Machinability of materials is an important engineering parameter of machining, which is the deformation ability of materials without damage in the process of plastic deformation.The machinability of materials is divided into two independent parts, namely, stress state machinability and internal machinability.The stress state machinability is mainly controlled by the applied stress and the geometric shape of the deformation zone.Therefore, it is mainly aimed at the machining process and has nothing to do with the material characteristics.The internal machinability depends on the alloy composition, the microstructure determined by the previous processing history and the response to temperature, strain rate and strain during processing.[1]
Chinese name
Machinability
Foreign name
processibility
Substantive
Difficulty in cutting workpiece materials
influence factor
Material chemical composition, mechanical and physical properties, etc
Processability is used in many senses;There is no exact definition.However, in a word, it refers to the difficulty of cutting workpiece materials.In other words, it refers to the degree to which the material is easy to cut.
One of the physical, chemical or mechanical properties of materials is used to characterize the properties of products. Therefore, the properties of materials are excellent, and the quality of industrial products is also good.However, modern industry is based on mass production, and materials with low productivity are not suitable for industrial products.In addition to the above properties, the difficulty of material mechanical manufacturing cannot be ignored, that is, the difficulty of mechanical manufacturing is an important thing in characterizing the performance of materials.
In the degree of difficulty of this kind of machine manufacturing, there is castability that represents the degree of difficulty of casting, and the deformation performance of deformation processing, while for cutting processing, it is machinability.The majority of machine parts are machined under arbitrary shape, so machinability is an important issue for general industrial materials.
Although the so-called processability has the above meaning, it usually considers the following properties:
1) The tool wear should be small, and cutting can be carried out at a high cutting speed.
2) The cutting force should be small.
3) The cutting surface must be good.
4) The cutting temperature should be low.
5) Chips must be long and discontinuous and easy to handle.
Materials with all the above properties are the ones with good workability.However, people's interpretations are different. Some use 1, 2, 3 or 1, 3, 6 to indicate the processability, while the simplest one only uses item 1 to indicate the processability of materials.
However, no matter what is used to represent machinability, the most important thing is that the tool wear of item 1 should be small, and cutting can be carried out at a high cutting speed. Generally speaking, as machinability in a narrow sense, only this item is often valued.Machinability, as the property of materials, has recently been widely regarded as important. Research on the methods of testing machinability, or materials with excellent machinability, namely free cutting steels and free cutting alloys, has achieved results.[2]
Factors affecting material processability
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Mechanical and physical properties of materials
hardnessThe higher the hardness of materials at room temperature, the worse the machinability. For example, white cast iron is more difficult to machine than gray cast iron;The material has high high temperature hardness and poor machinability, for example, heat-resistant steel is difficult to machine;The more serious the work hardening of materials is, the worse the workability is. For example, one of the reasons why austenitic stainless steel is difficult to machine is the serious work hardening, which aggravates tool wear.
strengthThe machinability decreases with the increase of material strength, especially the higher the high temperature strength, the worse the machinability.For example, the strength of 45 steel at room temperature is higher than that of 20CrMo steel, but the strength of 20CrMo steel is higher than that of 45 steel at 600 ℃, so the machinability of 20CrMo steel is worse than that of 45 steel.
plasticityThe material has large plasticity, large cutting deformation, high cutting temperature, and is easy to produce chip accretion, which makes the machinability worse.
toughnessThe higher the material toughness, the greater the cutting force, the higher the cutting temperature, and the more difficult the machining.In addition, material toughness also has a significant impact on chip breaking. When the strength is similar, materials with high toughness are more difficult to break chips than materials with low toughness.
Thermal conductivityThe smaller the thermal conductivity of the material is, the less easily the cutting heat is transmitted, the higher the cutting temperature is, the more serious the tool wear is, and the worse the machinability is.
Chemical composition of materials
carbonThe strength and hardness of carbon steel increase with the increase of carbon content, while the plasticity and toughness decrease with the increase of carbon content.The plasticity and toughness of low carbon steel are higher, and the strength and hardness of high carbon steel are higher, which bring some difficulties to machining.Medium carbon steel has moderate strength, hardness, plasticity and toughness, so it has good workability.
Chromium, nickel, vanadium, molybdenum, tungsten, manganeseThey are the main elements of alloy steel, which can improve the strength and hardness of steel, and some can also improve the toughness of steel.The more these alloy elements, the worse the workability.Therefore, under the same hardness, the machinability of alloy steel is worse than that of carbon steel.
