霸刀分享-刀具在机加工中的磨损问题及应对办法

1 刀具寿命

　　对于刀具寿命，通常取决于不同的工件和刀具材料，以及不同的切削工艺。定量分析刀具寿命终止点的一种方式是设定一个可以接受的最大后刀面磨损极限值（用VB或VBmax表示）。刀具寿命可用预期刀具寿命的泰勒公式表示，即VcTn=C，该公式的一种更常用的形式为VcTn×Dxfy=C式中，Vc为切削速度；T为刀具寿命；D为切削深度；f为进给率；x和y由实验确定；n和C是根据实验或已发表的技术资料确定的常数，它们表示刀具材料、工件和进给率的特性。

　　不断发展的最佳刀具基体、涂层和切削刃制备技术对于限制刀具磨损和抵抗切削高温至关重要。这些要素，加上在可转位刀片上采用的断屑槽和转角圆弧半径，决定了每种刀具对于不同的工件和切削加工的适用性。所有这些要素的最佳组合能够延长刀具寿命，使切削加工更经济、更可靠。

2 改变基体

　　通过在1－5μm范围内改变碳化钨的粒度，刀具制造商可以改变硬质合金刀具的基体性能。基体材料的粒度对于切削性能和刀具寿命起着重要作用。粒度越小，刀具的耐磨性越好。反之，粒度越大，刀具的强韧性越好。细颗粒基体主要用于加工航空牌号材料（如钛合金、Inconel合金和其他高温合金）的刀片。

　　此外，将硬质合金刀具材料的钴含量提高6％－12％，可以获得更好的韧性。因此，可以通过调整钴含量来满足特定切削加工的要求，无论这种要求是韧性还是耐磨性。

　　刀具基体的性能还可以通过在接近外表面处形成富钴层，或者通过在硬质合金材料中有选择性地添加其他合金元素（如钛、钽、钒、铌等）而获得增强。富钴层可以显著提高切削刃强度，从而提高粗加工和断续切削刀具的性能。

　　此外，在选择与工件材料和加工方式相匹配的刀具基体时，还表现考虑另外5种基体特性——断裂韧性、横向断裂强度、抗压强度、硬度和耐热冲击性能。例如，如果硬质合金刀具出现沿切削刃崩刃的现象，就应该选用具有较高断裂韧性的基体材料。而在刀具出现切削刃直接失效或破损的情况下，可能采用的解决方案是选用具有较高横向断裂强度或较高抗压强度的基体材料。对于切削温度较高的加工场合（如干式切削），通常应该首选硬度较高的刀具材料。在可以观察到刀具产生热裂纹的加工场合（在铣削加工中最常见），建议选用耐热冲击性能较好的刀具材料。

3 涂层选择

　　涂层也有助于提高刀具的切削性能。目前的涂层技术包括：

　　①氮化钛（TiN）涂层：这是一种通用型PVD和CVD涂层，可以提高刀具的硬度和氧化温度。

　　碳氮化钛（TiCN）涂层：通过在TiN中添加碳元素，提高了涂层的硬度和表面粗糙度等级。

　　氮铝钛（TiAlN）和氮钛铝（AlTiN）涂层：氧化铝（Al2O3）层与这些涂层的复合应用可以提高高温切削加工的刀具寿命。氧化铝涂层尤其适合干式切削和近干切削。AlTiN涂层的铝含量较高，与钛含量较高的TiAlN涂层相比，具有更高的表面硬度。AlTiN涂层通常用于高速切削加工。

　　氮化铬（CrN）涂层：这种涂层具有较好的抗粘结性能，是对抗积屑瘤的首选解决方案。

　　金刚石涂层：金刚石涂层可以显著提高加工非铁族材料刀具的切削性能，非常适合加工石墨、金属基复合材料、高硅铝合金和其他高磨蚀性材料。但金刚石涂层不适合加工钢件，因为它与钢的化学反应会破坏涂层与基体的粘附性能。

　　近年来，PVD涂层刀具的市场份额有所扩大，其价格也与CVD涂层刀具不相上下。CVD涂层的厚度通常为5－15μm，而PVD涂层的厚度约为2－6μm。在涂覆到刀具基体上时，CVD涂层会产生不受欢迎的拉应力；而PVD涂层则有助于对基体形成有益的压应力。较厚的CVD涂层通常会显著降低刀具切削刃的强度。因此，CVD涂层不能用于要求切削刃非常锋利的刀具。

4 切削刃制备

　　在许多情况下，刀片切削刃的制备（或称刃口钝化）已成为决定加工成败的分水岭。钝化工艺参数需根据特定的加工要求而定。例如，用于高速精加工钢件的刀片对刃口钝化的要求就与用于粗加工的刀片有所不同。刃口钝化可应用于加工几乎任何类型碳钢或合金钢的刀片，而在加工不锈钢和特殊合金材料的刀片上，其应用则有一定限制。钝化量可以小至0.007mm，也可以大到0.05mm。为了在条件恶劣的加工中起到增强切削刃的作用，还可以通过刃口钝化形成微小的T型棱带。

　　一般来说，用于连续车削加工以及铣削大部分钢和铸铁的刀片需要进行较大程度的刃口钝化。钝化量取决于硬质合金牌号和涂层类型（CVD或PCD涂层）。对于重度断续切削加工刀片，对刃口进行重度钝化或加工出T型棱带已成为一种先决条件。根据不同的涂层类型，钝化量可接近0.05mm。

　　与此相反，由于加工不锈钢和高温合金的刀片容易形成积屑瘤，因此要求切削刃保持锋利，只能进行轻微钝化（可小至0.01mm），甚至还可以定制更小的钝化量。同样，加工铝合金的刀片也要求具有锋利的切削刃。

Ba Dao Sharing - Wear Problems of Tools in Machining and Solutions

1 Tool life

Tool life 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 limit value of rear face wear (expressed in VB or VBmax). Tool life can be expressed by the Taylor formula of the expected tool life, that is, VcTn= C. A more commonly used form of this formula is VcTn×Dxfy=C, where Vc is the cutting speed. T represents the tool life; D represents the cutting depth; f represents the feed rate; x and y are determined by experiments. n and C are constants determined based on experiments or published technical data, which represent the characteristics of tool material, workpiece and feed rate.

