[Edit this paragraph] Fatigue of metal materials
Many mechanical parts and engineering parts work under alternating loads. Under the action of alternating load, although the stress level is lower than the yield limit of the material, after a long period of repeated stress cycles, a sudden brittle fracture will occur, which is called fatigue of metal materials.
The fatigue fracture of metallic materials is characterized by:
(1) The load stress is alternating; (2) The action time of the load is long;
(3) The fracture is instantaneous;
(4) Both plastic and brittle materials are brittle in the fatigue fracture zone.
Therefore, fatigue fracture is the most common and dangerous fracture form in engineering.
According to different conditions, the fatigue phenomenon of metal materials can be divided into the following types:
(1) High cycle fatigue: refers to the fatigue with stress cycle above 100000 under the condition of low stress (working stress is lower than the yield limit of materials, even lower than the elastic limit). This is the most common fatigue failure. High cycle fatigue is generally called fatigue.
(2) Low cycle fatigue: refers to fatigue with stress cycle lower than 10000~ 100000 under high stress (working stress is close to the yield limit of materials) or high strain conditions. Because alternating plastic strain plays a major role in this fatigue failure, it is also called plastic fatigue or strain fatigue.
(3) Thermal fatigue: refers to fatigue damage caused by repeated thermal stress caused by temperature change.
(4) Corrosion fatigue: refers to the fatigue damage of machine parts under the combined action of alternating load and corrosive medium (such as acid, alkali, seawater and active gas).
(5) Contact fatigue: This refers to the contact surface of machine parts, which is pitted and peeled off or the surface is crushed and peeled off under the repeated action of contact stress, resulting in the failure and damage of the parts.
Plasticity of metal materials
Plasticity refers to the ability of metal materials to produce permanent deformation (plastic deformation) under the action of external force without being destroyed. When a metal material is stretched, its length and cross-sectional area will change. Therefore, the plasticity of metal can be measured by two indicators: elongation of length (elongation) and shrinkage of section (shrinkage of section).
The greater the elongation and area shrinkage of metal materials, the better the plasticity of materials, that is, materials can withstand greater plastic deformation without damage. Generally, metal materials with elongation greater than 5% are called plastic materials (such as low carbon steel), and metal materials with elongation less than 5% are called brittle materials (such as gray cast iron). Materials with good plasticity can produce plastic deformation in a larger macro range, and at the same time, metal materials can be strengthened through plastic deformation, thus improving the strength of materials and ensuring the safe use of parts. In addition, materials with good plasticity can be successfully processed in some molding processes, such as stamping, cold bending, cold drawing and straightening. Therefore, when selecting metal materials as mechanical parts, certain plasticity indexes must be met. String 2
Hardness of metal materials
Hardness indicates the ability of a material to resist hard objects pressing into its surface. It is one of the important performance indexes of metal materials. Generally, the higher the hardness, the better the wear resistance. Commonly used hardness indexes include Brinell hardness, Rockwell hardness and Vickers hardness.
1. Brinell hardness
A hardened steel ball with a certain size (generally diameter 10mm) is pressed into the surface of the material with a certain load (generally 3000kg) and kept for a period of time. The ratio of load to indentation area after unloading is Brinell hardness (HB), and the unit is kilogram force/square millimeter (N/mm2).
2. Rockwell hardness (hours)
When HB & gt450 or the sample is too small, the Brinell hardness test cannot be used, and Rockwell hardness measurement can be used instead. It uses a diamond cone with a vertex angle of120 or steel balls with diameters of 1.59 and 3. 18mm to press into the surface of the measured material under a certain load, and the hardness of the material can be obtained from the indentation depth. According to the hardness of the test materials, there are three different types of clay buildings. HRA: It is the hardness measured by a diamond cone press under a load of 60kg. Used for extremely hard materials (such as cemented carbide).
HRB: Hardness is obtained from hardened steel balls with a load of 100kg and a diameter of 1.58mm, and is used for materials with low hardness (such as annealed steel and cast iron).
HRC: It is the hardness measured by a diamond cone press with a load of 150kg. Used for materials with high hardness (such as hardened steel).
3 vickers hardness (HV)
Vickers hardness value (HV) is obtained by pressing the surface of the material with a load within 120kg by a diamond square cone press with the apex angle of 136, and dividing the surface area of the indentation pit by the load value.
