A combination of energy forms and innovative processes can generate the best method for laser material processing
By Wenwu Zhang
Lasers play an increasingly important role in modern science and engineering. To better uncover lasers' potential, laser energy should be treated as a common energy field used in engineering. This paper will reflect on the process of liquid core fiber laser machining, followed by a brief introduction on the methodology of Intelligent Energy Field Manufacturing. Finally, future trends in laser material processing will be discussed. Lasers are an amazing tool, but marketwise, they have not reached our expectations.
2010 marked the 50th anniversary of the first functioning laser. Laser energy is amazing in many ways. It can be highly collimated; thus, one can send a signal to the moon and still measure the reflected signal. It is monochromatic, which allows it to be used in precision metrology. It can easily be focused down to sub-100 micron spot sizes or operated on the femtosecond (10-15 s) time scale, which enables an individual laser pulse to locally exceed the material damage threshold and be used for material processing. Multiple laser beams can be combined into a single beam so that the resultant beam can be used to study nuclear fusion in the national labs. Beyond these facts, the military has recently used lasers to test the missile defense system, and optical fiber-based communication has enabled a flatter world.
There are plenty of achievements one should feel proud of in laser technology. For laser material processing in particular, few other energy forms can compare with laser in its versatility, its flexibility, its quality, or its spatial resolution. Laser is a source for concentrated and coherent photon energy. As long as the laser energy can be absorbed by a target material, this target will be heated or ablated independent of how hard it is or how soft it is. To attain the required spatial resolution, one can use ultraviolet (UV) or infrared (IR) laser beam or femtosecond pulse durations to achieve millimeter to nanometer spot sizes. When processing speed is needed, one can use high power and/or high repetition rate systems. With this versatility, it is not surprising that laser processing has achieved widespread applications in cutting, welding, marking, drilling, and surface texturing. There are also other applications that have shown the laser to be a very competitive tool to accomplish improved speed and quality in three-dimensional (3D) manufacturing, surface treatment, and surface cleaning.
Given its flexibility, laser still has not penetrated into many adjacent arenas. While the world laser market is expected to reach 8 billion in 2012 according to a recent projection from Photonics West 2011, what is limiting the speed of market penetration?
One potential factor affecting the speed may be that the cost of implementing laser technology is high, both financially and in terms of the skills required to develop and insert laser solutions into production. In addition, the broader laser community has often been accused of not aligning either laser technology or laser processing solutions with customer needs.
The laser lab in GE Global Research serves all of the GE businesses, including GE Aviation, GE Energy, GE Oil & Gas, and GE Healthcare. Based on experience working with such a diverse customer base, lasers often compete against more mature, often less expensive processes. As a result, laser-based solutions are most successful when they accomplish a goal that cannot be reached another way. In the following, two illustrative examples of how lasers have fared against competing technologies – laser hole drilling of acoustic panels and laser dicing of cadmium zinc telluride (CZT) wafers – will be discussed.
Reflection on laser acoustic hole drilling
In 2005, the laser lab was approached to investigate the feasibility of high speed drilling of composite acoustic panels. These holes are used as damping structures in aircraft engines. In an acoustic panel, there can be as many as 500,000 holes that have to be drilled into the 0.09 inch thick polymer matrix composite (PMC) panels. With a special CNC machine, one can drill ~2 holes per second when multiple drill heads are used. The process that is used induces substantial tool wear and generates back side delamination. Additionally, the drill bits used to produce these holes have to be replaced approximately every 200 holes.
Using lasers, it was possible to demonstrate suitable hole drilling quality at a rate of 2.2 holes per second. So, lasers could drill at comparable speed to the conventional process and demonstrated negligible tool wear and heat affected zones. Unfortunately, the process also induced discoloration on the samples, which the customer disliked. In addition, the substantiation procedure to qualify the laser process for production was lengthy. Also, the customer was concerned about practically implanting the technology.
In short, laser processing was still viewed as a "high risk" given that there was already a process that could do the job. Also, they did not have people trained to operate high power lasers. Thus, while laser drilling of acoustic holes showed good promise, it was not adopted as a solution yet.
