Why Some Metals Warp During Laser Cutting-And How To Prevent It
Laser cutting warping prevention is necessary to provide high standards of components that are used in precision products and assemblies. A warped metal alloy is of no use, even of the features cut are within design specifications. Metallics are unforgiving when it comes to their integrity in that that is no leeway for even slightly skewed profiles. Flexibility and the characteristics of formability do not occur in most metal alloys, and those that come close will be ultra-thin. For most metal precision parts cut by a laser process, not lying flat, will mean a non-conformance. While all metal alloys have characteristics that promote use in specific applications and environments, they can be damaged if the heat affect zones or HAZ are not controlled. Let’s review some alloys and how laser cutting overcomes potential warpage.
Metal Alloys and Laser Cutting
Metal alloys come in different raw forms for use in manufacturing such as blocks, tubes, rods, plates, rolls, sheets, and foils. When laser cut, the most common forms used are sheets and foils because the bulk of precision laser cut components are flat, 2D profiles. There are systems dedicated to cutting tubes and cylinders to form medical stents and other tools, but flat parts remain the niche that laser cutting supports. With a focus on producing high precision components that include challenging profiles, laser cutting warping prevention is as much an artform as it is a technological solution. Why do some metal alloys warp and compared to others and how it is controlled is not as straight forward as it may seem. Metal alloys have characteristics that give them strength, high thermal thresholds, conductivity, corrosion resistance, and machinability. These variables however are subject to the design constraints, the method used to machine, and other factors. For laser cutting specialists, what has worked for one project, may not be the best process for another of the same material.
To compare, a head-to-head review of different metal alloys cutting the same design, of 20mil thick stainless steel, titanium, and nickel with an array of 0.76mm holes with 1mm pitch (0.24mm spacing) for a total of about 3000 cut apertures. These were the results of using a fiber laser:
Nickel: This faired the best out of the three with minimal effect to the array flatness.
Titanium: The warping was considerable and enough to create unevenness where the laser focus was affected.
Stainless Steel: The results of cutting the array out of stainless steel indicated some warpage and in some areas. Potentially making a component non-conforming.
The result of these three metal alloys is surprising when it comes to the titanium, as this material has characterized by having a thermal threshold, but it does seem affected by the intense laser pulses that does did not allow heat dissipation but possibly intensified. This one example does not mean that the three metal alloys tested will always react as observed but does really show every project can have its challenges. To remedy warpage, we need to understand why metals can be prone during the laser cutting process.
Factors Affecting Metal Alloys During Laser Cutting
Metal alloys can warp during laser cutting due to the intense heat generated by the laser beam, which causes localized melting and rapid cooling. This rapid thermal cycling induces significant thermal stresses within the material. If these stresses exceed the yield strength of the alloy, permanent deformation, or warpage, can occur. Laser cutting warpage prevention depends on a complex interplay of material properties, cutting parameters, and part geometry. Here are the factors that contribute to warpage during laser cutting of metal alloys:
- Factors Causing Warpage and Adjustments Made:
- High Heat Input: The concentrated energy of the laser beam introduces a significant amount of heat into a small area, leading to rapid expansion and contraction.
- Rapid Cooling: The material surrounding the cut area acts as a heat sink, causing the molten metal and the heat-affected zone to cool quickly, resulting in uneven shrinkage.
- Thermal Conductivity: Alloys with lower thermal conductivity tend to retain heat longer, leading to a larger heat-affected zone and increased thermal stress.
- Material Thickness: Thicker materials require more laser power and multiple passes, increasing the overall heat input and the potential for warpage.
- Cutting Speed: Slower cutting speeds increase the dwell time of the laser on the material, leading to higher heat input.
- Part Geometry: Complex or intricate designs with thin sections or large unsupported areas are more susceptible to distortion. Designs with concentrated features such holes, slots, or any other areas where cutting will be prominent.
- Clamping and Fixturing: Inadequate or improper clamping can allow the material to move and deform under thermal stress.
- Internal Stresses in the Material: Pre-existing internal stresses from manufacturing processes like rolling or welding can be released during laser cutting, contributing to warpage.
