Laser surface treatment of hot work tool steel

As an important part of die steel, hot work die steel is often used in high temperature applications such as stamping press dies and forms.  The hot die is subjected to friction caused by repeated metal deformation and temperature variation of forced cooling, resulting in surface wear, thermal fatigue crack and corrosion.

Damage of hot die steel usually starts from the surface, and the service life of hot die steel can effectively improved by surface strengthening technology.  Common surface modification methods include chemical heat treatment, diffusion/ infiltration, ion implantation and vapor phase deposition, which all have disadvantages such as long treatment period, high technical requirements, poor surface binding and thin coating.

The laser surface modification technology is to change the physical and chemical properties of metal materials through the action of high-energy density laser beam on the surface, so as to improve abrasive resistance, corrosion resistance, thermal fatigue resistance and oxidation resistance.  There are different process applicable:

1. Laser phase change hardening

Laser beam with energy density of 104 ~ 105W·cm² was directed onto steel surface to instantly raise local temperature above the phase transition temperature.  High temperature surface layer was austenised while the core or sublayer temperature remained unchanged.  After the heating is stopped, the surface cools itself at 104 ~ 106℃/s to achieve the self-hardening effect and form a strengthened layer.

2. Laser melting

The laser beam with an energy density of 105 ~ 106W· cm² was applied onto metal surface to melt temperature, it is then cooled rapidly through its own heat conduction.  On the surface of the material, a fusion layer with the same composition as the matrix but different alloy element saturation and grain size was formed.  It can effectively remove impurity elements, cracks and pores on the surface of materials and improve the surface properties.

3. Laser cladding

The laser beam with an energy density of 105 ~ 106W·cm² was applied on the steel surface to make the substrate surface thin layer melt together with the cladding material to form the metallurgical bonding layer. Compared with laser alloying, laser cladding has a lower dilution rate and can maintain the properties of the cladding material.

4. Laser alloying

The laser beam with an energy density of 105 ~ 106W·cm² was applied on the steel surface to make alloying elements and matrix materials melt, mix and solidify rapidly at the same time to form an alloying layer with required depth and chemical composition.

Among the four methods, laser phase change hardening improves the surface properties by heating metal materials, while laser melting, laser cladding and laser alloying change the surface properties by melting.

The future development direction of laser surface modification technology is: laser surface modification technology and automatic control technology development, laser surface modification equipment is moving towards high-power, automation and intelligent development; The combination of laser surface modification technology and remanufacturing can develop and utilize waste products with value, reduce waste pollution, prolong parts life and increase economic benefits. The combination of computer science and material science can effectively improve the research efficiency of laser surface modification technology and save the research cost.