Fiber lasers with an average power of 2 kW have been commercially available for more than 10 years with widespread use by automotive parts manufacturers. The majority of fiber lasers produced in this power range have continuous wave (CW) output and are well suited for automotive metal cutting and welding applications.
However, quasi continuous wave (QCW) lasers produce peak power 10 times greater in pulsed mode than CW mode, providing new process opportunities in automotive. Major QCW laser enhancements in processing speed and quality have been made recently in both aerospace and medical device manufacturing. These are now being adopted in other industries, particularly in the automotive sector.
Prima Power Laserdyne has taken advantage of the technology to develop its SmartTechniques that enables unique capabilities for high-power laser cutting, welding and drilling using CW and QCW fiber laser systems. SmartTechniques is a suite of hardware, software and control capabilities that improve productivity and quality in laser processing. Originally developed for aerospace applications, it is now being adapted to meet the needs of the automotive industry. This article covers the piercing and measurement tools along with a unique, patent-pending gas assist nozzle that all help with the laser cutting process.
Automotive applications differ from aerospace applications in a few important ways. The major difference is the type of material that is laser processed. Generally, aerospace parts are produced from stainless steel, nickel-based superalloys and titanium alloys. These parts usually require high tolerances and strict control of metallurgical properties, such as the heat-affected zone and the recast layer thickness.
Typically, producing aerospace parts requires very lengthy processes. For example, laser processing an aerospace combustor requires drilling thousands, sometimes tens of thousands, of cooling holes in a single part. The size and location of the required holes are measured in thousandths of an inch (fractions of a millimeter) in order to provide the precise airflow needed over the surface of the combustor.
In contrast, automotive parts are generally fabricated from cold- or hot-rolled steel. Cutting speed and throughput are the most critical requirements because of the quantities of the parts required. While automotive parts also require quality and precision, they are more often secondary to processing speed. For many automotive parts, edge quality and feature size don’t have the same impact on the performance of the part as they do for aerospace parts. On the other hand, cycle times often determine process viability for automotive parts.
For laser processing to be acceptable in an automotive application, the parts manufacturer must establish aggressive goals. As an example, for 5-mm to 10-mm-thick sections of low carbon steel, cut quality, cycle times, part quality and operating cost must be optimized. Cut quality requires rapid pierce without any spatter, and cuts must be be dross free with minimum taper. Features, such as slots and holes, must be located within the required precision, despite the fact that formed blanks could vary significantly from the design shape. Part-to-part cycle time is a key goal while minimizing cost of optics, assist gases and other utilities.
Laser cutting of thick-section carbon steel is, traditionally, a gas-assisted process using oxygen or an inert gas such as nitrogen. Variables related to the assist gas have a big influence on the cut quality. These include assist gas pressure, nozzle design and standoff. All play an important role in governing the gas dynamics while significantly influencing the cut quality and cycle time.
One of the benefits of using an oxygen assist gas compared to air for automotive applications is the ability to clear the cut of molten material and produce a dross-free cut. The pressure of the gas is important, as well. Too little pressure and the molten material may adhere to the parent material forming dross and, at times, seal and ruin the cut. Also, too much oxygen can burn and significantly degrade the cut quality.
To avoid failures in these applications, oxygen assist gas is preferred to achieve a clean cut. Most important, oxygen assist gas helps achieve faster cutting speeds. Also, the consumption of oxygen is much lower in these applications than the consumption of compressed air or nitrogen, thus reducing costs.
Regardless of material type and thickness, the laser cutting operation begins with a piercing process that governs the overall cut quality. In other words, if the pierce is clean, the stage has been set for a clean cut. However, if the pierce is poor or incomplete, there is significant opportunity for cut quality to be poor or, in some cases, for the cut to fail.
Prima Power Laserdyne cutting tests carried out using a 20-kW QCW fiber laser and oxygen assist gas revealed that cut quality was acceptable in terms of dross, taper and cutting speed. However, piercing with oxygen proved to be very difficult and unsuitable for this application because faster pierce times and spatter-free pierces were required. Repeatable oxygen pierces were possible using low peak power. However, the pierce time of 0.8 sec for 7.5-mm-thick steel plate was far too high and the spatter accumulated at low gas pressure required the process to be stopped and nozzle to be cleaned after only 15 pierces.
Once it was decided to develop a better process using the two gases – compressed air for piercing and oxygen for cutting – optimizing the piercing process ensured 100 percent reliability while minimizing piercing time.
With the high-power QCW fiber laser as the source, the Laserdyne SmartPierce process was applied. (SmartPierce is a technique that involves pulse-by-pulse changes in any or all of peak power, pulse width and pulse frequency. Direct control of the laser with the Laserdyne control provides this capability.) Using SmartPierce, cycle time to provide consistent pierces in 7.5-mm-thick steel plate using compressed air was reduced to just 0.4 sec.
The most robust process involving oxygen assist cutting uses high-pressure compressed air for piercing and low-pressure oxygen for cutting. Changing between gases within a process is relatively common, even in aerospace applications. However, piercing with compressed air requires purging the nozzle before introducing the oxygen.
To purge the assist gas delivery lines for consistent cutting, a minimum dwell time of 2 sec. was required. With modification to the assist gas hardware, the changeover dwell time between the two assist gases was reduced to 0.7 sec. Although this may not seem like a major time savings, the total time savings is quite significant and can add up to hours per day when multiplied by hundreds of cut features required in multiple automotive parts.
Those familiar with laser cutting understand that laser system nozzle design is key to the overall process of piercing and cutting. Nozzle design can influence cut quality, protect optics, affect cycle time and regulate gas consumption cost.
To achieve the stated goals by minimizing changeover time between the two assist gases (compressed air and oxygen), Prima Power Laserdyne developed and extensively tested its new dual-gas delivery cutting nozzle. The nozzle is designed to deliver coaxial and directional non-coaxial assist gas for piercing thick steel sections followed by fast laser cutting of the material.
The directional non-coaxial gas provides the assist gas for piercing while protecting the laser optics and nozzle assembly during piercing. The coaxial gas is used for the cutting process. By utilizing both gases, piercing is accomplished quickly and cleanly through the thick sections. And the need to purge gases when transitioning from piercing to cutting is eliminated.
A comparison of cycle times for the three processes evaluated for this article is shown in Table 1.
The final innovation in this new laser process was to create a routine for mapping selected surfaces of the actual 3-D blank to adjust for its imperfect shape before piercing and cutting the various features. This mapping process was required to meet the feature location tolerances for the part.
The Prima Power Laserdyne answer is SmartSense, the laser based, non-contact measurement tool that operates coaxial with the cutting laser beam and software. It collects and analyzes measurement data from the surface. The mapping results are used to adjust the planes of processing to reflect the real part location and shape in order to achieve laser processed feature precision. The SmartSense technique provides proper location of holes and other cut features within the part despite the less-than-perfect shape of the blank.