Identifying Techniques While Using A Laser Marking System
This is a fundamental guide to the common laser techniques of ablation, annealing, bonding, discolorization and engraving/etching.
Ablation: the simplest description of laser ablation is to selectively remove material from a solid surface in a controlled fashion.
Annealing: as defined by laser processing is the controlled heating of a substrate followed by cooling to produce a definitive micro-structure.
Bonding: is a laser processing technique that relies on the heat generated by the laser beam to “set” or fuse additives.
Carbon migration: occurs in metals and metal alloys when rapid heating causes nearby carbon molecules to be migrated into the specified area.
Discolorization: is most often associated with laser direct part marking of plastics where the laser burns or foams the substrate resulting in a color contrast from the original substrate.
Engraving and Etching: are terms that are used to express marking with depth.
In North America annealing and carbon migration are used more often when describing direct part marking on metals in general and steels specifically. While discolorization can indeed be used to describe metal (especially titanium) when a color change is affected by the heat applied through a laser beam laser, more often discolorization is associated with the direct part marking of plastics.
As a generalization of annealing, carbon migration and laser discolorization, these direct laser part marking techniques use lower energies at the work piece than ablation, cutting or engraving / etching which removes material.
In the best examples of direct part surface marking the heat from the laser causes an apparent color change from the background material which gives a mark that is flush and smooth. The trade off of these techniques may be long term permanence in high wear applications. Carbon migration (AIAG Spec B-17) and laser discolorization (SAE AS9132) are standard terms in our industry to describe the effects of darkening materials. Annealing is a wide-ranging term and is often used to describe the same technique.
SAE AS9132 makes the following statement regarding these techniques:
“As the mark relies on thermally induced surface discoloration, it is unsuitable for applications where the component operating temperature results in significant oxidation of the part, parts which are exposed to an aggressive environment in operation or rework, or where the risk of fretting of the mark is present. Due to its relatively non-aggressive application it can be considered suitable for thin sections and cooler components.”
Laser Annealing is a comprehensive term associated with heating. The practice of such a heat treatment may be: to remove stresses; to induce softness; to alter ductility; toughness; electrical magnetic, or other physical properties; to refine the crystalline structure; to remove gases; to produce a definite micro-structure.
Laser Annealing can be used to describe heating and cooling processes for metals and non-metals alike and appears to have no direct relationship to the presence or absence of carbon. In the broadest sense, the term annealing is correctly used when described in laser marking as heating followed by cooling to produce a definitive micro-structure.
Laser Ablation the simplest description of laser ablation is to remove material from a solid surface in a controlled fashion. By this description, it is more closely associated with micromachining than with marking. Laser ablation techniques are used to clean surfaces, remove paint or coating, or prepare surfaces for painting without damaging the underlying surface.
Laser Bonding is a laser marking technique that uses laser generated heat applied to a part to bond a contrasting additive substance to a wide range of substrates. This technology delivers permanent marks on metals, glass and ceramic parts for a diverse range of decorative and part traceability applications, ranging from aerospace to the awards & engraving industries. It differs from the more widely known techniques of laser ablation, engraving and etching in that it is an additive process, adding material to the substrate to form the marking instead of removing it.
Laser Carbon Migration is borrowed from welding where carbon migration is considered to be a significant factor in determining the quality of a weld and appears to be a well studied phenomenon. Carbon migration (AIAG Spec B-17) occurs when molecules such as chromium are boiled off during heating and nearby carbon molecules are migrated into the area.
Laser Discolorization (SAE AS9132) is a laser marking technique used extensively in the marking of plastic substrates. This technique causes the surface substrate to burn, foam or bubble, causing contrast against the original color of the part.
“Discolorization” correctly describes the look of the resulting laser direct part mark compared to the original substrate. “Discolorization” was selected to differentiate this technique from laser colorization bonding techniques. Laser bonding of colored material is typically a patented method that uses materials suspended in a slurry that bind to the surface when heated by the laser beam or instead uses laser enhancers to increase contrast.
Laser etching or engraving describes a direct part marking technique that uses the laser to remove and vaporize material, creating a laser mark that has both width, depth and the remnants of resolidified material (recast) around the mark. It is also characterized by a change in the localized structural features of the material identified as the “heat affected zone” (HAZ).
Laser etching is generally agreed to be a shallow cutting mark with depth less than 0.001 inch while laser engraving is a typically agreed upon as a deeper cutting mark over 0.001 inch. While there are no universally applied U.S. Government standards as to what defines a deep and permanent mark some indication can be obtained from ATF Federal Firearms Regulations Reference Guide (ATF Publication 5300.4 Revised September 2005) which states a minimum depth for serial numbers on weapons as 0.003 inch and in a print size of 1/16 inch.
As a generalization this direct part marking technique uses higher energy at the work piece than an annealing or discolorization mark.
Laser engraving is the most permanent of marking techniques and will withstand harsher environments because of the depth and the resulting HAZ, however it should be noted that SAE AS9132 makes this warning: “An increase in the depth of mark will tend to improve in-service readability. However it may have a detrimental effect on local surface integrity, which is also affected by extent of recast layer, heat-affected zone and micro cracking. It is therefore the responsibility of the design authority to define the acceptable depth limits dependent on component usage”.