Using Direct Diode Lasers for the Heat-Assisted Forming of Sheet Metal

There is a growing demand for products fabricated from ultra-high-strength steels. While traditional ‘cold’ sheet metal forming methods, like roll forming and stamping, are ideally suited to mild steels, utilizing them for ultra-high-strength steels can lead to spring-back, cracking or distortion.1

Image Credit: Shutterstock/Roman Zaiets

These issues can be circumvented by raising the temperature of the steel before forming, but heating steel sheets can be very energy-intensive.

Monocrom edge-emitting laser diodes permit precise, efficient and targeted heating. This enhances sheet metal formability without the huge energy losses which comes with other heating methods.

The Benefits of Hot Forming High Strength Sheet Metals

A fundamental form of metal from which products and components are created is sheet metal. Applications include household appliances, buildings and body panels for aircraft and vehicles.

Sheet metal is transformed into these items by utilizing a variety of stretching and bending processes collectively known as sheet metal forming methods.

Stamping is one of these methods, whereby sheet metal is formed by pressing it against a die in a stamping press. Roll-forming is another technique where sheet metal is bent into a specific cross-section as it passes through a series of rollers.

Although these methods are ideal for shaping a wide range of different sheet metals, they are of limited use in processing ultra-high-strength steels. Global industry is moving towards enlarged use of ultra-high-strength steels, especially for automotive applications where weight reduction is crucial.2–4

The relatively high brittleness of these materials and the forces needed to shape them mean that traditional sheet metal forming methods, such as roll-forming and stamping, frequently lead to spring-back, cracking or distortion of the final product.5

‘Hot forming’ is the solution to this problem. This involves heating the steel plate prior to forming it. Heating can enhance the ductility of ultra-high-strength steels dramatically, making it possible to roll-form large bend angles and stamp high-strength steel sheets without springback.2,4

In industry, there are a variety of heating methods which are commonly used, including the use of electric furnaces and electromagnetic induction (‘inductive heating’). These heating methods have their own benefits and drawbacks, but they also share some common problems.

The main issue is that they are highly energy-intensive. Such heating methods usually heat the entire metal sheet which needs a huge amount of energy, and in turn, raises the costs. Further to this, heating some types of steel (particularly ultra-high-strength steel) can introduce weaknesses.

Luckily, these weaknesses are usually counteracted by work-hardening that happens during the forming process; but unworked areas can remain weakened from the heating process if the entire sheet is heated.2

Laser-Assisted Roll-Forming of High Strength Steels

By utilizing high-efficiency lasers to selectively heat only the areas of the steel which are to be worked, Monocrom direct diode lasers provide an alternative solution to the hot forming of steel.

Monocrom MET series lasers are based on edge-emitting diode technology and allow the advantages of hot forming to be realized without the related energy losses and weakening that occur when unworked areas of the sheet are heated.

The ductility of the workpiece can be fine-tuned by just heating the areas of the sheet that are to be worked. This allows the formation of complex structures with small bending radii and large bending angles without spring-back or cracks.

The lifetime of forming tools is also increased because of decreased forming forces; and residual stress in the metal during the forming process can be released, resulting in higher production accuracy. Controlled energy deposition and high efficiency minimize energy usage and lifetime costs.

Monocrom supplies two direct diode lasers which are optimized for forming ultra-high-strength steel: the MET R-9 and the MET S-9.

MET R-9

The MET R-9 is a direct diode laser that is designed for heat-assisted roll forming. It is made up of numerous laser bar stacks that are arranged in a horizontal array. It concentrates up to 5 kW of power onto a tightly focused straight line on the workpiece, with customizable line width and length.

MET S-9

The MET S-9 is also made up of a horizontal array of edge-emitting laser diodes and provides more flexibility and power for heat-assisted stamping of ultra-high-strength steels. Providing up to 10 kW of power, the MET S-9 enables the intensity distribution along the line to be fully customized, allowing more control of workpiece ductility.

The R-9 and S-9 both provide high electro-optical efficiency and extremely high maximum power density, permitting rapid and controlled heating with low power consumption.

The unique patented solder-free mounting technology means that Monocrom’s MET series lasers have a lifetime of more than 20,000 hours. That equates to over two years of constant usage or over five years of use at 10 hours per day.

MET Series Lasers from Monocrom

Monocrom’s Met DoForce is a versatile product in this product category and convinces through its long working distance, modularity and depth of field. A single module illuminates an area of 40 × 34 mm2 and power densities are as high as 1.2 – 1.5 W/mm2.

It comprises two laser diode stacks, which are individually controllable and can heat an area of 20 × 34 mm2. Depending on the total heated length needed, these modules can be mounted side by side, up to 5 in total, to increase it up to 200 mm.

Some other semiconductor laser arrays are prone to near-field distortion (known as the ‘smile’ effect) but the MET series lasers supply high power intensity with under 0.3 microns of smile distortion, allowing accurate and reliable power distribution.6

Monocrom MET series lasers offer a high performance have customizable dimensions and optical properties and are a low cost in-line solution for heat-assisted forming of sheet metal.

References

  1. Halmos, G. T. Roll Forming Handbook. (CRC Press, 2005).
  2. Lindgren, M., Bexell, U. & Wikström, L. Roll forming of partially heated cold rolled stainless steel. Journal of Materials Processing Technology 209, 3117–3124 (2009).
  3. Olsson, K. & Sperle, J.-O. New advanced ultra-high-strength steels for the automotive industry. AutoTechnology 6, 46–49 (2006).
  4. Mori, K. et al. Hot stamping of ultra-high-strength steel parts. CIRP Annals 66, 755–777 (2017).
  5. SSAB. Does roll forming work for advanced high-strength steel? https://www.youtube.com/watch?v=yDh7Kc6rSmU (2013).
  6. Wang, J. et al. Study of the mechanism of “Smile” in high power diode laser arrays and strategies in improving near-field linearity. in 2009 59th Electronic Components and Technology Conference 837–842 (IEEE, 2009). doi:10.1109/ECTC.2009.5074109

This information has been sourced, reviewed and adapted from materials provided by Monocrom.

For more information on this source, please visit Monocrom.

 

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