- Email: [email protected]
Wear resistance of machine tools' bionic linear rolling guides by laser cladding
Optics & Laser Technology 91 (2017) 55–62
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Wear resistance of machine tools' bionic linear rolling guides by laser cladding
MARK
⁎
Yiqiang Wanga, Botao Liub, , Zhengcai Guoc a b c
College of Mechanical and Energy Engineering, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, China School of Mechanical Engineering, Zhejiang University, Hangzhou 310000, China School of Mechanical Engineering, Taiyuan University of Science and Technology, Taiyuan 030024,China
A R T I C L E I N F O
A BS T RAC T
Keywords: Rolling wear resistance Bionic linear rolling guide Laser cladding
In order to improve the rolling wear resistance (RWR) of linear rolling guides (LRG) as well as prolong the life of machine tools, various shape samples with different units spaces ranged from 1 to 5 mm are designed through the observation of animals in the desert and manufactured by laser cladding. Wear resistance tests reproducing closely the real operational condition are conducted by using a homemade linear reciprocating wear test machine, and wear resistance is evaluated by means of weight loss measurement. Results indicate that the samples with bionic units have better RWR than the untreated one, of which the reticulate treated sample with unit space 3 mm present the best RWR. More specifically, among the punctuate treated samples, the mass loss increases with the increase of unit space; among the striate treated samples, the mass loss changes slightly with the increase of unit space, attaining a minimum at the unit space of 4 mm; among the reticulate treated samples, with the increase of unit space, the mass loss initially decreases, but turns to increase after reaching a minimum at the unit space of 3 mm. Additionally, the samples with striate shape perform better wear resistance than the other shape groups on the whole. From the ratio value of laser treated area to contacted area perspective, that the samples with ratio value between 0.15 and 0.3 possess better wear resistance is concluded.
1. Introduction High precision, efficiency and automation are the features that machine tools face with the rapid development of modern industry, which make a higher requirement on the precision and service life of machine tools. As LRG have many advantages such as high stiffness, smooth motion and low friction coefficient compared with sliding contact guides, they have been widely used for precise positioning devices to transport machine parts through a linear path in machine tools and X-Y tables etc. As the key part of machine tool, LRG's wear resistance level determines the precision level of machine tools to great extent. Due to the excellent properties such as cast-ability, low melting point, low cost and toughness, grey cast iron (GCI) has been utilized as an engineering material for applications where the applied loads are extremely high and cyclic. Additionally, GCI is better as a shockabsorber owing to the amount of flake graphite spread in the iron matrix. But as the presence of graphite, component such as rolling guide made by GCI, which is typically suffered high alternating stress, will be easy to fail if it's used directly without any treatment. Based on survival of the fittest, nature provides species a multi-
⁎
functional structure so that they can survive in the complex environment. The function adapt to living environment is the result of coupling factors on morphologies, structures and materials of living things to the optimization of the biological function. During the past many years, great progress has been made on applied sciences by imitating the biological structures or habits of plants and animals in nature [1–3]. Evidence is provided to demonstrate that the biomimetic structure possesses excellent properties on mechanical behavior [4,5]. Moreover, scientists have carried on many experiments concerning the effect of bionic structure on wear resistance, and the experiment results manifest that the wear resistance of bionic coupling samples is better than that of untreated sample [6–10]. Previous theoretical and experimental study [11–13] suggests that GCI performs satisfactorily under rolling contact fatigue after laser remelting. However, a considerable drawback to laser remelting is the formation of heterogeneous microstructures due to the non-uniform distribution of carbon, which limits wear resistance improvement. Laser cladding, as a new laser surface modification technology, sample surface covered with WC is coupled by high-energy laser beam, has a better performance in many applications compared with laser remelting. It is thus signifi-
Corresponding author. E-mail address: [email protected] (B. Liu).
http://dx.doi.org/10.1016/j.optlastec.2016.12.015 Received 11 May 2016; Received in revised form 15 August 2016; Accepted 17 December 2016 0030-3992/ © 2016 Elsevier Ltd. All rights reserved.
