Choosing the right tool coating for different applications is one of the most important factors in ensuring efficient machining. Proper selection of coatings can reduce the cost per project or piece, as this reduces friction; reduces heat and shrinkage, reduces cutting forces; increases feed rate and speed; improves workpiece surface quality and reduces downtime. Choosing the right coating for a particular application can be a complicated and laborious task. Every coating that can be seen on the market has its advantages and disadvantages, and improper selection can shorten tool life and make the problem worse.
Decisive factor in the coating
Hardness is critical in the coating because the harder the material or the surface, the longer the tool life. The surface hardness of the CVD diamond coating is close to 9,000 HV, which is 20 times longer than the PVD coating. The coating is suitable for cutting non-ferrous materials because of its high hardness and cutting speeds of 2 to 3 times that of uncoated tools. Abrasion resistance, the ability of the coating to prevent wear. Although a material may not be very hard, the elements and processes added during the production process may increase the likelihood of blade breakage or protrusion formation. Surface lubricity is very important because high friction coefficient increases heat, shortens coating life or causes coating failure. The low coefficient of friction increases tool life. The table is smooth – not rough or irregular – and causes the chips to slip off the tool table, producing less heat. High surface lubricity also increases speed compared to uncoated tools, helping to prevent plastic deformation of the workpiece.
Oxidation temperature, which is the temperature at which the coating begins to fail. High oxidation temperatures increase the success rate in high temperature applications. Spot welding
Anti-blocking ability prevents the processing material from adhering to the tool by preventing chemical reactions between the tool and the cutting material. This ability reduces built-up edge (BUE), a common problem in the processing of non-ferrous materials such as aluminum or brass, resulting in splitting of the tool or overcutting of the part. Once the material begins to attach to the tool, continue to bond more material.
Common coating
Titanium nitride (TIN), a general purpose coating, is obtained by physical vapor deposition (PVD). The coating has a surface hardness of 81 HRC and a coefficient of friction of 0.4. It is thermally stable at 1,000°F (550°C) and is suitable for high speed steel (HSS) tools for cutting a variety of materials, including iron-based materials, non-alloy and alloy steels, and hardened steels.
Titanium carbonitride (TICN), which has more carbon than titanium nitride, has a higher hardness (90HRC) and a better surface lubricity (0.3 friction coefficient). The coating is also very suitable for HSS cutting tools and has a higher wear resistance, making it suitable for processing materials that are more difficult to machine, such as cast iron, aluminum alloys, tool steels, copper, nickel-chromium alloys, titanium alloys and non-ferrous metals.
Titanium aluminum nitride (TIALN) is a high performance coating with a surface hardness exceeding 80HRC. Due to the formation of a layer of alumina between the tool and the chip – the heat is transferred away from the tool and transferred to the part or chip – it remains at high temperatures. Its superior oxidation resistance enables unparalleled performance in high temperature processing. Because carbide tools can use much less cutting speed than HSS with little or no coolant, TIALN is preferred when coating carbide tools. TIALN has a coefficient of friction less than TIN and has excellent properties for machining wear resistant and difficult-to-machine materials such as cast iron, aluminum alloys, tool steels and nickel alloys. The coating is highly ductile and is ideal for interrupted cutting operations.
Titanium aluminum nitride (ALTIN), which has a higher content than TIALN, has a higher surface hardness and an aluminum oxide layer. It has a good tool life in high temperature processing and is a good choice for high speed machining.
Titanium boride (TIB2) coating is harder than TIN and TIALN coatings, and has an extremely smooth surface, so the surface friction is low, the chip flow rate is fast, and the wear resistance is good. In addition, since this coating has a low affinity for aluminum, it can prevent built-up edge. Recommended for processing aluminum, titanium, magnesium and copper alloys.
Chromium nitride (CRN), also known as rainbow coating. High corrosion resistance, high hardness (90 ~ 92HRC), good wear elasticity and thermal conversion, almost suitable for processing all materials, including high silicon aluminum, stainless steel, high nickel alloy, titanium and composite materials. The coefficient of friction is exceptionally low at 0.027 and its lubricity exceeds that of TIN, TICN, TIALN and ZRN coatings. The coating has an excellent anti-blocking property and is therefore the first choice in applications where built-up edge is likely to occur. The CRN coating has a thickness of 0.00065 inches (0.01651 MM) to ensure that the workpiece is held to the required tolerances.
Diamond coating, produced by chemical vapor deposition (CVD), achieves the highest processing performance for non-ferrous materials. They are suitable for cutting graphite, metal matrix composites (MMC), high silicon aluminum and many other wear resistant materials. Diamond coatings must not be used to machine steel because the heat generated will cause a chemical reaction that will break the bond between the coating and the tool base.
Coating for hard processing
Under the global competition of low labor costs, many mold factories turned to hard milling. Hard milling, combined with high-speed machining (HSM) technology, processes high tensile strength materials, while economical machining is not possible with high-speed steel tools. The hard milling threshold starts with materials with tensile strengths in excess of 1,800 MPA or 52 HRC. The cutting material is typically microcrystalline cemented carbide, ceramic metal or cubic boron nitride (CBN), and the cutting is usually done dry air using compressed air.
In order to protect the substrate of the cemented carbide tool from the high temperature caused by hard machining, it is best to use a high performance TIALN coating. The coating structure (hierarchy) and composition are related to the specific application, such as the tool geometry. The TIALN coating can be composed of a single layer, a single component layer, a plurality of layers of multiple coating structures, such as alternating layers of TIN and TIALN coatings or very thin nanolayers. The one-component layer coating improves lubricity and the multilayer structure ensures higher heat resistance.
Diamond coating solves the problem of composite processing
An advanced composite machine for the F-35 Joint Strike Fighter is prone to excessive delamination when processing surface materials. The National Defense Manufacturing and Processing Center and its alliance partners - AMAMCO Tool Company, Diamond Tool Coating, Kennametal, Inc., MCCULLOUGHMACHINE and MOT - have developed a design to improve the geometry and materials of special milling cutters. Tool coating solution.
A DIATIGER multi-layer diamond coating produced by Diamond Tool Coating LLC was chosen as the coating for the special milling cutter. Uniquely structured diamond crystals form an interlocking layer. Wear, plastic deformation, adhesive wear, and cutting edge damage due to mechanical shock can be well prevented. DIATIGER coatings are recommended for the processing of difficult-to-process non-ferrous metals and composites.
The new tool strategy greatly increases the life of the tool. The number of tools required for each wing surface is reduced from the original 24 to 2 – 1 for roughing. 1 is used for finishing. The cutting distance has been increased by a factor of six – from the original 9 feet (about 2.74M), from 1/3 inch of total material thickness to 57 inches (1.448M) full material thickness. These improvements have saved $80,000 per aircraft.
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