Hardness test is the simplest test method in mechanical properties test. In order to replace some mechanical properties tests with hardness tests, more accurate conversion relationship between hardness and strength is needed in production.
Practice has proved that there is an approximate correspondence between various hardness values of metal materials and between hardness values and strength values. Because the hardness value is determined by the initial plastic deformation resistance and the continuous plastic deformation resistance, the higher the strength of the material, the higher the plastic deformation resistance and the higher the hardness value.
[Edit this paragraph] Characteristics of metallic materials
The performance of metal materials determines the scope and rationality of application. The properties of metal materials are mainly divided into four aspects, namely, mechanical properties, chemical properties, physical properties and technological properties. Metal is hard.
[Edit this paragraph] Mechanical properties
(a) the concept of stress, the force on the unit cross-sectional area inside an object is called stress. The stress caused by external force is called working stress, and the stress balanced in an object without external force is called internal stress (such as organizational stress, thermal stress, residual stress after machining, etc.). ).
(2) Mechanical properties, the ability of metal to resist deformation and fracture under the action of external force (load) at a certain temperature, is called the mechanical properties of metal materials. There are many forms of load borne by metal materials, which can be static load or dynamic load, including tensile stress, compressive stress, bending stress, shear stress, torsional stress, friction, vibration and impact. Alone or at the same time. Therefore, the indicators to measure the mechanical properties of metal materials mainly include the following items:
1. Power
This represents the maximum ability of materials to resist deformation and damage under the action of external force, and can be divided into tensile strength limit (σb), bending strength limit (σbb), compressive strength limit (σbc) and so on. Because metal materials have certain rules to follow from deformation to failure under the action of external force, tensile test is usually used to judge, that is, metal materials are made into samples of certain specifications and stretched on a tensile testing machine until the samples break. The determined strength indicators mainly include:
(1) strength limit: the maximum stress that a material can resist fracture under the action of external force, generally refers to the tensile strength limit under the action of tensile force, which is expressed by σb, such as the strength limit corresponding to the highest point b in the tensile test diagram, which is commonly used in megapascals (MPa), and the conversion relationship is: 1mpa = 1n/m2 = (9.8. C to the maximum stress when the material breaks (or the maximum load that the sample can bear); Fo？ C the original cross-sectional area of the tensile specimen.
(2) Yield strength limit: When the external force borne by the metal material sample exceeds the elastic limit of the material, although the stress no longer increases, the sample still undergoes obvious plastic deformation. This phenomenon is called yield, that is, when the material bears an external force to a certain extent, its deformation is no longer directly proportional to the external force, resulting in obvious plastic deformation. The stress when yielding occurs is called yield strength limit, which is expressed by σs, and the point S corresponding to the tensile test curve is called yield point. For materials with high plasticity, there will be obvious yield point on the tensile curve, while materials with low plasticity have no obvious yield point, so it is difficult to find the yield limit according to the external force of yield point. Therefore, in the tensile test method, the stress when the gauge length on the specimen undergoes 0.2% plastic deformation is usually defined as the conditional yield limit, which is expressed as σ0.2. The yield limit index can be used as a design basis for requiring parts not to produce obvious plastic deformation when working. However, for some important parts, the yield ratio (σs/σb) is also considered to be smaller to improve its safety and reliability, but the material utilization rate is also low at this time.
(3) Elastic limit: the ability of a material to deform under the action of an external force, but to return to its original state after the external force is removed, which is called elasticity. The maximum stress that a metal material can maintain elastic deformation is the elastic limit, which corresponds to point E in the tensile test diagram and is expressed by σe, and the unit is MPa): σ e = Pe/FO, where Pe is the maximum external force (or the load when the material is in the maximum elastic deformation).
2. plasticity,
(1) Brinell hardness (code HB). Under a specified load P, a hardened steel ball with a certain diameter D is pressed into the surface of the sample. After a certain time, the load is released, and the surface of the sample will leave an indentation with a surface area of F, and the hardness of the sample is expressed by the size of the load on the unit surface area of the sample: HB = P/F. In practical application, the diameter of the pit is usually measured directly. According to the load P and the diameter D of the steel ball, Brinell hardness has a certain relationship with the tensile strength of the material: σb≈KHB, and k is the coefficient, for example, for low carbon steel, K≈0.36, for high carbon steel, K≈0.34, for quenched and tempered alloy steel, K≈0.325, … and so on.
