What is Heat Affected Zone (HAZ) ?
The heat-affected zone (HAZ) is the area of base material, either a metal or a thermoplastic, which has had its microstructure and properties altered by welding or heat intensive cutting operations. The heat from the welding process and subsequent re-cooling causes this change from the weld interface to the termination of the sensitizing temperature in the base metal. The extent and magnitude of property change depends primarily on the base material, the weld filler metal, and the amount and concentration of heat input by the welding process.
Cutting processes that use intense heat, like oxyfuel cutting and plasma arc cutting, produce thermal effects near the edge of the cut that lead to microstructural and metallurgical changes in the metal. The portion of a metal work-piece that has been so altered by heat is termed the “heat-affected zone” or HAZ. All thermal cutting processes create an HAZ in the cut metal.
The changes induced by heat can include :
- Altering the microstructure of particular steels, leading to an increase in the hardness of the cut edge relative to the un-cut metal.
- Altering the microstructure of particular steels, leading to a decrease in the strength of the cut edge.
- The formation of nitrides on the cut edge, which can affect the weldability of the cut face. Darkening or discoloration of the surface of the metal next to the cut face (“heat-tint”). Distortion of the metal being cut.
One disadvantage of the laser ablation process is that a heat affected zone is left behind where molten material re-solidified in situ or where material was sufficiently heated and cooled rapidly enough to result in embrittlement. This change in material properties can alter subsequent laser ablation and material performance. The advantage of ablating with a short duration excimer laser is that the pulse width is short enough to greatly reduce the transfer of heat out of the ablation zone.
This tends to localize the heat more and reduces the extent of the heat affected zone. The size of the heat affected zone is a function of the laser pulse duration and the material parameters such as thermal conductivity and specific heat. The heat affected zone will depend on the distance the heat is conducted within the material and varies with material and laser wavelength. A plot of the heat conduction distance as a function of wavelength is shown below. The better the material conduction (thermal diffusivity) the greater is the extent of the heat affected zone. The effect of the HAZ on the material (embrittlement, for example) is more a function of the material thermomechanical properties.
The width of the HAZ is influenced by:
- Cut speed – in general, faster speeds result in a smaller HAZ.
- Amperage (when using plasma) – for a given thickness of metal, a higher amperage (and consequently a faster cut speed) results in a smaller HAZ.
- The type of metal being cut. Different metals transfer heat at different rates and respond to differently to elevated temperatures. Increased temperatures and longer cutting times will result in a wider HAZ. As an example, a Plasma arc cutter can be used to cut any electrically-conductive material, but all things being equal it will create a different width HAZ on aluminum than on mild steel of the same thickness.
Associated with the heat affected zone is recast and formation of burrs. During the ablation process, the expulsion of material in the plasma jet creates a compressive on the molten pool of material under the laser spot. This will cause a portion of the liquified material to be forced out of the ablation zone and it will deposit onto the surrounding region. The amount of recast material can be minimized by a small absorption depth which will reduce the melted volume and a high laser power which will convert more of the melted material into vapor faster. Coupled with a the higher power is a shorter pulse duration which will have less dynamic impact on the relatively massive liquid. The duration of the forcing function is small compared to the dynamic response time of the liquid.