A new method of laser cutting superhard materials proves to be both economical and environmentally friendly
How “really hard” is a material object? How “really hard” is it to carry out a physical act? The micromachining industry is being confronted increasingly with having to use or produce parts from materials that are intrinsically superhard. This, in turn, raises the question: how can these materials be either processed or used for manufacturing high-quality products in an economical manner while, at the same time, satisfying today’s increasingly environmentally conscious world?
Hard in the materials context means pure diamond (C), polycrystalline diamond (PcD), cubic boron nitride (cBN), and silicon nitride (Si3N4)-four of the hardest materials previously known to mankind. Only now are they being overtaken by newly discovered and still exotic materials, such as aggregated diamond nanorods (ADNRs) and ultrahard fullerite (C60). Hard in the physical sense refers to forming or shaping these superhard materials for the required application, such as for cutting tools for micromachining, possibly with very complex contours, and at the same time using an economical process.
Currently, the prevalent manufacturing methods include lasers, grinding, electrical discharge machining (EDM), or possibly electrochemical machining (ECM), also called chemical milling or wet etching. All of these methods, however, have distinct disadvantages. Thick materials cannot be cut with dry lasers, as they leave behind conical kerfs and a heat-affected zone (HAZ) around the cut area, thereby lowering the quality and usefulness of the finished article. Grinding is an accepted method, but is slow and costly in terms of grinding media. It also leaves a HAZ and is limited to linear formations, making it unsuitable for manufacturing complex shapes. EDM is a proven method, but only works with electrically conductive materials. In addition, EDM is slow and does not leave a smooth finish in the workpiece. Perhaps more important, ECM involves the use of highly corrosive chemicals, which pose health, safety, and environmental problems and like EDM, only works with electrically conductive materials.
As diamond is not subject to processing with any of these methods, with the exception of laser drilling, we will concentrate on the three most commonly used materials in the tooling industry: PcD, cBN, and Si3N4. PcD is a synthetic material using diamond particles mixed with a chemical binder and sintered into a coherent structure under high temperature and pressure. It is used for machining a wide range of nonferrous materials, such as aluminum (Al) and Al alloys, tungsten carbide (WC), ceramics, stone, plastics, fiberglass, and wood-based materials. Pure cBN, or its polycrystalline form PcBN (a composite formed by sintering cBN particles with a ceramic or metal catalyst binder at extremely high temperature and pressure), is mainly used for machining ferrous materials, where high temperatures and an oxidizing atmosphere are present. Si3N4, a ceramic material with a high fracture resistance, is also used as a material for cutting tools due to its hardness, thermal stability, and wear resistance. It is especially used for high-speed machining of cast iron. When used for machining steel, it is usually coated with titanium nitride, usually by chemical-vapor deposition (cvd), for increased chemical resistance. Si3N4 also finds applications in items such as high-quality skateboard bearings and ignition devices for gas appliances.
Preparing these materials for use in the tooling industry has been carried out using one or more of the methods mentioned previously. Now there is a highly successful alternative with the arrival of the water-jet-guided laser. Better known as the laser microjet (LMJ), the patented technology is designed and manufactured by the Swiss company Synova.
The LMJ technology uses a low-pressure, hair-thin water jet (which functions similarly to an optical fiber) to guide a tightly focused short-pulsed and highly efficient diode-pumped solid-state (DPSS) Nd:YAG laser beam with a wavelength of 1064 or 532 nm onto the workpiece. This combination produces an effective cutting range up to several centimeters and a cutting kerf width in the range of 25 to 120 µm (see Fig.1). The water jet not only guides the laser beam-avoiding the beam-divergence problem inherent with conventional lasers-but also provides adequate cooling to the cut edges and surrounding area between pulses, so that the workpiece exhibits little or no HAZ.
FIGURE 1. The Laser MicroJet principle.
Simultaneously, the water jet ejects ablated material from the kerf and flushes it clear of the cut area. The water film formed on the material surface serves as a protective layer preventing any redeposition or adhesion of the ablated material on the workpiece. Deionized water is the only consumable, requiring on average ~1 l/h at 300 bar, an extremely important environmental aspect when operating in a 24/7 production facility.
