Regarding tool life, it usually depends on different workpieces and tool materials, as well as different cutting processes. One way to quantitatively analyze the end point of tool life is to set an acceptable maximum flank wear limit value (indicated by VB or VBmax). Tool life can be expressed by Taylor's formula for expected tool life, that is, VcTn=C. A more common form of this formula is VcTn×Dxfy=C where Vc is the cutting speed; T is the tool life; D is the depth of cut; f Is the feed rate; x and y are determined by experiments; n and C are constants determined based on experiments or published technical data, and they represent the characteristics of the tool material, workpiece and feed rate.
The continuous development of the best tool matrix, coating and cutting edge preparation technology is essential to limit tool wear and resist cutting high temperatures. These elements, coupled with the chip breaker and corner arc radius used on the indexable insert, determine the applicability of each tool to different workpieces and cutting processes. The best combination of all these elements can extend tool life and make cutting more economical and more reliable.
Change the matrix
By changing the particle size of tungsten carbide in the range of 1-5 μm, tool manufacturers can change the matrix properties of cemented carbide tools. The particle size of the matrix material plays an important role in cutting performance and tool life. The smaller the particle size, the better the wear resistance of the tool. Conversely, the larger the particle size, the better the toughness of the tool. The fine-grained matrix is mainly used to process the blades of aviation grade materials (such as titanium alloy, Inconel alloy and other high-temperature alloys).
In addition, increasing the cobalt content of cemented carbide tool materials by 6%-12% can achieve better toughness. Therefore, the cobalt content can be adjusted to meet the requirements of a specific cutting process, regardless of whether the requirement is toughness or wear resistance.
The performance of the tool matrix can also be enhanced by forming a cobalt-rich layer near the outer surface, or by selectively adding other alloying elements (such as titanium, tantalum, vanadium, niobium, etc.) to the cemented carbide material. The cobalt-rich layer can significantly increase the strength of the cutting edge, thereby improving the performance of roughing and interrupted cutting tools.
In addition, when selecting a tool matrix that matches the workpiece material and processing method, it also considers the other five matrix properties-fracture toughness, transverse fracture strength, compressive strength, hardness and thermal shock resistance. For example, if cemented carbide tools appear to chip along the cutting edge, a matrix material with higher fracture toughness should be used. In the case of direct failure or breakage of the cutting edge of the tool, the possible solution is to select a matrix material with higher transverse fracture strength or higher compressive strength. For machining occasions with higher cutting temperature (such as dry cutting), tool materials with higher hardness should usually be preferred. In the processing occasions where hot cracking of the tool can be observed (the most common in milling processing), it is recommended to choose a tool material with better thermal shock resistance.
Coating selection
The coating also helps to improve the cutting performance of the tool. Current coating technologies include:
①Titanium Nitride (TiN) coating: This is a general-purpose PVD and CVD coating that can increase the hardness and oxidation temperature of the tool.
②Titanium carbonitride (TiCN) coating: By adding carbon element to TiN, the hardness and surface finish of the coating are improved.
③TiAlN and AlTiN coatings: The composite application of the aluminum oxide (Al2O3) layer and these coatings can increase the tool life of high-temperature cutting. Alumina coating is especially suitable for dry cutting and near-dry cutting. The AlTiN coating has a higher aluminum content and has a higher surface hardness than the TiAlN coating with a higher titanium content. AlTiN coating is usually used for high-speed machining.
④ Chromium Nitride (CrN) Coating: This coating has good anti-adhesion performance and is the preferred solution to resist built-up edge.
⑤Diamond coating: Diamond coating can significantly improve the cutting performance of cutting tools for processing non-ferrous materials, and is very suitable for processing graphite, metal matrix composite materials, high silicon aluminum alloys and other highly abrasive materials. However, diamond coating is not suitable for processing steel parts, because its chemical reaction with steel will destroy the adhesion of the coating to the substrate.
In recent years, the market share of PVD-coated tools has expanded, and their prices are comparable to those of CVD-coated tools. The thickness of CVD coating is usually 5-15μm, while the thickness of PVD coating is about 2-6μm. When applied to the tool substrate, the CVD coating will generate undesirable tensile stress; while the PVD coating helps to form beneficial compressive stress on the substrate. Thicker CVD coatings usually significantly reduce the strength of the cutting edge of the tool. Therefore, CVD coatings cannot be used for tools that require very sharp cutting edges.
Cutting edge preparation
In many cases, the preparation of the cutting edge of the blade (or dullness of the edge) has become a watershed that determines the success or failure of machining. The passivation process parameters need to be determined according to specific processing requirements. For example, blades used for high-speed finish machining of steel have different requirements for cutting edge passivation than those used for rough machining. Cutting edge passivation can be used to process almost any type of carbon steel or alloy steel blades, but there are certain limitations in the processing of stainless steel and special alloy blades. The amount of passivation can be as small as 0.007mm or as large as 0.05mm. In order to enhance the cutting edge in harsh processing conditions, it is also possible to form tiny T-shaped ridges through edge passivation.
Generally speaking, inserts used for continuous turning and milling of most steel and cast iron require a greater degree of cutting edge passivation. The amount of passivation depends on the grade of cemented carbide and the type of coating (CVD or PCD coating). For heavily interrupted cutting inserts, it has become a prerequisite to severely passivate the cutting edge or machine a T-shaped ridge. According to different coating types, the passivation amount can be close to 0.05mm.
In contrast, since the blades of stainless steel and high-temperature alloys are prone to forming built-up edge, the cutting edge is required to be sharp and can only be slightly passivated (as small as 0.01mm), and even a smaller amount of passivation can be customized. Similarly, blades for machining aluminum alloys also require sharp cutting edges.
Analysis of methods to improve tool life from three aspects
2021 12/27