Experimental research of ploughing extrusion forming multi-tooth tool wear and damage mechanism and workpiece defects

Abstract

According to the characteristics of ploughing extrusion forming and the theory of metal cutting, the wear and damage mechanism, common wear and damage forms and causes of multi-tooth tools in ploughing extrusion forming are analyzed. Then, the correctness of the analysis of tool wear and damage mechanism and common wear and damage forms is verified by field cutting experiments. At the same time, the phenomena and causes of workpiece stagnation and cracks in the process of ploughing and extrusion forming were obtained through experiments. Finally, the reasonable processing parameters of ploughing and extrusion forming multi-tooth cutting tool were obtained.

Keywords

Ploughing extrusion forming multi-tooth tool wear mechanism microgroove

1. Introduction

With the fast development of major industrial pillar industries such as shipbuilding, national energy, engineering machinery, military industry, aerospace, metallurgy and transportation, the machining quality requirements for large and heavy parts are getting higher and higher [1, 2, 3, 4]. Therefore, improving the rotation speed, machining efficiency and machining accuracy of large/heavy CNC vertical Machine tool workbenches are key issues that urgently need to be resolved in the heavy equipment design and manufacturing industry [5, 6, 7, 8]. In addition, hydro-static rolling/sliding thrust bearings are the crucial components of the large/heavy CNC vertical machine tool. Its working performance directly affects the machining efficiency and machining quality of the equipment [9, 10, 11, 12]. The prime reason is that the temperature of the oil film rises rapidly with the increasing of the table rotation speed, which makes the bearing oil film get thinner and easier to rupture, leading to lubrication failure [13, 14, 15]. Therefore, it is very important to deal with the problem of oil film temperature rising of the hydro-static thrust bearings. The Heat pipe with fast thermal response, good isotherm, high thermal conductivity, simple structure and no additional electric drive becomes an ideal component for heat conduction in a high heat flux environment [16, 17, 18]. The processing of groove structure on the inner wall of micro heat pipe is the most urgent problem to be solved. The spinning forming process of South China University of technology is too complex. Therefore, the fabrication of micro grooves in grooved micro heat pipes with good wick performance is an urgent problem to be solved. Due to the characteristics of heat pipe material and the structure of micro groove, ploughing extrusion forming is a reasonable method to form micro groove of heat pipe [19, 20].

2. Friction and wear mechanism analysis of multi-tooth tools

In the process of ploughing and extrusion copper material with multi-tooth tool, friction occurs between the tool material and copper material, which causes the surface of the tool to be worn to a certain extent by the influence of force, heat and other factors. The friction and wear of multi-tooth tools were analyzed by observing the surface of the tools under scanning electron microscopy and combining the theoretical knowledge of friction and wear after machining micro-grooves in the inner hole with multi-tooth tools.

The ploughing extrusion machining test was completed on the XK5032 CNC vertical lifting platform machine tool. The cutting tool materials were high speed steel (W18Cr4V) and bearing steel (GCr15) and the workpiece material was pure copper. The hardness of ploughing extrusion forming multi-tooth tool is HRC60-62, Ploughing extrusion speed is set at 10–20 mm/min, Scanning electron microscopy was used to observe the tool morphology after the experiment. In the process of machining, the copper plate was fixed on the workbench, the tool moved in a straight line along the Y axis, and the raised fins were ploughed and extruded on the surface of the copper plate. The ploughing extrusion forming is similar to broaching, but no chips are produced during the machining. The ploughing process is shown in Fig. 1.

Figure 1.

Schematic diagram of copper block processing for multi-tooth ploughing tools.

2.1 Friction and wear mechanism of multi-tooth tools

Friction is the tangential resistance that prevents the relative sliding of two contacting objects when the contact surface slides or has a tendency. During the ploughing process, the multi-tooth tool contacts with the surface of copper material, makes relative motion and interacts with each other. Because of the large force and thermal stress at the contact point of multi-tooth tool surface, the particles of tool material are taken away by chips or workpieces, so the cutting part of the tool gradually loses, and the shape of multi-tooth tool changes relative to the original shape, resulting in wear and tear. According to the analysis of the working mechanism of multi-tooth tools and the characteristics of copper materials processed, the main wear forms of multi-tooth tools are bond wear and so on.

2.2 Bond wear

When a certain cutting temperature and contact pressure are reached, a micro-molecular force is produced between the multi-tooth cutting tool material and the workpiece material, which adsorbs between two different materials. This phenomenon is called bond wear (also known as “cold welding”). According to the principle of bond wear, when the multi-tooth tool moves relative to the workpiece continuously, the particle of the multi-tooth tool material will be taken away by the workpiece material.

