HOT-STAMPED STEEL CREATES NEW APPLICATIONS FOR PRODUCTION LASER CUTTING

With the escalating energy crisis and environmental problems, energy saving and safety have become very important issues for the automotive industry.

At the end of 2007 a package of energy measures was approved in the U.S., including a 40 percent increase by 2020 in fuel economy standards for cars and light trucks sold in this country. One of the most effective ways to achieve fuel efficiency is by weight reduction. Considering that most vehicles are at least 70 percent metal, the property of AHSS (advanced high strength steel) and hot stamped steel are very interesting to most automotive manufacturers.

Driven by the automotive industry?s need to improve passenger protection while reducing vehicle weight and increasing fuel economy, the steel industry has been highly successful at developing advanced-high-strength steels and hot-formable steels.

Hot stamping with die quenching of boron steels appeared at the end of the 1990s for producing some simple automotive parts like door beams and bumper beams. Since then, hot stamping (also called press hardening or hot forming) is the preferred process for complex, high-strength components that require high levels of precision while maintaining manufacturing affordability.

In contrast, the forming process of traditional ultra-high strength steels (UHSS) considerably limits part complexity due to spring back conditions. Intricate shapes with no spring back issues are being accomplished by hot-forming steels at temperatures above the austenitic region (1652-1742 deg F/900-950 deg C).

The hot-formed steel achieves tensile strengths above 1300 Mega Pascals (MPa) after quenching in the die, providing the necessary strength to be used for safety and structural parts while reducing a vehicle's body structure mass by as much as 25 percent. With so many advantages and very few drawbacks, it is easy to predict that the use of hot-formed steels in the automotive and possibly in other industries, will dramatically increase in the next few years.

Hot-forming is a temperature and time dependent process and consists of several well optimized steps. It also requires a high level of expertise and proprietary knowledge of the process to achieve excellent product quality. The required capital investment, in the order of multi-million dollars for the press, the furnace and the relevant automation, has limited the number of players in this highly technical arena.

In hot stamping, forming and hardening are combined in a single operation. Two different methods are used: direct and indirect.

DIRECT METHOD
In the direct method, the blanks are heated at temperatures between 1652 and 1742 deg F (900 and 950 deg C) for four to ten minutes inside a continuous feed furnace and subsequently transferred to an internally cooled die set. The blanks are stamped and cooled down under pressure for a specific amount of time according to the sheet thickness and the profile complexity.

During this period, the formed part is cooled in the closed die set at a gradient of 68 deg to 86 deg F/s (20 to 30 deg C/s), completing the quenching process. The total cycle time for transferring, stamping, and cooling in the die is 15 to 30 seconds. The part leaves the hot-stamping line at about 302 deg F (150 deg C) with high mechanical properties, an ultimate tensile strength of 1,400 to 1,600 MPa and a yield strength between1,000 and 1,200 MPa.

INDIRECT METHOD
The indirect method requires a part to be drawn unheated, to about 90 percent to 95 percent of its final shape in a conventional die, followed by a partial trimming operation, depending on edge tolerance. The preformed part is then heated and hardened in the die. The additional step extends the forming limits for very complex shapes by heat-treating the cold-formed parts.

LASERS TO THE RESCUE
This new type of steel processing requires a new generation of 3D processing equipment. The CO2 laser is uniquely positioned and is the most efficient tool available today to post-process hot-formed steel, since the thermal reaction is unaffected by the steel strength.

Conversely, traditional mechanical means to trim and pierce the formed parts will produce extremely high tool wear. In addition, the flexibility provided by the laser allows for easy changes of accurate trim lines and features size or location adjustments, which is unattainable by mechanical process.

At present, only a handful of Tier 1 suppliers have mastered the hot-forming process and they have not entrusted job shops for the laser processing of the parts. These companies have elected to do the final laser processing in-house to have tighter quality control, timely supplies of parts as soon as they are stamped, and to reduce the cost of massive material handling that would otherwise be required by shipping thousands of parts to other laser processing houses. This approach provides for parts produced at a reasonable market price with the added bonus for the OEMs of part traceability.

In the absence of suitable mechanical means, the laser processing of hot-formed materials has progressively improved in terms of the maximum achievable cutting speed. In 1995, hot-formed parts were cut using oxygen as the assist gas. The typical laser source was below 2500 watts and general cutting speeds were around 100 ipm. The exothermic reaction using oxygen on thickness below 0.080 in is a limiting factor and, even today, maximum cutting speeds are around 400 ipm.

Now lasers with a higher beam quality, capable of being focused to small spots of high power density and power up to 5000 watts, make it possible to use an inert assist gas like nitrogen and achieve cutting speeds in excess of 780 ipm. An added benefit of using N2, other than being able to achieve higher speed, is that the cut edge is free from oxide and is therefore weld and paint ready with no secondary operation. A typical automotive part, such as a B-Pillar, has a cycle time of less than 50 seconds when cut with N2. There is currently no alternative process capable of providing this sort of result with the same operating cost as a laser.

The growing demand for hot-formed parts has positioned the laser as a viable process for such volumes, requiring multi-axis machines to be capable of sustained operation in 24/7 production environments. This new environment is very different from that of a few years ago, where lasers were generally utilized for prototyping low volume batches, and it has required a new generation of multi-axis laser systems.

Development of the mechanical structure, the aid of FEM (finite element method) during the mechanical design phase, the use of exotic materials like carbon fiber to provide high rigidity and low weight, high speed processors and servo systems with fiber optics feedback, have created laser machines with acceleration of 0.8g and rapid speeds of 5,511 ipm while maintaining accuracy of 0.001 in all on a large, Cartesian machine tool.

Laser power up to 5,000 watts and an adaptive sensor with 4g acceleration, capable of maintaining a constant focal point position in any condition, have increased the cutting speed well above 780 ipm in real production environments. Clever solutions like a turntable or the separation of the work envelope in two distinct areas have allowed for no-idle time operation, maximizing machining productivity. These features and designs have made the laser process viable in a mass production environment. They are already widely accepted in some of the largest automotive suppliers in Europe, for example.

We started to process hot-formed parts in 1995, when three 5-axis laser machines were installed at SSAB HardTech in Sweden. From 1995 to 2003, each of the three machines worked over 60,000 hours (approximately 7,500 hours per year, totaling 8 years of continuous operation). In the meantime, several other players have entered this market and today our machines are dedicated to hot-formed parts cutting all around the word. The majority of these machines currently work on a 24/7 schedule, reaching and exceeding the 8,000 hours per year target with a system availability that is well over 93 percent.

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Antonio Rotunno is the director of technical services for Prima North America Inc., 711 East Main Street, Chicopee, MA 01020, www.prima-na.com.