(2) Rockwell hardness (HR) A diamond cone indenter with a certain vertex angle (such as 120) or a hardened steel ball with a certain diameter d is pressed into the surface of the sample under a certain load p, and the load is removed after a certain time, leaving an indentation with a certain depth on the surface of the sample. The pit depth is automatically measured by Rockwell Hardness Machine and displayed by hardness reading (obviously, the deeper the pit, the lower the hardness and the smaller the Rockwell Hardness value). According to the different indenter and load, Rockwell hardness can be divided into HRA, HRB and HRC, among which HRC is the most commonly used. There is the following conversion relationship between Rockwell hardness HRC and Brinell hardness HB: HRC≈0. 1HB. In addition to the most commonly used Rockwell hardness HRC and Brinell hardness HB, there are Vickers hardness (HV), Shore hardness (HS), microhardness and Richter scale hardness (HL). Here, I want to explain the Richter scale hardness, which is the most novel hardness characterization method at present. It was measured with a Richter hardness tester. Its detection principle is: the impact device of Richter hardness tester releases the punch from a fixed position, and the punch quickly impacts the sample surface. The impact speed and rebound speed (induced as impact voltage and rebound voltage) of the punch at the distance of 65438±0mm from the sample surface are measured by electromagnetic induction of the coil, and the Richter hardness value is expressed by the ratio of the rebound speed and impact speed of the punch. 1000 formula: HL- Richter scale hardness; Vr- punching rebound speed; Vi- Impact speed of the punch (Note: In practical devices, the impact voltage and rebound voltage induced by the closed coil in the impact device represent the impact speed and rebound speed). The structure of impact device mainly includes built-in spring (loading sleeve, different types of impact devices have different impact energy), conduit, release button, built-in coil and skeleton, support ring and punch. The punch mainly adopts two kinds of extremely hard spheres, diamond and tungsten carbide (different types of impactors have different punch diameters). Advantages: The host computer of Richter hardness tester receives the signal from the impact device for processing and calculation, and then directly displays the Richter hardness value on the screen. Portable Richter hardness tester can be converted into Brinell (HB), Rockwell (HRC), Vickers (HV) and Shore (HS) hardness after being measured by Richter hardness tester (HL). Or directly use Brinell hardness (HB), Rockwell hardness (HRC), Vickers hardness (HV), Richter scale hardness (HL) and Shore hardness (HS) to measure the hardness value according to the Richter scale hardness principle, and at the same time, the tensile strength σb of the material can be converted, and the measurement results can also be stored, directly printed or sent to a computer for further data processing. 4. Toughness
The ability of metal materials to resist damage under impact load is called toughness. Usually, the impact test is adopted, that is, when a metal sample with a certain size and shape is broken under impact load on a specified impact testing machine, the impact work consumed per unit cross-sectional area on the fracture surface represents the toughness of the material: αk=Ak/F unit J/cm2 or Kg? m/cm2， 1Kg？ M/cm2=9.8J/cm2αk is called the impact toughness of metal materials, Ak is the impact work, and f is the original cross-sectional area of fracture. 5. Fatigue strength limit Under the action of long-term repeated stress or alternating stress (stress is generally less than the yield limit strength σs), the phenomenon that metal materials break without obvious deformation is called fatigue failure or fatigue fracture. This is because of various reasons, the stress (stress concentration) on the surface of the part is greater than σs, or even σb, which leads to plastic deformation or microcrack of the part. With the increase of repeated alternating stress, the crack gradually expands and deepens (stress concentration at the crack tip), which leads to this. In practical application, under the action of repeated or alternating stresses (tensile stress, compressive stress, bending or torsion stress, etc.), the maximum stress that a sample can withstand without fracture within a specified number of cycles (106~ 107 for general steel and 108 for non-ferrous metals). ) is generally regarded as the fatigue strength limit, and σ-6508 is used. In addition to the five most commonly used mechanical properties mentioned above, for some materials with particularly strict requirements, such as metal materials used in aerospace, nuclear industry and power plants, the following mechanical properties are also required: creep limit: under a certain temperature and constant tensile load, the phenomenon that the material slowly produces plastic deformation over time is called creep. Usually, high-temperature tensile creep test is adopted, that is, under constant temperature and constant tensile load, the maximum stress when the creep elongation (total elongation or residual elongation) or creep elongation speed of the sample does not exceed the specified value in a relatively constant stage is expressed as MPa, where τ is the test duration, t is the temperature, δ is the elongation and σ is the stress; Or, v is the crawling speed. Tensile endurance strength limit at high temperature: the maximum stress of a specimen under constant temperature and constant tensile load for a specified time without breaking, expressed in MPa, where τ is the duration, t is the temperature and σ is the stress. Metal notch sensitivity coefficient: expressed by Kτ as the ratio of stress between notch sample and smooth sample without notch under the same duration (high temperature tensile endurance test), where τ is the test duration, stress of notch sample and stress of smooth sample. Or expressed as the ratio of the duration of notched samples to the duration of smooth samples under the same stress σ. Heat resistance: the ability of a material to resist mechanical load at high temperature.
