement of microstructure and dissolution/redistribution of intermetallics or inclusions, leading to improved corrosion resistance. For example, aluminium alloy surfaces contain numerous intermetallic particles with dimensions up to tens of microns, providing driving force for microgalvanic activity and increased susceptibility to localised corrosion of the alloys in aggressive environments. Excimer laser surface melting of aerospace aluminium AA2024-T351 alloy can be applied for improved localised corrosion resistance by the removal of the intermetallics within the melted layer (Figure 2).
Figure 2: Typical cross section of excimer laser-melted AA2024-T351 alloy
LSM can be applied directly for corrosion protection. More importantly, LSM can also been used as a pre-treatment or post-treatment method for other surface coating techniques. The most commonly encountered example is LSM of thermally sprayed coatings to eliminate the inherent defects presented in thermal spray coatings, such as porosity, splat-structures and relatively weak mechanical bonding with substrate, for further enhanced corrosion/oxidation properties. In addition, our recent research has shown that the excimer LSM of aluminium alloys, as described above, can be successfully applied as a pre-treatment prior to conventional anodising for significant improvement of anodising efficiency and corrosion performance.
Laser shock peening (LSP): LSP is a relatively new process for inducing compressive residual stresses beneath the treated surface of metallic materials, by high magnitude shock waves induced by high-energy laser pulses. Compared with conventional shot peening, LSP produces higher magnitude compressive stresses of more than 1 mm in depth that is 4 times deeper than traditional shot peening. This process has been successfully applied to improving component resistance to crack initiation/propagation during cyclic loading and fatigue. LSP has been also applied to demonstrate great potentials for enhanced resistance to stress corrosion cracking and corrosion fatigue.
Laser surface alloying (LSA): LSA is to incorporate additional alloying elements into the surface of a component, by laser melting a coating and a portion of the underlying substrate. Typical thickness of the alloyed layer ranges from 1 mm to 2 mm. Due to the high cooling rates, non-equilibrium phases and supersaturated solid solutions can be readily achieved, which has beneficial effects on corrosion properties.
Laser cladding (LC) and pulsed laser deposition (PLD): LC is to produce a relatively thick, typically from 50 mm to 2 mm, and homogeneous overlay of coating material on a substrate with a fusion bond; whilst, PLD is a thin film deposition technique using high-energy laser pulses to vaporize the surface of a solid target inside a vacuum chamber and condensing the vapour on a substrate to form a thin film up to a few microns in thickness. Both techniques can produce protective barrier coatings/thin films against environmental degradation. LC has found wide uses for the protection of materials against corrosion and oxidation in a variety of applications.
Laser-assisted thermal spray: Improvement of integrity and performance of thermal-sprayed coatings can be achieved by LSM. However, such process requires additional steps. An effective combination of laser processing and conventional thermal spray systems, namely laser-assisted thermal spray, has been developed, that provides the opportunity for depositing dense coatings on large surface areas with acceptable processing efficiencies for industrial applications. The laser in such a combined system is used to reduce microstructural defects, improve cohesion within the coatings and achieve fusion bonding between the coating and the substrate.
In summary, LSE offers certain advantages over more conventional surface engineering techniques. The laser techniques can be used to selectively modify surface composition and microstructure without affecting surrounding material properties or causing serious thermal distortion. However, a laser beam with sufficient energy density has a limited beam size. Therefore LSM is less efficient than other conventional methods like thermal spraying, in terms of coverage rate. Generally, LSE techniques, except PLD, require no vacuum conditions; thereby substantial manufacturing flexibility can be gained. They provide chemically clean, non-contact, possibly remotely operated procedures that can be automated and integrated into other conventional production processes. The major challenge in LSE might be concerned to converting scientific feasibility into commercial reality.
Author:
Dr. Zhu Liu
Senior Lecturer,
Corrosion and Protection Centre,
School of Materials,
The University of Manchester, UK.