One thing this example teaches about impediments to introducing laser material processing solutions is that the final technology decision is never a simple capital issue or process issue. It is always an engineering system issue involving more than laser process considerations. It is the total solution that competes against or works with other energy forms. As a result, an engineering system won’t be complete unless one considers all of the system elements: energy, materials, information, people, and planning.
Just making better lasers is not sufficient. Neither is proving the feasibility of the laser process. One must instead prove that the engineering system, which includes laser material processing, is more competitive than other engineering systems to win the assignment.
This deficiency in systems thinking is not unique to laser materials processing. In fact, this attitude holds back many new innovative solutions with regard to other processes or solutions. Whenever one focuses solely on the process or technology or equipment in isolation rather than the benefit the user will experience, the full promise of the innovation will not be fulfilled. By contrast, when one views energy fields in tandem, a useful system solution emerges. Laser dicing of CZT wafers illustrates this point.
Liquid fiber based laser dicing of CZT wafers
This is a good example of the teamwork of energy fields to solve a challenging engineering task. Cadmium zinc telluride (CZT) is used as a nuclear detector material. A GE Global Research team was formed to develop a cost-effective CZT dicing process . Single crystal CZT wafers are very expensive to grow. CZT is also prone to defect generation during crystal growth. As shown in FIGURE 1, the wafer has some good areas and some defective areas, as revealed by ultrasonic imaging. The marked squares are free from defects and are potential zones from which to dice the detector material.
|FIGURE 1. Ultrasonic imaging of a CZT single crystal wafer. Marked zones are potential areas to get qualified detector elements.|
Why is CZT machining challenging? Due to its brittleness, edge chipping and side cracking must be minimized. Due to its toxicity, all of the machined CZT material must be properly handled throughout the manufacturing process. Also, due to its thickness (3-10 mm) and the goal of minimizing the expense of CZT, the process should be able to dice such depths with a ~0.5 mm kerf. Finally, the process must be able to cut out useful regions of the CZT without damaging adjacent material.
Silicon wafer dicing using diamond wire saws is an established technique, but wire saw cutting may result in excessive waste due to its inability to perform the random cuts required to dice all of the good zones.
FIGURE 2 shows the various processes that were considered to perform CZT dicing. While waterjet machining and laser machining enable access to random portions of the wafer, waterjet machining produced random chipping. Laser machining appeared to be a promising approach; however, it may introduce thermal damage into the crystal when machining 3-10 mm thick CZT.
|FIGURE 2. Process down-select in CZT dicing.|
Direct dicing with ultrashort pulsed laser was initially thought to be the simplest and most promising solution, but for large depth (>1 mm), such lasers are slow. Furthermore, the laser generated plasma defeated the promise of short pulses. This also produced taper and sidewall damage, as shown in FIGURE 3. As a result, the depth capability is limited to <6 mm.
|FIGURE 3. CZT wafer cut by ps laser showed side wall striation, taper, and some chipping.|
Attempting to dice CZT presented two engineering contradictions: how to ablate material without thermal damage to adjacent material and how to machine thick material without redeposition of the material on the side walls or top of the wafer.
Finding a solution to these engineering contradictions is a topic the author has been studying for many years. It is a general challenge for laser material processing. As shown in FIGURE 4, nanosecond laser machining of aluminum in air showed strong melting and surface re-deposition, while drilling underwater showed clean features and produced smooth side walls. Thus, using water cooling in combination with laser machining can potentially solve the quality issues.
|FIGURE 4. Difference between open-air machining (left) and underwater machining (right).|
To machine large depths, a new machining mechanism was needed. One approach that was tested was to inject laser energy into a fiber and feed the fiber into the material like a mechanical drill bit. FIGURE 5 illustrated the basic idea. The process that was developed is referred to as Liquid Core Fiber Laser Material Processing, which was developed at GE Global Research from 2004 to 2010. Water under pressure is fed into a special tube. Because the tube has a lower optical index than water, when the laser is coupled into the tube, total internal reflection occurs. Ultimately, laser intensities of >1 GW/cm2 were passed through this fiber and water when green light (532 nm) was used. Finally, both metals and ceramics were successfully machined.