In the theoretical study of bond friction, two metal surfaces contact each other and are subjected to normal force. Because the contact surface has some slightly raised points, the contact normal force at the raised point is quite large. Once the stress exceeds the material yield limit, the material undergoes plastic deformation and turns to plane contact until the contact surface increases to be able to withstand all loads. In the ideal case, the relationship between load and real contact area is as follows:

$\displaystyle F_{N}=A_{r}\cdot\sigma_{s}$ (1)

Where, $F_{N}$ is load, $A_{r}$ is real contact area, and $\sigma_{s}$ is Compression yield limit.

In this case, the metal contact point produces bonding. The two metal materials slip relatively and the bond is cut off due to the tangential force. The process of bonding and friction is the process of ceaseless bonding and cutting. Bonding friction is to overcome the shear force produced by cutting. When the hardness of the two materials is different and friction occurs, the harder metal will cut off the harder metal, so this ploughing effect increases the friction force by a force $F_{e}$ . Researchers have found that close contact between metals results in strong bonding (i.e., bonding at bumps). Assuming that the shear strength at the bond is $\tau^{\prime}_{b}$ , the friction force $F_{f}$ can be expressed as follows:

$\displaystyle F_{f}=A_{r}\tau^{\prime}_{b}+F_{e}$ (2)

In general, the value of $F_{e}$ is very small and has little influence on the whole friction force, which can be neglected in calculation. Therefore, the friction force can be approximated to:

$\displaystyle F_{f}=A_{r}\tau^{\prime}_{b}$ (3)

Based on the above formulas, the formula of bond friction force is deduced:

$\displaystyle F_{f}=\frac{F_{N}}{\sigma_{s}}\tau^{\prime}_{b}$ (4)

In the study of bond wear, it is assumed that the distance between atoms on the tool-workpiece contact surface is less than the distance between atoms of the object, and there will be strong molecular force between different atoms. When the bonded two metal surfaces are forced to separate, a certain amount of atoms will be taken away from the metal surface, which is bond wear. If each hard particle is set as a micro-convex body, the total wear amount after sliding for such a long distance of L is as follows:

$\displaystyle Q=k_{m}\beta\frac{N}{\sigma_{b}}(1+\alpha\mu^{2})^{\frac{1}{2}}L$ (5)

Where, $k_{m}$ is probability coefficient that the geometric factors of the surface layer are mainly considered. $\mu$ is friction coefficient, $\alpha$ is coefficient of contact area increase caused by shear force; $\sigma_{b}$ is yield strength.

According to Eq. (5), the tool wear is proportional to the normal pressure, and the yield strength of the tool decreases with the increase of temperature. Therefore, the higher the cutting temperature, the greater the bond wear of the tool.

Tool material is high speed steel (W18Cr4V) and bearing steel (GCr15). After finishing the machining under the same geometric parameters and processing parameters of the tool structure, the wear situation of the two different materials of the tool is compared and analyzed. In the process of ploughing and extrusion micro-grooves in the inner hole of flat plate with multi-tooth tools, friction between tool material and workpiece material produces a lot of heat. Because the small hole is not conducive to heat dissipation, the heat generated by multi-tooth tools can not flow out in time. A large amount of cutting heat accumulates in the cutting area, and the cutting temperature rises sharply, which reduces the yield limit of the tool. Moreover, the copper material itself is very viscous and easy to adhere to the surface of the tool. With the continuous ploughing and extrusion of the tool, the wear of multi-tooth tool increases. The structure of tool material is different from that of tool material, and the internal stress distribution is uneven due to the internal defects and cracks, which results in the different performance of each part of the tool. Therefore, bond wear is more likely to occur in parts with poor tool performance [10, 11, 12, 13, 14]. The local bond wear morphology of multi-tooth tools under electron microscopy is shown in Fig. 2. Figure 2a is the bond wear morphology of multi-tooth cutting tools for high-speed steel. The reason is that the copper material flows through the tool surface to form internal friction during the process of ploughing and extrusion of multi-tooth cutting tools, which results in friction deformation on the surface of the outflow copper material and hardens and adheres to the tool surface, which results in greater resistance in the direction of tool processing and in the tool itself. Due to the influence of stress and defect, the tool has bond wear during continuous machining. Figure 2b is the bond wear morphology of multi-tooth cutting tools for bearing steel is shown. The bond wear process is basically the same as that of high-speed steel. However, due to the lower yield limit of bearing steel, the time of oxide layer falling off on the surface of its own cutting tools is faster after the same processing time. After the oxide layer peeling off, the surface roughness of the new material exposed by the tool is larger, and the bond wear is more serious.

Figure 2.

Multi-tooth tool topical adhesive wear morphology.