[Edit this paragraph] Chemical properties
The chemical properties of metals are called the chemical properties of metals. In practical application, we mainly consider the corrosion resistance and oxidation resistance of metals (also called oxidation resistance, which refers to the resistance or stability of metals to oxidation at high temperature), and the influence of compounds formed between different metals and between metals and nonmetals on mechanical properties. Among the chemical properties of metals, especially the corrosion resistance is of great significance to the corrosion fatigue damage of metals.
[Edit this paragraph] Physical properties
The physical properties of metals are mainly considered as follows:
(1) density (specific gravity): ρ=P/V unit gram/cubic centimeter or ton/cubic meter, where p is weight and v is volume. In practical application, besides calculating the weight of metal parts according to the density, it is very important to consider the specific strength of metal (the ratio of strength σb to density ρ) to help material selection, as well as the acoustic impedance (the product of density ρ and sound velocity c) in acoustic testing related to nondestructive testing and the different absorption capacities of materials with different densities in radiographic testing.
(2) Melting point: The temperature at which a metal changes from solid to liquid directly affects the melting and hot working of metal materials, which is closely related to the high-temperature performance of the materials. (3) The phenomenon that thermal expansion changes with temperature (expansion or contraction) is called thermal expansion, which is often measured by linear expansion coefficient, that is, the ratio of the increase or decrease of material length when temperature changes 1℃ to its length at 0℃. Thermal expansion is related to the specific heat of materials. In practical application, specific volume (the increase or decrease of material volume per unit weight, that is, the ratio of volume to mass) should also be considered, especially for metal parts working in high temperature environment or alternating cold and hot environment, the influence of its expansion performance must be considered.
(4) The nature of magnetically attracted ferromagnetic objects is magnetism, which is reflected in parameters such as magnetic permeability, hysteresis loss, residual magnetic induction intensity and coercivity. In this way, metal materials can be divided into paramagnetic and diamagnetic, soft magnetic and hard magnetic materials.
(5) Electrical properties mainly consider its conductivity, which affects its resistivity and eddy current loss in electromagnetic nondestructive testing.
[Edit this paragraph] Process performance
The adaptability of metals to various processing methods is called process performance, which mainly includes the following four aspects:
(1) Machinability: It reflects the difficulty of cutting metal materials with cutting tools (such as turning, milling, planing and grinding).
(2) Malleability: It reflects the difficulty of forming metal materials in the process of pressure working, such as the plasticity of materials when heated to a certain temperature (represented by plastic deformation resistance), the allowable temperature range of hot pressure working, the characteristics of thermal expansion and cold contraction, the critical deformation boundary related to microstructure and mechanical properties, and the fluidity and thermal conductivity of metals during thermal deformation.
(3) Castability: it reflects the difficulty of melting and casting metal materials into castings, which is characterized by fluidity, gas absorption, oxidation, melting point, uniformity and compactness of casting microstructure, and cold shrinkage.
(4) Weldability: It reflects the difficulty of local rapid heating of metal materials, so that the connection parts can be quickly melted or semi-melted (requiring pressure), and the connection parts can be firmly combined into a whole, which is manifested in melting point, gettering, oxidation, heat conduction, thermal expansion and contraction characteristics, plasticity, correlation with the microstructure of the connection parts and nearby materials, and influence on mechanical properties.