Optics & Laser Technology 91 (2017) 55–62
Y. Wang et al.
compared with other materials shown in Fig. 1 [14], GCI is chosen as the raw material. The used GCI is cut from the rail of a machine tool by electric spark machine. The nominal chemical compositions are given in Table 1. Samples are cut with dimensions of 60×50×8 mm (L×W×H) by electric spark machine which is equipped with a computer numerical control system, then removed cutting trace and oil on the surface of the sample by sand papers to obtain a smooth and clean surface. 2.2. Bionic shape design By studying the cuticle morphologies of some animals in desert such as the snake, lizard and camel's sole, the authors find that these structures on their surfaces are different. Enlightened by them, samples with different bionic shapes are concluded and designed. The bionic shapes are divided into punctuate, striate and reticulate as shown in Fig. 2. In order to study the effect of unit space on the wear resistance of treated samples, each kind of shape is divided into five samples ranged from 1 to 5 mm. Among the classification, as shown in Figs. 3– 5, the punctuate shape with units space ranged from 1 to 5 mm are marked Nos. 1–5, the striate shape with units space ranged from 1 to 5 mm are marked Nos. 6–10, the reticulate shape with units space ranged from 1 to 5 mm are marked Nos. 11–15, the untreated sample used as the reference is marked No. 16, respectively.
Fig. 1. Vibration attenuation of different materials. Table 1 The nominal chemical compositions of HT300 (wt%). Elements
C
Si
Mn
P
S
Cu
Cr
Fe
Content (%)
3.200
1.690
0.940
0.130
0.096
0.500
0.270
Bal
2.3. Laser cladding The laser cladding experiments are performed with a 300 W Nd:YAG laser, which is equipped with CNC and cooling system as shown in Fig. 6. The laser head is vertically mounted in the Z-direction, while the workbench is able to move along X, Y and Z axes, or rotate on a given X-Y plane. The WC powders are pre-coated on the substrate to form a pre-layer with the thickness of 0.3 mm. During the laser cladding, the pre-coated samples are placed on the workbench. Movement along X and Y axes is used to process the bionic units. Due to the variety of processing parameters set, accordingly, the characteristics of bionic unit present diversity, which increase the difficulty of quality control in the manufacturing process. The criteria for determining the optimum quality for the coating were based on a compromise of hardness, volume, homogeneity and pores [15]. On the base of these criteria, the best laser cladding processing parameters are listed in Table 2.
cance to examine the wear resistance of machine tools' bionic LRG by laser cladding. Through the observation of some sand creatures such as the snake, lizard and camel's sole, whose cuticles moving against sands possess high favorable wear resistance, the authors find that the apparently common characteristic of those creatures’ cuticles is soft-hard alternately. Enlightened by it, samples with different bionic coupling units are designed and manufactured by laser cladding. As we know, wear resistance is not an intrinsic property of the materials, but a tribosystem property where the materials in contact, type of reciprocal movement, relative speed, load levels, environmental conditions, presence of abrasive particles and lubricants can play crucial roles in the wear behavior of a component. Based on the truth, in order to improve the RWR of LRG as well as prolong the life of machine tools, it is advisable test the wear resistance of LRG reproducing closely the real operational condition from an engineering point of view. Subsequently, a linear reciprocating wear test machine made by the ball screw feed drive system is designed and manufactured. Both the wear pressure and speed are adjustable.
2.4. Wear tests The rolling contact wear tests are performed by using a rolling wear facility at room temperature. An illustration of the wear tester is shown in Fig. 7. The samples are fixed on the base by the screws passing through the transition pieces as shown in Fig. 8. Rolling slider whose 9 rollers (φ10×38) are always supported by samples, is mounted under the load platform as shown in Fig. 9. The photo of IKO SR rolling slider composed by 26 rollers is given as shown in Fig. 10. The space between rollers is 0.2 mm. In the accelerated wear test, a constant load of 250 N
2. Experimental 2.1. Preparation of original samples As GCI presents excellent performance on the vibration attenuation
Fig. 2. Three kinds of bionic shapes: punctuate shape, striate shape, reticulate shape.
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Fig. 3. Samples of punctuate shape with unit space ranged from 1 mm to 5 mm.
Fig. 4. Samples of the striate shape with unit space ranged from 1 mm to 5 mm.