The continuous development of the best tool substrate, coating and cutting edge preparation technologies is crucial for limiting tool wear and resisting high cutting temperatures. These elements, along with the chip-breaking grooves and the radius of the turning arc adopted on the indexable inserts, determine the applicability of each tool for different workpieces and cutting processes. The best combination of all these elements can extend tool life and make cutting processes more economical and reliable.

2. Alter the matrix

By altering the particle size of tungsten carbide within the range of 1-5μm, tool manufacturers can change the matrix properties of cemented carbide tools. The particle size of the base material plays a significant role in cutting performance and tool life. The smaller the particle size, the better the wear resistance of the cutting tool. Conversely, the larger the particle size, the better the strength and toughness of the cutting tool. Fine-grained matrix is mainly used for processing blades of aviation grade materials (such as titanium alloys, Inconel alloys and other high-temperature alloys).

In addition, increasing the cobalt content of cemented carbide cutting tool materials by 6% to 12% can achieve better toughness. Therefore, the cobalt content can be adjusted to meet the requirements of specific cutting processes, whether it 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 enhance the strength of the cutting edge, thereby improving the performance of rough machining and intermittent cutting tools.

In addition, when choosing a tool base that matches the material of the workpiece and the processing method, five other base characteristics should also be taken into consideration - fracture toughness, transverse fracture strength, compressive strength, hardness and thermal shock resistance. For instance, if a hard alloy cutting tool experiences chipping along the cutting edge, a base material with higher fracture toughness should be selected. When the cutting edge of the tool directly fails or breaks, a possible solution is to select a base material with a higher transverse fracture strength or higher compressive strength. For processing scenarios with high cutting temperatures (such as dry cutting), tool materials with higher hardness should usually be preferred. In processing scenarios where hot cracks can be observed in the cutting tool (most common in milling operations), it is recommended to select tool materials with better thermal shock resistance.

3 Coating Selection

The coating also helps to improve the cutting performance of the tool. The current coating technologies include:

① Titanium nitride (TiN) coating: This is a universal PVD and CVD coating that can enhance the hardness and oxidation temperature of cutting tools.

Titanium nitride (TiCN) coating: By adding carbon to TiN, the hardness and surface roughness grade of the coating are enhanced.

Titanium aluminum nitride (TiAlN) and aluminum titanium nitride (AlTiN) coatings: The composite application of aluminum oxide (Al2O3) layers with these coatings can enhance the tool life in high-temperature cutting processes. Alumina coating is particularly suitable for dry cutting and near-dry cutting. The AlTiN coating has a higher aluminum content and, compared with the TiAlN coating which has a higher titanium content, it has a higher surface hardness. AlTiN coatings are typically used in high-speed cutting processes.

Chromium nitride (CrN) coating: This coating has excellent anti-adhesion performance and is the preferred solution for combating built-up edge.

Diamond coating: Diamond coating can significantly enhance the cutting performance of tools for processing non-ferrous materials and is highly suitable for processing graphite, metal matrix composites, high-silicon aluminum alloys and other highly abrasive materials. However, diamond coatings are not suitable for processing steel parts because their chemical reaction with steel can damage the adhesion between the coating and the substrate.

In recent years, the market share of PVD-coated cutting tools has expanded, and their prices are also comparable to those of CVD-coated cutting tools. The thickness of CVD coatings is usually 5 to 15μm, while that of PVD coatings is approximately 2 to 6μm. When applied to the substrate of the cutting tool, the CVD coating will generate unwanted tensile stress. The PVD coating, on the other hand, helps to exert 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 on cutting tools that require extremely sharp cutting edges.

4. Preparation of cutting edges

In many cases, the preparation of the cutting edge of a blade (or edge blunting) has become the dividing line that determines the success or failure of the processing. The passivation process parameters need to be determined according to specific processing requirements. For instance, the requirements for blunting the cutting edge of blades used for high-speed finishing of steel parts are different from those for rough machining. Edge passivation can be applied to the processing of almost any type of carbon steel or alloy steel blades, but its application in the processing of stainless steel and special alloy material blades is somewhat limited. The passivation amount can be as small as 0.007mm or as large as 0.05mm. To enhance the cutting edge in harsh processing conditions, a tiny T-shaped edge band can also be formed by blunting the cutting edge.

Generally speaking, the inserts used for continuous turning and milling most steel and cast iron need to undergo a considerable degree of edge blunting. The amount of passivation depends on the grade of cemented carbide and the type of coating (CVD or PCD coating). For heavy-duty intermittent cutting inserts, severely blunting the cutting edge or machining T-shaped edge bands has become a prerequisite. Depending on different coating types, the passivation amount can approach 0.05mm.

On the contrary, since the blades used for processing stainless steel and high-temperature alloys are prone to forming built-up edge, it is required that the cutting edge remain sharp and only undergo slight passivation (as small as 0.01mm), and even a smaller amount of passivation can be customized. Similarly, the blades used for processing aluminum alloys are also required to have sharp cutting edges.

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