Principle, process flow and technical characteristics of rapid prototyping technology;
Rapid prototyping belongs to discrete/cumulative molding. Based on the forming principle, a new thinking mode size model is proposed, that is, the three-dimensional model of parts made on the computer is stored in grids and layered to obtain the two-dimensional contour information of each layer section. According to these contour information, the machining path is automatically generated. Under the control of the control system, the molding head selectively solidifies or cuts the molding material layer by layer to form profile sheets of various sections, and sequentially adds three-dimensional blanks layer by layer. Then, the blank is post-processed to form parts.
The process of rapid prototyping is as follows:
L) Establish a three-dimensional model of the product. Because the RP system is directly driven by the 3D CAD model, the 3D CAD model of the machined workpiece should be constructed first. Three-dimensional CAD model can be directly constructed by computer-aided design software (such as Pro/E, I-DEAS, Solid Works, UG, etc.). ), or you can convert the two-dimensional drawings of existing products into three-dimensional models, or you can scan product entities by laser and ct to obtain point cloud data, and then build three-dimensional models by reverse engineering.
2) Approximate processing of 3D model. Because products often have some irregular free-form surfaces, it is necessary to approximate the model before machining to facilitate the subsequent data processing. STL format file has become a quasi-standard interface file in the field of rapid prototyping because of its simple and practical format. It uses a series of small triangular planes to approximate the original model. Each small triangle is described by three vertex coordinates and a normal vector, and the size of the triangle can be selected according to the accuracy requirements. STL file has two output forms: binary code and ASCll code. The output form of binary code takes up much less space than the file output form of ASCII code, but the output form of ASCII code can be read and checked. Typical CAD software has the function of converting and outputting STL format files.
3) Slice the 3D model. According to the characteristics of the processed model, the appropriate processing direction is selected, and the approximate model is cut with a series of planes at certain intervals in the molding height direction, so as to extract the profile information of the section. The interval is generally 0.05mm~0.5mm, and 0. 1mm is commonly used. The shorter the interval, the higher the molding accuracy, but the longer the molding time, the lower the efficiency. Conversely, the lower the accuracy, the higher the efficiency.
4) forming and processing. According to the cross-sectional profile of the slice, under the control of the computer, the corresponding forming head (laser head or spray head) does scanning motion according to the cross-sectional profile information, accumulates materials layer by layer on the workbench, and then bonds the layers together to finally get the prototype product.
5) Post-treatment of molded parts. Take the molded part out of the molding system, grind, polish and coat it, or put it in a high-temperature furnace for post-sintering, so as to further improve the strength.
Classification of rapid prototyping technology;
Rapid prototyping technology can be divided into two categories according to the molding methods: laser technology based on laser and other light sources, such as three-dimensional mask aligner (SLA), layered solid manufacturing (LOM), selective laser powder sintering (SLS), shape deposition molding (SDM) and so on. Spray technology, such as fused deposition molding (FDM), three-dimensional printing (3DP) and multiphase spray deposition (MJD). Here is a brief introduction to the more mature technology.
1, SLA(stereolithography Apparatus) process SLA process, also known as light modeling or stereolithography, was patented by Charles Hull of the United States in 1984. 1988 American 3D systems company launched commercial prototype SLA-I, which is the first rapid prototyping machine in the world. SLA molding machine occupies a large share of RP equipment market.
SLA technology is based on the photopolymerization principle of liquid photosensitive resin. This liquid material can quickly undergo photopolymerization under the irradiation of ultraviolet light with a certain wavelength and intensity, and its molecular weight increases sharply, and the material changes from liquid to solid.
Working principle of SLA: The laser beam filled with liquid photocurable resin in the liquid tank can be scanned on the liquid surface under the action of deflection mirror, and the scanning trajectory and the presence or absence of light are controlled by computer. Where the light spot hits, the liquid will solidify. At the beginning of molding, the working platform is at a certain depth below the liquid level. The focused light spot is scanned point by point on the liquid surface according to the instruction of the computer, that is, solidified point by point. When a layer of scanning is completed, the unirradiated area is still liquid resin. Then the lifting platform drives the platform to descend one layer, and the molded layer is covered with a layer of resin. Scraper will scrape the resin with high viscosity, and then scan the next layer, and the newly circulated layer will be firmly bonded with the upper layer, and so on until the whole part is manufactured, and a three-dimensional solid model will be obtained.