Fig. 5. Samples of reticulate shape with unit space ranged from 1 mm to 5 mm.
actuated on the samples’ surface back and forth. The rotating speed of electric motor is kept to be an average value of 1200 rpm that the equivalent horizontal velocity of the roller is 10 cm/s. During the accelerated lifetime test, the operational conditions (static load, temperature, speed) are kept constant. Prior to the test, the weight of each sample is measured by electronic balance with a precision of 0.1 mg. Similarly, the test is interrupted to measure the mass of sample after 60 h, as well. Particularly, samples are ultrasonically cleaned with anhydrous alcohol and blow-dried before mass measurement. The Dvalues of the mass of samples before and after the test are taken as the data to assess the RWR. 3. Result and discussion 3.1. The microstructure characteristics of bionic unit and X-ray diffraction
Fig. 6. Equipment of making bionic samples.
According to Li et al. [16] who reports the temperature distribution during laser processing by finite element, the isotherms present an inverted parabola shape and the temperature gradually decreases outward from the heat source. After the irradiation of high-energy laser beam, GCI is heated and subsequently resulted in phase transformation. In light of this, the cross-section of created unit shows inverted parabola shape, where the top of the unit is wider than its bottom, implying the temperature field feature. According to the temperature field, the region under temperature higher than the melting point is defined as laser cladding zone (LCZ) or laser remelting zone (LRZ), while the region under temperature lower than the melting point but higher than 727 °C is defined as heat-affected zone (HAZ). Additionally, the tempering effect is ignored in the zone under
Table 2 Laser cladding processing parameters. Pulse output power (W)
Scanning speed (mm/s)
Diameter of spot size (mm)
Frequency (Hz)
Current (A)
Pulse duration (ms)
233
0.3
2
9
100
5
is transformed to samples’ surface through a narrow contact area under the non-lube condition. The load platform and nut block are connected by screws. By the driving of electric motor, the rolling sliders are 57
Optics & Laser Technology 91 (2017) 55–62
Y. Wang et al.
Fig. 7. Homemade linear reciprocating wear test machine.
Fig. 10. The photo of IKO SR rolling slider. Fig. 8. The schematic of assembly relationship among sample, transition piece and base.
trapping carbon, and subsequently transform to martensite or retained austenite. During the phase transition process, some α-Fe keeps its original phase in the absence of carbon diffusion. WC is retained in the melt pool because of its extra high melting point.
temperature lower than 727 °C. By the changed temperature in crosssection, the distinct regions in units are apparent in visual. Fig. 11 shows the cross-sectional SEM micrographs of laser remelting and laser cladding. More specifically, LRZ and LCZ are experienced rapidly melting and solidification causing phase transformation. Fig. 12 shows the microstructure of LRZ and LCZ. During the laser cladding process, WC is retained in the melt pool because of its extra high melting point. The typical WC particle microstructure can be found clearly and distributed evenly as shown in Fig. 12b. The boundary is clearly observed because of the distinct microstructure. LCZ, HAZ and substrate are combined by metallurgical bonding as shown in Fig. 13a. The microstructure of the GCI substrate contains pearlite and graphite as shown in Fig. 13b. The phases of substrate, laser remelting zone and laser cladding zone are detected by means of X-ray diffraction as shown in Fig. 14. The new phases in laser cladding units, such as martensite, retained austenite, Fe3C, and WC, performing the role of reinforced phase are validated. After laser treatment, most of α-Fe transform to γ-Fe by
3.2. Results of the RCF wear tests The results of the rolling wear tests are given in Table 3. Base on the wear test results, the histogram shown in Fig. 15 is the recorded mass losses for samples marked Nos. 1–16 at the end of test. According to the figure, it can be clearly seen that the mass loss of untreated sample is always higher than that of the treated samples. Moreover, the reticulate sample with unit space 3 mm possesses the least mass loss, a reduction of 86.38%. It intensively demonstrates laser cladding to be effective on the improvement of rolling contact wear resistance. The histogram shown in Fig. 16 is the wear mass loss of three shape groups with the unit space from 1 to 5 mm, respectively. From the perspective of average mass loss, that the samples with striate shape present better wear resistance than the other shape groups on the whole is concluded.
Fig. 9. The schematic of assembly relationship among sample, rolling slider and load platform.
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Fig. 11. The micrographs of sample cross-section: (a) the cross-sectional SEM micrograph of laser remelting, (b) the cross-sectional SEM micrograph of laser cladding.
Fig. 12. The microstructure of LRZ and LCZ.
Fig. 13. The boundary of LCZ, HAZ and Substrate, and the microstructure of the GCI substrate.
