Laser-beam machining thermally removes material by applying a coherent beam of monochromatic light on a part that needs to be processed. When the laser beam touches the workpiece surface, the material melts and “splatters” around the cut (or around the hole in case of laser-beam drilling). Unlike conventional machining, the material removal by laser does not leave a chip behind. Figure 1 shows the splattering of melted material around a laser-drilled hole.
Once the melted material solidifies, it will form a so-called “recast layer” around the cut or around the drilled hole. The thickness of the recast layer depends on the type of material the workpiece is made of, type of laser, laser-beam parameters and thickness of the cut. Figure 2 shows the recast layer resulted from LBM.
This recast layer is detrimental to the part. It creates tensile stress on the surface of the part that may initiate a crack, or it will propagate further into the base material if the crack already exists. A thicker, continuous and more homogenous recast layer will increase the stress on the workpiece surface, negatively affecting the life of the part.
Typically, either the recast layer is removed by mechanical means (e.g., grinding, deburring, etc.) or a maximum thickness of the recast is allowed based on the part application. However, there are some cases when mechanical removal of the recast is not entirely possible due to a difficult configuration of the part.
Therefore, some other means of recast removal or decreasing the thickness of it to an allowed limit may be necessary. This study looks into the effect of heat treatment performed after laser cut on the thickness of the recast layer.
Three types of alloys were used in this experiment: 6061 aluminum, 321 stainless and Incoloy 931. Samples were cut from each alloy using a fiber-optic laser machine (Figs. 3, 6 & 9), and the recast layer was measured in the etched condition.
For aluminum alloy 6061 (AMS 4025) at 0.063 inch thick, samples were solution heat treated at 985°F for 1 hour, water cooled and naturally aged to condition T4. Samples were etched using Kellers agent (Figs. 4 & 5).
For stainless steel CRES 321 (AMS 5510) at 0.023 inch thick, samples were annealed at 1800°F for 20 minutes in a vacuum furnace with a maximum pressure of 1 micron (µm) and cooled down to 175°F in argon fan gas. Samples were etched using Fry’s agent, and no oxidation was observed on the sample surface (Figs. 7 & 8).
For Incoloy 903 (UNS N19903) at 0.082 inch thick, samples were solution heat treated and precipitation hardened. Samples were etched using modified Kalling’s agent (Fig. 10).
The recast layer resulting from the laser cut was measured before and after heat treatment (in the worst-case condition). The results are summarized in the table.
The thickness of the recast layer significantly decreased after heat treatment of that material. Furthermore, the transition from the recast to the base material becomes less distinctive, showing a strong bond between recast and base metal.
In conclusion, heat treatment is able to improve the surface quality of a part that was processed by nonconventional machining, and it can be used in some particular applications where the recast layer cannot be removed or reduced by mechanical means.