How Warpage Can Be Controlled by Laser Cutting Specialists
Heat affected zones (HAZ), is a recurring factor in warpage of metal precision parts. Laser cutting warping prevention takes on several forms such as the programing of the cutting sequence, to the mounting of the metal sheet, and to how the profile is designed. As stated earlier, minor differences can make dramatic changes to the outcome of the final cut piece and by using both computer software and physical measures, warpage can be controlled as shown in the following list:
- Optimizing Laser Parameters:
- Reducing Laser Power: Using the minimum power necessary for a clean cut minimizes heat input.
- Increasing Cutting Speed: Faster cutting reduces the dwell time of the laser, limiting heat buildup.
- Pulse Mode Cutting: Using pulsed laser output can reduce the average heat input compared to continuous wave mode.
- Implementing Effective Cooling Strategies:
- Assist Gas: Using appropriate assist gases (e.g., nitrogen, argon) not only helps remove molten material but also provides some cooling.
- Optimizing Cutting Path and Sequence:
- Common-Line Cutting: Cutting adjacent parts with a shared line can help distribute heat more evenly.
- Tab and Slot Designs: Incorporating tabs and slots that are cut last can help maintain part stability during the main cutting process.
- Optimized Cutting Order: Cutting internal features before the outer contour can help manage stress buildup.
- Multiple Cutting Files: By programing the laser to cut in multiple files can allow the metal to cool while another area is cut.
- Employing Proper Clamping and Fixturing:
- Using a sufficient number of clamps: Securely holding the material prevents movement and deformation during cutting.
- Utilizing vacuum tables: Providing uniform support across the material surface can minimize distortion.
- Designing custom fixtures: Creating specialized fixtures can support complex geometries and prevent warpage.
Recommended Materials To Prevent Warpage
Choosing a metal alloy for your project can be a key factor, especially if it concerns warpage or oil canning. While there is not one metal that can be safe from HAZ, there are grades that are used for the ability to withstand heat while exhibiting other positive characteristics such indicated in this table:
| Alloy Name | Key Properties Preventing Warpage | Considerations For Laser Cutting | Common Applications |
|---|---|---|---|
| Aluminum 5052 | Good thermal conductivity, relatively low thermal expansion, good stiffness. | Fiber laser preferred optimized power, speed, with nitrogen assist gas. | Sheet metal work, automotive parts, marine applications, appliance components. |
| Aluminum 6061 | Good strength, good thermal conductivity, relatively low thermal expansion, high stiffness. | Fiber preferred optimize power and speed, use nitrogen assist gas. | Aerospace components, bicycle frames, automotive parts, structural components. |
| Stainless Steel 304 | Moderate thermal conductivity, good stiffness. | Fiber or ultraviolet laser -careful control of laser parameters. Assist gas (nitrogen or argon) | Kitchenware, food processing equipment, architectural panels, medical devices. |
| Stainless Steel 316 | Moderate thermal conductivity, good stiffness, excellent corrosion resistance. | Fiber or ultraviolet laser-preferred, careful control of laser parameters. Inert assist gas (nitrogen or argon). Depending on system type. | Chemical processing equipment, marine hardware, medical implants. |
| Titanium Alloys | High strength-to-weight ratio, low thermal expansion (some grades). | Requires careful control of laser power and speed, use of inert assist gas (argon or nitrogen), and proper fixturing to manage heat and prevent oxidation. | Aerospace components, medical implants, high-performance automotive parts. |
| Invar | Extremely low coefficient of thermal expansion. | The primary advantage is the inherent minimal thermal expansion reducing the driving force for warpage. | Precision instruments. |
This table lists alloys that generally exhibit better resistance to warpage compared to others under optimized laser cutting conditions. Actual warpage experienced will depend heavily on the specific laser cutting parameters used, the thickness and geometry of the part, and the fixture employed. The choice of alloy also depends on other required properties beyond warpage resistance, such as strength, corrosion resistance, and cost.
Conclusion
Metals warp during laser cutting primarily due to thermal expansion and the stress it induces. Alloys like titanium and Invar show better resistance to warpage under optimized conditions. Preventing warpage involves careful control of laser power and speed, using inert assist gases to manage heat and prevent oxidation, and employing proper fixturing. Additionally, selecting alloys that minimize thermal expansion and meet other required properties like strength and corrosion resistance is crucial.