5 Key Technologies for Strengthening the Surface of Aluminum

Common aluminum surface treatment methods include anodizing, chemical conversion coating, electroplating, and micro-arc oxidation. As aluminum alloys are used in increasingly demanding environments and broader applications, the requirements for surface performance continue to rise. As a result, aluminum surface engineering technologies are constantly evolving, with a number of new research developments emerging.

To improve wear resistance and corrosion resistance, various surface strengthening technologies are widely used, including thermal spraying, surface coating, surface alloying, high-energy surface modification, and composite surface strengthening methods.

Arc Spraying

Arc spraying is a thermal spray process in which metal wires are melted using an electric arc. High-pressure gas is then used to atomize the molten metal into fine droplets, which are sprayed onto the aluminum substrate to form a wear- and corrosion-resistant coating.

The idea of this technology was first suggested in 1913 by the Swiss engineer M.U. Schoop. Arc spraying has evolved rapidly since the 1980s, with constant improvement of the technique through the use of newer variations like high-speed arc spraying, composite arc spraying, protective-atmosphere arc spraying, vacuum arc spraying, and plasma transferred arc spraying.

One of the most important thermal spraying technologies during the last 30 years has been arc spraying. It has been partially substituted in many applications by flame spraying and plasma spraying, particularly in long-term corrosion protection systems.

When applied to aluminum 6061 machined parts, arc spraying can produce a dense, uniform aluminum coating with low porosity. The coating is mechanically attached to the substrate. Sealing treatment has a significant effect on corrosion resistance, but it will not protect the base material from corrosion.

Plasma Spraying

Plasma spraying is a coating technique that utilizes a plasma arc to supply the heat source. Plasma temperature is up to 104 K, which is high enough to heat, accelerate,e and partly and/or fully melt the powder particle(s). These particles then collide with the surface of the substrate, flatten, solidify, and stack to create a coating.

In particular, this process is suitable for high melting point materials like ceramics, and the surface properties of Al alloys can be greatly enhanced.

Al₂O₃/TiO₂ nano and micro ceramic coatings were prepared on the surface of 6063 aluminum alloy, for example. The nano-ceramic coating had 3.5 times the hardness of the micro-ceramic coating. It has lowered the friction coefficient by 12.5%, and wear volume was only 60% of the micro-coating, which is much better than that of the aluminum substrate.

High-Velocity Flame Spraying

Powder particles can be given very high velocities using high-velocity flame spraying. The particles are deposited on the substrate with increased kinetic energy, thereby enhancing adhesion to the substrate and minimizing oxidation and decomposition of the sprayed materials.

For example, a WC/Co-NiCr coating with a hardness of 818 HV and a porosity of only 0.43% (on an aluminum alloy substrate) proved to be very resistant to wear and very integral.

Electroplating

Electroplating is a surface strengthening process that applies a relatively low-stress, fine-grained, smooth, and bright coating on aluminum machined parts.

Aluminum alloys, however, have a very negative electrochemical potential and a high affinity for oxygen, so pre-treatment is necessary before electroplating. Examples are zinc plating and copper or zinc underlayers for better bonding of the coatings.

Electroless Plating

Electroless plating is based on Ni-P alloy systems. It can also add other elements to create a functional composite coating without the need for external current, which can enhance the hardness, corrosion resistance, and wear resistance.

Electrical Discharge Deposition

Electrical discharge deposition involves subjecting the aluminum surface to high-energy electrical pulses between an electrode. When the spark is discharged, the wear and corrosion-resistant materials in the electrode are melted and diffused with the surface of the substrate, forming an alloyed layer that is metallurgically bonded.

For instance, when taking TC4 titanium alloy as the electrode, a surface film of Ti–Al alloy can be produced on the surface of the Al. The layer formed is made of intermetallic compounds (TiAl3, Ti3Al5, TiAl, TiN, TiO2, Al2O3) and is approximately 30 μm thick. Hardness can be up to 596 HV, and the volume of wear is about one-seventh that of the base material.

Surface quality and hardness can be further enhanced by improving processing conditions, such as adopting an oil immersion processing environment and special preparation electrode materials, such as tungsten carbide materials and titanium carbide materials,s that are prone to carbide formation under discharge.

Laser Surface Alloying

Laser surface alloying is based on the application of very high laser intensity in order to melt the surface of the aluminum substrate in a short time. The alloying elements of a coating layer dissolve in the molten surface layer and create a new surface layer with a different composition, structure, re and properties that substantially enhance the performance.

As an example, a Ni/WC alloyed coating was fabricated on ZL108 aluminium alloy by the laser alloying technology. The strengthening layer consisted of intermetallic compounds (Al₃Ni, AlNi₃, Ni₂Al) that provide strong metallurgical bonding with the substrate. Its depth of alloyed layer was up to 1.2 mm, while its hardness was nearly 8 times higher than the base alloy,y with greatly enhanced wear resistance.

Micro-Arc Oxidation

The process of Micro-arc oxidation forms a high-temperature and high-pressure environment on the surface of the aluminum material through micro-scale arc discharge. This changes the natural oxide film into a high-hardness & wear-resistant ceramic layer.

It is one of the widely used surface strengthening techniques for Aluminium Alloys.

In the case of aluminum alloy 7A52, micro-arc oxidation creates a ceramic film primarily consisting of α-Al₂O₃ and β-Al₂O₃. Surface hardness as high as 21Gpa, and wear resistance is about 109 times higher than that of untreated aluminum.

Laser Shock Peening

Laser shock peening changes the surface micro-structure by a high-energy laser-induced shock wave. It provides lower surface roughness and a deeper strengthening layer than traditional shot peening.

The maximum compressive residual stress and peak microhardness value are located in the vicinity of the surface layer in the case of LY12 aluminum alloy. The plastic zone is about 2 mm deep. The fatigue life has been improved by 131.4% and 132.5% following one and three laser impacts, respectively.

Ion Implantation

Ion implantation is a method of implanting ions into the surface layer without being constrained by solubility or diffusion problems. This process can be used to create non-equilibrium structures, which result in higher hardness and wear resistance.

For instance, ion nitriding treatment of aluminium alloys has the effect of creating AlN hard phases close to the surface. Microhardness is 228 HV, which is 1.4 times higher than that of the untreated samples. Mass losses are decreased 75%, and adhesive wear is also greatly reduced.

Composite Surface Strengthening Technologies

Surface strengthening methods, such as composite surface strengthening, are a group of surface modification technologies used in combination to obtain a composite effect that goes beyond the properties of individual processes. It combines various technologies, resulting in synergistic effects of the technologies.

In a coating and heat treatment process, for instance, the aluminum surface is coated with a layer of zinc, copper, and steel (20-30 μm thick). The subsequent heat diffusion treatment at approximately 150°C further improves bonding between the coating and the substrate, as well as increases the wear resistance.