|FIGURE 5. a) The principle of liquid core fiber laser machining and the b) fiber in machining of a CZT wafer.|
FIGURE 5b showed liquid core fiber machining of a CZT wafer. The water jet carried the laser energy that was used to ablate the CZT substrate. It also helped flush and contain the ablated CZT materials. The fiber was inserted up to 3 mm into the wafer. As the fiber was inserted even deeper into the wafer, it produced a sharper angle of the side jet. Actually, the side jet was so strong that tissue paper had to be placed over the cell to prevent splashing. Later, attempts were made to perform totally immersed machining with the liquid core fibers, which worked. This eliminated splashing and contained all of the toxic CZT material, which could then be properly disposed of or recycled.
Beyond the application of CZT machining, when laser energy is condensed into a solid needle that can do clean machining, deep machining, and immersed machining, this enables many additional applications.
Despite the promise that liquid core fiber machining demonstrated, there were still some issues that needed to be addressed; so, a similar, more robust process was eventually adopted as the production process for CZT dicing. As shown in FIGURE 6, random access dicing with high edge quality and little HAZ was achieved.
|FIGURE 6. Wafers successfully diced with liquid fiber-assisted laser machining.|
Reflecting on this experience, laser alone was not directly applicable, but when it was combined with water jet, it solved multiple problems: machining depth, cooling, and flushing. By combining the ability to finely control the energy during laser processing with water jet machining and the ability to insert the fiber into the kerf, as is done in mechanical machining, it was possible to solve the challenging task.
Introduction to Intelligent Energy Field Manufacturing
When solving real-world manufacturing problems, the most suitable solution is rarely an engineering system with a single energy source. Instead, it is the combination of multiple energy forms that generates the most suitable solution.
Unfortunately, the search for hybrid approaches runs counter to how engineers and scientists are trained. Usually, people focus on the application of work to a component or the resultant material's behavior as a result of the manufacturing operation. Furthermore, even manufacturing engineers divide machining processes into two categories: traditional and non-traditional machining. Traditional machining processes are processes with direct mechanical contact, such as turning, drilling, grinding or milling. Non-traditional processes involve the application of electrical, chemical, or optical energy to perform the machining. These processes would include EDM, ECM, laser drilling, and water jet machining. Furthermore, traditional processes are considered mainstream processes due to their technical maturity, while laser and other non-traditional processes are thus considered as niche application processes.
Dividing processes into traditional and non-traditional is a historically biased approach. In reality, separating processes into traditional and non-traditional categories is relative in nature. In reality, it divides engineers into expert groups. This situation actually impedes the full potential of process innovation because it imposes unnecessary barriers that impede the integration of different energy fields.
Following this thought, a branch of new engineering methodology was developed by the author, which is called Intelligent Energy Field Manufacturing (IEFM) . This methodology considers all energy forms to be equivalent in the sense that they are simply tools that an engineer can deploy. In this framework, the engineer uses human intelligence to control the application of these energy fields to convert or combine materials to produce the desired end state or configuration.
In this sense, there is much common ground between all processes – whether non-mechanical or mechanical. The task of the engineer is to optimize the integration of energy fields to solve given tasks, whether they fall into the realm of mechanical engineering or bio-engineering.
Laser is just one energy field that can be used. Other energy fields include mechanical work, electromagnetic radiation, gravity, thermal energy, or plasmas, etc. As a result, one should avoid thinking of laser as a special kind of energy field. Recognizing that each energy form has its relative strengths and weaknesses, all energy forms should be considered as potential solutions when starting to solve a problem. To find the solution, one should seek to find the optimal ways to integrate these energy fields. Similar to the liquid core fiber example presented earlier, multiple forms can be utilized to provide a robust solution. Many innovative processes, such as abrasive water jet machining or rotary ultrasonic machining, were developed using this approach.
In IEFM, the concept of a general energy field was proposed, along with general logic functional materials and general intelligence. Furthermore, a new criterion of engineering optimization (CEO) was introduced.