With all of these unique capabilities, the laser beam can cut into and through even complex 2-D shapes in ferrous or nonferrous materials, producing clean, perfectly cut parallel kerf walls, with no burrs, contaminants, or redeposition. Notably, the LMJ is not reliant on electrical conductivity of the workpiece. For cutting hard materials, machines with processing tables of 300 × 300 mm are available with cutting speeds of up to 1000 mm/s, an absolute precision of ±3 µm, and repeatability of ±1 µm. The LMJ machining capabilities were carried out using Synova’s Laser Cutting System LCS 300 machine.
Using the LMJ
For example, the LMJ was used to cut a 59-mm-diameter disk, 1.6 mm thick, consisting of a layer of PCD on a backing of tungsten carbide (WC). It was cut into 36 quarter- and half-sized circles. For this application a dual-pulsed DPSS laser with wavelength of 532 nm was used, operating at an average power of 140 W and with an 80-µm nozzle fitted to the LMJ. The cutting carried out at 30 mm/s in multipass mode-required a total of 60 passes-with overall speeds of 14 and 16 mm/min for the straight and rounded cuts, respectively (see Figs. 2 and 3).
FIGURE 2. Microscopic image of topside rounded edge, inset shows separated pieces and remnant disk after cutting.
FIGURE 3. Microscopic image of sidewall after cutting.
FIGURE 4. Microscopic top view image of the cBN insert tip and the completed part (inset image).
Another example is the cutting of a Christmas tree-shaped tooling insert from a 5-mm-thick WC disk with a cBN insert (see Fig. 4). The cutting was again performed using a dual-pulsed DPSS laser with wavelength of 532 nm, operating at an average power of 140 W, and with an 80-µm nozzle fitted to the LMJ. The speed was 10 mm/s, requiring 220 passes, resulting in an overall speed of 6 min/insert.
A third illustration is the drilling of Si3N4 material. The 8-mm-thick material was cut to produce a Ø3mm hole. This was carried out with a dual-pulsed DPSS 532-nm-wavelength laser, with an average power of 150 W and a 75-µm LMJ nozzle. The cutting was carried out in ~2 min at a speed of 20 mm/s and required 250 passes (see Figs. 5 and 6).
FIGURE 5. Microscopic images of Ø3mm hole viewed from the front side.
FIGURE 6. Enlarged view of Ø3mm hole viewed from the back surface.
Finally, the LMJ was used to cut a test sample in-house for demonstrative purposes (see Fig. 7). The material was a 6-cm-diameter disk with a cBN layer of ~0.8 mm on a WC backing with a total material thickness of 3.8 mm. The 85-µm-wide cuts were made to a depth of 2.6 mm through the cBN layer into the WC with a pitch of 250 µm between the slots. This application also used a dual-pulsed DPSS 532nm wavelength laser, with an average power of 140 W and with an 80-µm LMJ nozzle. Each slot, cut at a speed of 30 mm/s, required 100 passes to complete. This example specifically illustrates the extremely high depth-to-width aspect ratios that can be achieved with the LMJ and the resulting absolutely parallel kerf walls.
FIGURE 7. Demonstration article cut from 3.8-mm-thick cBN/WC layered material.
These four examples demonstrate the remarkable capabilities of the LMJ for cutting superhard materials. The results are fast, precision cutting, leaving perfectly parallel kerf walls with negligible HAZ. Moreover, the finished workpieces are free of chipping, burrs, contamination, and deposition, which would have otherwise required additional processing steps.
Adding to these advantages is the LMJ’s capability to cut directly any desired 2-D geometrical shape. Overall, these results illustrate the LMJ’s ability to overcome the shortfalls of current hard-materials manufacturing processes. These advantages in flexibility and speed, coupled with the superior cutting quality and workpiece integrity, enable manufacturers with a new technology approach that is both economical and environmentally friendly.
Keith Stay is a technical writer at Synova (Ecublens, Switzerland; www.synova.ch).