According to the comparison of friction and wear profiles of multi-tooth tools with two materials and the same structure, under the same processing conditions, the wear of multi-tooth tools for high-speed steel is less than 0.01 mm per ploughing extrusion 3 m. While the wear rate of multi-tooth cutting tools for bearing steel is 3–4 times higher than that of high-speed steel. In conclusion, the multi-tooth cutting tools of high-speed steel have better anti-bond wear effect in the process of processing.

2.3 Other wear

Bond wear occurs along with oxidation wear and diffusion wear, which affects the wear of multi-tooth cutting tools. In the process of ploughing and extrusion, the higher the temperature, the oxidation of W, C and other elements in the tool material with oxygen in the air to form oxide film, the lower the hardness and strength, resulting in the friction between the multi-tooth tool and the material more vulnerable to wear. However, due to the difficulty of air entering in the ploughing extrusion environment of multi-tooth tool, the amount of oxide layer on the surface of the tool can be neglected, so it can be neglected when considering the wear of multi-tooth tool. In the process of ploughing and extrusion, W will diffuse by displacement, while C and other elements will diffuse by gap, but because the processed material is copper, its chemical activity is not high, so the diffusion speed of W, C and other elements is very slow even if plastic deformation occurs in the process of processing. Therefore, the wear of multi-tooth tools can also be neglected.

3. Study on the damage mechanism of multi-tooth cutting tool

The damage of multi-tooth tools affects the service life, production cost, processing efficiency and quality of multi-tooth tools. Therefore, it is of great significance to study the damage forms of multi-tooth tools.

3.1 Damage analysis of multi-tooth tool

When the multi-tooth tool is ploughing and extrusion the inner hole of the flat plate, the metal will be plastic deformed and then the groove and the top of the teeth of the multi-tooth tool will flow. The flowing metal and the multi-tooth tool surface will produce friction and contact stress. At the same time, the tool will be subjected to the high temperature, the impact caused by vibration and the uneven force caused by the process system. Moreover, the structure of each tool teeth on multi-tooth tool is relatively small, and it is easy to be damaged when it is subjected to the above conditions.

3.2 Damage analysis of bearing steel multi-tooth tool

When multi-tooth tools are made of bearing steel (GCr15), it is mainly analyzed that it has the characteristics of high fatigue strength and high elastic limit. However, after quenching and tempering, the yield strength is usually between 1574 and 1837 MPa, and the material strength is obviously lower. When machining the inner hole of flat plate, the multi-tooth tools are subjected to friction stress, contact stress and thermal stress when they are all in the process. When the multi-tooth tool plows and extrudes the workpiece, the plastic deformation of the workpiece material occurs. As the multi-tooth tool advances along the working direction, part of the metal flows into the groove of the multi-tooth tool. The tool teeth have resistance to the plastic deformation of the flowing metal, so there is a large friction stress and contact stress between the tool teeth and the flowing metal. Because there are some defects and cracks in the tool material, when the tool is continuously subjected to various stresses, there will be contact fatigue cracks in the material at a certain depth on the surface of the tool teeth, and there will be obvious cracks at the root of the tool teeth, and mechanical impact and vibration will inevitably occur in the mechanical processing system. When the allowable stress of multi-tooth tool exceeds, the tool will break teeth and crush. Figure 3 shows the damage form of bearing steel tool. When the multi-tooth tool of bearing steel (GCr15) is used to process the micro-grooves in the inner hole of flat plate, the tool will crush or break its teeth when the average machining length is 2–2.5 m.

Figure 3.

GCr15 multi-tooth tool failure mode electron micrographs.

3.3 Damage analysis of high speed steel multi-tooth tool

High-speed steel (W18Cr4V) is chosen as tool material for multi-tooth tools. The main characteristics of multi-tooth tools are high strength, high wear resistance and red hardness. The bending strength of multi-tooth tools is very high, generally reaching 3–3.2 GPa. In ploughing process, the stress situation is the same as that of bearing steel multi-tooth tools, but because of its superior material performance, the damage phenomenon is not obvious in the long-term processing process, and its failure mode is mainly wear and tear. After many times of ploughing and extrusion processing, the tool diameter has little change with that before machining. Figure 4 is the scanning image of high-speed steel multi-tooth tool after machining. The tool wear is 0.01–0.03 mm when the multi-tooth tool of high speed steel is used to process the inner hole of the flat plate for 6 m.

Figure 4.

Electron microscopic scanning chart of high speed steel multi-tooth tool.

Figure 5.

Complete micro-groove structure.