Sulfur, phosphorus, selenium, lead, bismuthAdding a small amount of these elements into steel can reduce the strength and plasticity of steel.Sulfur and manganese and iron in steel form MnS and FeS, which are very soft and have weak adhesion with the metal matrix. It can reduce the cutting force and has a certain lubrication effect. It can reduce the tool wear and surface roughness Ra value.Lead, selenium and bismuth also have similar effects of sulfur.Phosphorus can reduce the plasticity of ferrite and make chips easy to break.These have led to a significant improvement in workability.
Ways to improve material processability
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The machinability of materials has a great impact on the productivity of cutting and the surface quality of parts. Therefore, on the premise of meeting the use requirements of parts, materials with good machinability should be selected as far as possible.At the same time, it should also be noted that the processability of materials is not unchangeable, but can be improved artificially within a certain range and to a certain extent. The improvement ways are as follows:
Adjust chemical composition
It can be seen from the above that the processability of steel can be improved by adding a small amount of sulfur, phosphorus, selenium, lead, bismuth and other elements into the steel.People use this method to smelt a class of materials called "free cutting steel".Although the strength of free cutting steel decreases slightlyTool durabilityHigh, easy to chip break, and can obtain good surface quality.Free cutting steel is mostly used forAutomatic machine toolStandard parts production on.
The machinability of cast iron mainly depends on the amount of free graphite.When the carbon content is constant, the more free graphite, the less cementite and the better workability.Appropriate elements to promote graphitization, such as silicon, aluminum, nickel, copper, titanium, etc., are often added to cast iron to improve its machinability.
Select corresponding heat treatment process
Through heat treatment, the metallographic structure that is not conducive to cutting can be transformed into the metallographic structure that is conducive to cutting, so as to improve the machinability of materials.For example, low carbon steel has poor workability due to its high plasticity and low hardness. Normalizing can reduce plasticity, increase hardness and improve its workability;High carbon steel has poor workability due to its high hardness.Spheroidizing annealing treatment can be adopted to transform the reticular or lamellar cementite structure into spherical, so as to improve the workability;The white cast iron has extremely high hardness and poor machinability. It can be annealed to decompose its cementite into flocculent graphite, which can significantly improve its machinability.[3]
Machinability evaluation
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Machinability evaluation is mainly aimed at the machinability of parts. The manufacturability indicators of parts mainly include economic indicators and technical indicators. For engineers and technicians, machinability is the most direct content of DFM, the main content that affects product technical indicators, and the most basic content of collaboration between design and manufacturing,The part design process is to select the required features (including non geometric information) from the feature library, and obtain the part model composed of features through Boolean operation. According to the factors affecting manufacturability, the manufacturability evaluation can be divided into three parts: feature relationship evaluation, unit feature evaluation, and overall part evaluation. In the feature relationship evaluation, there are tolerance relationship evaluationDimension relationship evaluation;The unit feature evaluation includes feature size accuracy evaluationSurface roughnessEvaluation and feature shape evaluation;In the overall evaluation of parts, there are overall structural process evaluation, overall dimensional accuracy evaluation and overall quality requirements evaluation.These evaluations give the constraints of manufacturing on design in concurrent design, and provide a reference model for the development of evaluation enabling tools in the design phase. Rule based technology can be used for the above evaluation factors, supported by rule base, process information database, manufacturing resource database, feature information database, etc,Determine whether the design scheme meets the manufacturing requirements through matching and reasoning, and give feedback information, so as to make a decision on whether to redesign. At the same time, to ensure the effectiveness of the system, it is necessary to regularly organize experts to verify the evaluation results with expert knowledge. If the verification results are inconsistent, it is necessary to adjust or improve the contents of the corresponding information database,To achieve the consistency of the verification results.[4]
Machinability classification
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In metal cutting, several main performance indicators (hardness, tensile strength, elongation, impact value, thermal conductivity) of common materials that affect the machinability are divided into 12 grades (see the table below) according to the changes in quantity to comprehensively measure the machinability.According to the mechanical and physical properties of the specific material, the machinability grade of the material can be found from the table, and the main factors affecting the machinability can be found, so as to take measures to solve the problem.
The machinability of the workpiece material directly affects the selection of cutting parameters, especially the cutting speed. The following table shows the relationship between the machinability of the workpiece material and the allowable cutting speed.[5]
