When cemented carbide tools began spreading throughout the world in 1989, Mitsubishi Materials launched TF15, a cemented carbide for solid end mills that have been used by a wide range of manufacturers. Since then, Mitsubishi Materials has continued technical innovations to minimize tool size that led to innovations such as extremely small diameter drills. In this feature, we take a look at the history of superfine cemented carbide for solid tools.
Cemented carbide is an alloy of tungsten carbide (WC) and cobalt (Co). WC is the main ingredient and the cobalt functions as an adhesive. Generally, as the WC particles become smaller, the material becomes harder. The more cobalt, the lower the level of hardness. Cemented carbide is hard but fragile, therefore it is important to consider the balance of hardness and toughness according to the intended use. Cemented carbide manufacturing starts with the recycling of tungsten ore. This is followed by carbonization, pressing and sintering. Mitsubishi Materials provides products with consistently stable capabilities through an integrated process that covers material design, manufacturing and production management. Furthermore, the company can reflect the intention of material design back to the raw materials, which expands flexibility of development and makes it possible to create new products that become market leaders.
In the early 1980s, solid end mills were made primarily of high-speed steel materials. This period was still at the beginning of cemented carbide end mill development and the share was only 5% of the 700,000 end mills being produced domestically per month. At that time Mitsubishi Materials’ first superfine cemented carbide UF20/UF30 series was being used. The series was selected because of its strength, which prevented breakage when used for high-speed steel materials. However, they proved inadequate for high cobalt content alloys. Abrasion resistance needed to be improved to make the widespread use of cemented carbide end mills practical. Each cemented carbide material manufacturer joined the competition for development of new fine particle cemented carbide alloys. By the end of the 1980s, each manufacturer had mostly decided which basic components would be used for the end mills that they were producing. Mitsubishi Materials sought versatility to respond to a wide range of cutting applications and chose a material design that would ensure toughness rather than hardness in the cutting edge. We also used superfine particle WC powder that was developed jointly with one of our group companies, Japan New Metals Co., Ltd.; and in 1989 the grade TF15 was produced, a strong cemented carbide alloy featuring outstanding balance between hardness and toughness.
In addition to use in its own products, Mitsubishi Materials introduced TF15 to other end mill manufacturers to promote the use of cemented carbide end mills and expand the market, it is noted that it was accepted immediately by manufacturers in Japan. For more than a quarter of a century, the use of TF15 in products other than end mills such as the solid drill WSTAR series and general-use VP15TF inserts coated with Miracle Coating, has increased, making it a major product in the cemented carbide business. In addition, TF15 serves as the main material in current cemented carbide end mills. We believe this shows that the original TF15 material design was excellent. We are proud that the quality and material design of our products, that is, the quality of the manufacturing technology that enables us to provide stable, high-performance products, is highly regarded by our customers.
MF10 was launched at almost the same time as TF15, targeting the already expanding market for miniature drills used to make holes on printed circuit boards. The characteristics of cemented carbide end mills differ from standard end mills. Being highly rigid and hard, they were suitable for application. The priority condition for tools used to make holes in expensive circuit boards is that they are strong and do not break easily. In addition, the accuracy of the holes they produce must be precise. Our standard-diameter tool was the HTi10, and the small-diameter tools were UF20. However, neither the strength of HTi10 nor the rigidity of UF20 proved adequate for the requirements of circuit boards. New materials that were strong enough when integrated were required. We focused on minimizing defects, the original goal in developing cemented carbide alloy. The strength of brittle cemented carbide alloy is affected by even tiny internal defects. Since cemented carbide alloys were manufactured using the powder metallurgical method, micropores would remain regardless of how much care was taken in the manufacturing process. Addressing this problem required significant improvement in sintering technology. Even if we could prevent such defects, it was very difficult to reduce variability in strength if components included uneven parts. To address this, we worked jointly with Japan New Metals Co., Ltd. to develop superfine particle WC powder, which has smaller particle size distribution than standard WC powder. At the same time, we also successfully improved our sintering technology to minimized micropores. The result was strong and rigid MF10. It has established a stable position in the small-diameter miniature drill market. Furthermore, its excellent performance in machining super hard steels addressed the weakness of TF15. Since then, MF10 has been used for super hard steels, and TF15 has been applied for general use.
In the late 1990s, along with the increasing use of electronic devices, demand for standard diameter miniature drills expanded compared with the smaller diameter MF10 grade drills. At the same time, circuit boards also became super hard, requiring improvement in the HTi10 carbide material. During the development of SF10; while the trend in miniature drill materials was superfine particles, we chose a rougher material design. This achieved greater stability in the material and with the advanced manufacturing technology used in the processing of the MF10 grade, these factors combined in reducing microchipping caused by circuit board fillers. In addition to Mitsubishi Materials standard-diameter, SF10 has been used by many other miniature drill manufacturers and continues to be employed as a main material.
In 2000, Kobelco’s Tool Department (current, Akashi Plant) was brought into Mitsubishi Materials group as Mitsubishi Materials Kobe Tools Corporation. Its strength was end mills for machining high hardness steels. In order to exercise the synergetic strengths and other advantages of material technology that Mitsubishi Materials had, we started a joint innovative project to improve end mills. Kobelco was using KRZX8, a superfine particle cemented carbide equivalent to Mitsubishi Materials’ MF10. It was necessary to improve the hardness of the material to respond to HRC60-class die steels. In addition, we needed to ensure the toughness of the cutting edge for use in end mills. We also needed to reduce the size of the cemented carbide particles by half. In order to achieve this, we needed to cut the size of WC powder particles in half, and the particle size distribution needed to be small. Because there were no WC powders on the market that satisfied the requirements, we worked with Japan New Metals to jointly develop superfine particle WC powders with a mean particle size of 0.1μm. This new powder exhibited both hardness that was significantly greater than that of MF10, and toughness that was equivalent to MF10. This powder was employed as the main material in the manufacture of the Impact Miracle series launched in 2005.
Current miniature drill development is two-pronged. While standard-diameter drills have become common, the sizes of smaller-diameter drills have decreased. We manufacture drills with diameters of less than 0.15 mm. Drills with extremely small diameters have centers that measure only a few μm across. It is impossible to place more than 100 WC particles when converting to MF10. The biggest issue here is the technology for mass production. It becomes increasingly difficult to manufacture when the WC particles are 0.1 μm. Smaller particles coagulate easily and their reactivity increases, which interferes with alloy uniformity.
Indirect influences on miniature drill materials also posed problems. The steep rise in WC ingredient prices in the early 2000s prompted a shift from solid drills made of cemented carbide to composite drills made of steel shank and cemented carbide cutting edges. In the late 2000s, with the exception of drills with 2 mm shank diameters, almost all of the drills that were being manufactured were composite. This accelerated the production of longer drills with smaller diameters. This also increased the difficulty of drill manufacture. As a result, we needed to significantly improve the technology we were employing in all processes; that is, mixing, extrusion, and sintering. The new materials we developed through this technical improvement were adopted by some manufacturers in 2012; but we need to work more to increase the popularity of our drill materials.
Looking back on the history of our product development, we realize our strength is in the ability to manufacture materials from raw ingredients. Our products are the result of raw-ingredient development that reflected material design. We also believe the popularity of our cemented carbide products is based on their consistently stable quality. High reliability requires severe quality management not only in material design, but also in the manufacture of raw ingredients, and high-precision product manufacturing. Ingredients are the basis of everything we produce, and we cannot hide defects or mistakes. This, however, is the real pleasure in the development of cemented carbide materials. Utilizing our accumulated strengths, we continue to seeking to fulfill the potential of cemented carbide.