According to the comparative analysis of the damage of two materials, the failure modes of bearing steel (GCr15) multi-tooth tools are damage and wear. The failure modes of high-speed steel (W18Cr4V) multi-tooth tools are mainly wear. Moreover, the service life of high-speed steel multi-tooth tools is much longer than that of bearing steel.

4. Analysis of machining microgrooves with multi-tooth tools

The purpose of multi-tooth cutting tool design is to process micro-groove structure of inner hole of flat plate. Whether continuous and complete micro-groove structure can be machined is a ruler to judge the performance of cutting tool. However, the formation of micro-groove structure is affected by the formulation of processing technology, the processing system and the performance of multi-tooth tools. Therefore, this paper combines with the analysis of some problems in processing micro-groove structure of plate inner hole with multi-tooth tool, and how to make multi-tooth tool process micro-groove structure better.

4.1 Incomplete micro-groove phenomenon and resolvent

When the processed plate is cut along the axial direction of the aperture, a complete micro-groove structure should be obtained according to the processing requirements as shown in Fig. 5. However, some or all slipping phenomena occur after the multi-tooth tool is used to process the plate, as shown in Fig. 6. According to the analysis of the processing process, the causes of incomplete groove and slippage are mainly due to the friction stress and contact stress of the multi-tooth tool which is resisted by copper material during the ploughing extrusion process driven by the tool rod. Because the surface roughness of each part of the multi-tooth tool is not uniform, the friction stress of each part of the multi-tooth tool is also unequal. In the process of ploughing, the ploughing direction of the multi-tooth tool inclines and the phenomenon of incomplete or slippery cutting occurs. In the process of processing, the feeding speed of ploughing should be lowered as far as possible to ensure the smooth processing. If there are difficulties in ploughing tools, the processing should be stopped in time, the workpiece should be dropped or remachined, and the phenomenon of slippery cutting should be eliminated.

Figure 6.

Incomplete groove phenomenon.

4.2 Hysteroma phenomenon and resolvent

The surface of the microgroove processed normally should be smooth as shown in Fig. 5, and no foreign body resides on it. However, in the process of processing, the phenomenon of stagnation appears as shown in Fig. 7. The main reason for this is that a small amount of chips will be produced by multi-tooth tools in the process of processing, and the chips can not be removed from the processing site in time. The chips adhere to the tool rake face and ploughing along with the tool. But when the chips accumulate to a certain height, the chips will be plastic hardened and bonded with the newly plastic deformed material, and will be removed from the tool surface and drop in the upper part of the groove. In the process, we should try our best to avoid the occurrence of stagnant tumors, such as long chips on the tool surface for a long time, which will make it difficult to ploughing the tool. When the tool surface falls off, the material on the tool surface will be peeled off. It is found that when ploughing speed f is 10 mm/min, it is not easy to stagnate in the process of processing.

Figure 7.

Microgroove forming stranded phenomenon.

Figure 8.

Microgroove forming crack phenomenon.

4.3 Crack phenomenon and resolvent

When vibration or shock occurs in the machining system, cracks will appear in the machined micro-groove structure as shown in Fig. 8. When vibration or impact occurs in machining, the tool runs radially, so there will be cracks on the surface of the micro groove. Vibration of the system can not be avoided in the process, but the processing feed can be adjusted properly. When the ploughing feed f is 10 mm/min, the processing of the mechanical system is more stable and the phenomenon of cracks is less. According to some problems of multi-tooth tool in the process of machining, it is concluded that ploughing speed f is 10 mm/min, which has a good influence on the quality of micro-groove structure. In this paper, the micro groove forming tools of different materials with fixed size are analyzed. However, the plough forming tool size parameters are also used as variables to analyze the optimal cutting parameters, which is the focus of the next research work of the project team.

5. Conclusion

In this paper, the failure modes of multi-tooth cutting tools of high-speed steel (W18Cr4V) and bearing steel (GCr15) are analyzed. It is concluded that the failure modes of multi-tooth cutting tools of high-speed steel are mainly wear (bond wear). The failure modes of bearing steel cutting tools are teeth breaking, crushing and wear. By comparison, the service life of multi-tooth cutting tools of high-speed steel is longer. The reasons of slipping, stagnation and cracks in the micro-groove structure of the inner hole of the flat plate machined by cutting tools are analyzed. Through repeated experiments, it is concluded that the bad phenomena can be avoided when the feed speed of ploughing is 10 mm/min.

Footnotes

Acknowledgments

This project is supported by National Natural Science Foundation of China (U2031142,11803013); Heilongjiang Provincial Natural Science Foundation of China (LC2017028); Basic Scientific Research Business Expense Research Project of Heilongjiang Provincial Colleges and Universities(135409102); Academic backbone project of Heilongjiang Provincial Department of Education (135509413).

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