Sub-surface laser engraving has been used for many years to burn images, patterns or words inside solid materials, typically glass. When a computer system is used to drive the movements of the laser head, the engraving process itself becomes extremely precise and fast. And because the process does not disturb the surface of the material, the images, patterns or words inside the material will never fade or deteriorate.
For a long time, sub-surface laser engraving has been used exclusively for producing markings inside transparent materials. Zhongli Li at A*STAR’s Singapore Institute of Manufacturing Technology and co-workers1 have now demonstrated the possibility of laser engraving inside metals. Using a ‘Nd-YAG’ laser, the team was able to create a series of small holes, or microcavities, below the surface of a block of stainless steel (Fig. 1). After surface polishing, the laser markings become completely invisible.
The new technology is useful for fabricating hidden markings that are difficult to remove and could therefore be used to make security watermarks in the automotive, aerospace and military industries. The markings themselves can be revealed using non-destructive techniques such as X-ray imaging.
An additional use of the technology is to create ‘closed’ porous metals surface. The desired density and distribution of pores, which determine the electrical resistivity, acoustic and electromagnetic properties of the patterned block, can be produced simply by programming the laser beam traces. The flexibility of patterning these closed porous metal surface could even open new avenues of research.
The researchers found that the working principle of sub-surface laser engraving in metals is very different from that in glass. When the laser pulse enters a slab of glass, the intense heat of the laser pulse causes changes to both the microstructure and the refractive index of the glass. It is these defects, or ‘cloud points’, that appear opaque under light. In contrast, the laser pulse is not able to penetrate a block of stainless steel and is instead absorbed at the surface. The intense heat generated by the absorption of the laser pulse causes the metal to melt, forming a microcavity when the laser point is moved and the molten material solidifies. The depth of the microcavity can be controlled by varying the peak power of the laser.
The researchers also demonstrated their technique using other metals, including mild steels and titanium alloy. “It will be interesting to see if the laser engraving technique is applicable to other materials. We plan to investigate this issue further,” says Li.
The A*STAR-affiliated researchers contributing to this research are from the Singapore Institute of Manufacturing Technology