SLA method is the most researched method in the field of rapid prototyping technology at present, and it is also the most mature method in technology. The parts formed by SLA process have high precision, the machining precision can generally reach 0. 1 mm, and the utilization rate of raw materials is close to 100%. However, this method also has some limitations, such as the need for support, the decline of precision caused by resin shrinkage, and the toxicity of light-cured resin.
2.LOM (Laminated Object Manufacturing) process The LOM process is called laminated entity manufacturing or layered entity manufacturing, which was successfully developed by Michael Feygin of Helisys Company in the United States in 1986. The LOM process uses thin materials such as paper and plastic film. The surface of the sheet is pre-coated with a layer of hot melt adhesive. In the process of processing, the hot-pressing roller carries out hot pressing on the plate to make it adhere to the workpiece formed below. The CO2 laser is used to cut the cross-sectional profile of the part and the outer frame of the workpiece on the newly bonded layer, and the grid aligned up and down is cut in the redundant area between the cross-sectional profile and the outer frame. After the laser cutting is completed, the workbench drives the formed workpiece to descend and separate from the strip plate. The feeding mechanism rotates the receiving shaft and the feeding shaft to drive the material belt to move and move the new layer to the processing area. The workpiece joint rises to the machining plane, and the hot roller is hot pressed, so that the number of layers of the workpiece is increased by one layer and the height is increased by one material thickness. Then cut the outline on the new layer. Repeat this process until all parts of the part are bonded and cut. Finally, the chopped redundant parts are removed, and the solid parts manufactured in layers are obtained.
LOM process only needs to cut the outline of the section of the part on the plate, and does not need to scan the whole section. Therefore, the forming speed of thick-walled parts is faster, and it is easy to manufacture large parts. There is no material phase change in the process, so it is not easy to cause warping deformation. The surplus material between the outer frame and the cross-sectional profile of the workpiece plays a supporting role in the machining, so the LOM process does not need support. The disadvantages are serious waste of materials and poor surface quality.
3.SLS (selective laser sintering) process SLS process is called selective laser sintering, which was successfully developed by C.R.Dechard of the University of Texas at Austin in 1989. SLS process is made of powder material. Spread the material powder on the upper surface of the molded part and scrape it flat. Scanning the section of the parts on the newly laid new layer with high-intensity CO2 laser. The material powder is sintered together under the irradiation of high-intensity laser to obtain the cross section of the part, which is connected with the part formed below. When sintering a section, a new layer of material powder is spread out and the lower section is selectively sintered.
After sintering, excess powder is removed, and then the parts are polished and dried.
SLS technology is characterized by a wide range of materials, which can not only make plastic parts, but also make parts made of ceramics, wax and other materials, especially metal parts. This makes SLS process attractive. SLS process does not need support, because there is no sintered powder to support it.
4.3DP (three-dimensional printing) process The three-dimensional printing process was developed by E-manual Sachs of Massachusetts Institute of Technology. It has been commercialized by Soligen Company in the United States, named DSPC (direct shell production casting), and is used to manufacture ceramic shells and cores for casting.
3DP process is similar to SLS process, using powder materials, such as ceramic powder, metal powder, etc. The difference is that the material powder is not connected by sintering, but the cross section of the part is "printed" on the material powder by the nozzle with adhesive (such as silica gel).
Parts bonded with adhesive have low strength and need post-treatment. Firstly, the binder is burned, and then the metal is infiltrated at high temperature, so that the parts are densified and the strength is improved.
5.FDM (Fused Deposition Forming) Process Fused Deposition Manufacturing (FDM) was successfully developed by American scholar Scott Crump on 1988. The materials of FDM are generally thermoplastic materials, such as wax, ABS, nylon and so on. Eat in a filamentous form. The material is heated and melted in the nozzle. The nozzle moves along the cross-sectional profile and filling trajectory of the part, and at the same time, the molten material is extruded, rapidly solidified and condensed with the surrounding materials.
FDM technology description
FDM technology is designed and manufactured by Stratasys and can be used in a series of systems. These systems are FDM Maxum, FDM Titan, Prodigy Plus and Dimension. FDM technology uses ABS, polycarbonate (PC), polyphenylene sulfone (PPSF) and other materials. These thermoplastic materials are extruded into filaments in semi-molten state. On the basis of stacking layer by layer, the prototype is directly constructed from 3D CAD data through deposition. This technology is usually used for molding, assembly, functional testing and conceptual design. In addition, FDM technology can be applied to proofing and rapid manufacturing.