All of this raises the question about the fundamental purpose of engineering. While different people may give different answers, this author suggests that engineers and engineering should help to sustain the healthy development of all cycles on earth and to extend understanding of the universe. Thus a new CEO should reflect the importance of sustainability. Unfortunately, engineering neglected sustainability for quite a long time. In recent years, some consideration of sustainability was introduced into engineering optimization. In IEFM, the emphasis is on the following:
New CEO = Current CEO x Sustainability
By this definition, a negative sustainability engineering activity will generate negative impact; the more efficient it is, the more negative impact it generate.
There is not enough space to explain the many other aspects of Intelligent EFM in detail in this report. Interested readers are encouraged to read reference . Multiple international symposiums have been held by ASME to propagate this methodology. Some universities will also start college courses based on this methodology in 2011. Over time, it is expected that the classification of machining into traditional and non-traditional approaches will pass, to be replaced by the era of IEFM.
Laser processing technology is still just one of the members of a larger list of energy fields. Even so, the many unique properties that lasers possess should position it to play a much bigger role in future manufacturing.
The era of high speed and cost effective laser material processing
Ten years ago, when one talked about laser micromachining, one tacitly assumed that laser machining, though capable of achieving high precision, was a slow process. At that time, only lower power laser systems were available. Today, many things have changed. There are multiple laser systems with >100 W power that are suitable for precision laser micromachining. As these high powers are combined with high speed motion systems and high repetition rate lasers, laser micromachining has gradually entered the era of high speed machining.
For example, there are commercially available picosecond (ps) laser systems producing 100 W of output power. Also, one can find 800 W nanosecond (ns) lasers. Even fs laser systems reached output powers greater than 1000 W. When scanners are used, travel speeds >10 m/s can easily be achieved. Currently, higher speed scanners are being developed to match the high frequency of short pulsed lasers.
When laser power goes beyond 50 W and still maintains high beam quality, the laser material removal rate or surface treatment speed can go beyond many other energy forms and still keep its high resolution and flexibility. When laser power reaches several hundred watts or even one kW, lasers start to compete against other high speed processes, including mainstream mechanical processes. In die-making, for example, high power ns or ps lasers can be used as a more cost effective tool than conventional milling systems.
For CW lasers, fiber lasers can offer single mode KW system, or >10kW multimode systems. Such systems can be used in high speed cutting, welding, or surface treatment.
Would kW laser systems eventually take over the market of traditional mechanical machining systems? For some materials, perhaps yes. What about laser machining of ceramics? What about combining laser machining with mechanical fine finishing?
With the increase in power and speed, the role of lasers will change from a niche application process to a major process in the future. To accelerate the coming of this era, users need to position lasers correctly to address the significant needs of the world, such as sustainability. In this regard, the methodology of Intelligent Energy Field Manufacturing applies well.
Finally, the laser business should learn from other well established industries, such as the automotive and computer industries. By paying attention to trends regarding the Diffusion of Innovation , the pace at which it gains acceptance will increase. Many laser vendors admit the current laser price is not its true manufacturing price; the price could be 20% less if mass production were used. Laser has to go through what the automobile and computer industries had to go through: standardize to lower cost, make it robust and easy to operate, and make it affordable to a large pool of customers. Only when these things are achieved can laser reveal its full potential.
Laser is an amazing tool, but its full potential is far from being realized. To further explore the benefits of laser technology, one should consider laser energy along with other energy fields and follow the methodology of Intelligent Energy Field Manufacturing. With the continued development of laser capability and the application of the technology to address societal needs such as sustainability, laser material processing will enter the era of high speed and low cost material processing. Eventually, it will become an important processing tool rather than a niche processing tool.
Special thanks go to Magdi Azer, who supported Intelligent EFM research in GE GRC and helped with editing this paper.
1. Wenwu Zhang, J. Eric Tkaczyk, Kristian Andreini, Steven R. Hayashi, Nitin Garg, Peter J. Bednarczyk, Haochuan Jiang, "Process Competition in the Micromachining of Brittle Components," PICALEO 2010 paper #303.
2. Wenwu Zhang, Intelligent Energy Field Manufacturing, CRC Press, 2010.
3. Everett M. Rogers, Diffusion of Innovations, Free Press, New York, 1983.
Dr. Wenwu Zhang (firstname.lastname@example.org) is with the Laser and Metrology Systems Lab., GE GRC, Schenectady, NY.