3.3. Effects of different laser treated area, shape and space
More specifically, among the punctuate treated samples, the mass loss increases with the increase of unit space as shown in Fig. 17; among the striate treated samples, the mass loss changes slightly with the increase of unit space, attaining a minimum at the unit space of 4 mm as shown in Fig. 18; among the reticulate treated samples, with the increase of unit space, the mass loss initially decreases, but turns to increase after reaching a minimum at the unit space of 3 mm as shown in Fig. 19. Additionally, from the ratio value of laser treated area to contacted area perspective, that the samples with ratio value between 0.15 and 0.3 has better wear resistance is concluded as shown in Fig. 20.
The average value of the hardness is got from six individual measurements. The results manifest that the hardness of the units processed by laser cladding WC powder is 1100 HV, the laser remelting one is 680 HV, and the substrate is 260 HV, respectively. It's obvious that the hardness of the units processed by laser cladding WC powder is greater than that of laser remelting ones and the substrate. Since the hardness of materials plays an important role in determining the wear characteristics. High hardness could restrain the surface plastic deformation and improve the wear properties. In addition, graphite is relatively softer than the substrate materials and it cyclically softened
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Fig. 14. X-ray diffraction of untreated sample and treated sample by laser cladding.
Fig. 16. Wear mass loss of three kinds of shape groups with different unit space.
Table 3 The wear tests results of samples marked Nos. 1–16. No.
Shape
Unit space (mm)
Treated to contacted area ratio
Mass loss (mg)
Mass loss decrease percent
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Punctuate Punctuate Punctuate Punctuate Punctuate Striate Striate Striate Striate Striate Reticulate Reticulate Reticulate Reticulate Reticulate Untreated
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
0.22 0.13 0.08 0.05 0.04 0.32 0.23 0.18 0.15 0.13 0.44 0.38 0.31 0.28 0.25 0
4.47 6.60 7.87 10.20 15.97 5.80 4.60 7.80 2.77 9.30 10.23 13.13 2.27 7.30 7.57 16.67
73.19% 60.41% 52.79% 38.81% 4.12% 65.21% 72.41% 53.21% 83.38% 44.21% 38.63% 21.24% 86.38% 56.21% 54.59% Fig. 17. Wear mass loss of punctuate shape with different unit space.
Fig. 15. Wear mass loss of samples marked Nos. 1–16.
Fig. 18. Wear mass loss of striate shape with different unit space.
to the substrate resulting in a concentration of the plastic strain. It means that graphite is a potential threat to GCI component in RCF conditions. After laser cladding process, graphite in LCZ is completely dissolved. The LCZ is mainly characterized of compact and refinement
grains, such as retain austenite, martensite, Fe3C and WC, which are benefit for the wear resistance. For microscopic behavior, the bionic units not only delay the initiation of micro-cracks, but also significantly
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concentration seduced by different unit shapes becomes the prominent factor when the proportion of laser treated area is more than 0.15. 4. Conclusions After laser cladding and rolling wear test on different samples, the following conclusions can be drawn from this study: a. Graphite in LCZ is completely dissolved and the laser treated unit is mainly characterized of compact and refinement grains, such as retain austenite, martensite, Fe3C and WC, which are benefit for the wear resistance. Hence, samples with bionic units present better rolling wear resistance than the untreated, of which the reticulate sample with unit space 3 mm possesses the least mass loss, a reduction of 86.38%. b. The treated portions with high hardness exhibit excellent deformation resistance and meanwhile the substrate acts as a soft phase to cushion shear stress coming from rollers, reducing the possibility of stress concentrations in the process of suffering high alternating load. From the ratio value of laser treated area to contacted area perspective, the samples with ratio value between 0.15 and 0.3 present better wear resistance. c. Various shapes of bionic units result in dissimilar stress situation and further affect the area of stress concentration. The wear resistance of treated sample changes along with the variety of unit shapes. After data analysis, that the samples with striate shape perform better wear resistance than the other shape groups on the whole is concluded. d. The unit space has great effect on rolling wear resistance. Among the punctuate treated samples, the mass loss increases with the increase of unit space; among the striate treated samples, the mass loss changes slightly with the increase of unit space, attaining a minimum at the unit space of 4 mm; among the reticulate treated samples, with the increase of unit space, the mass loss initially decreases, but turns to increase after reaching a minimum at the unit space of 3 mm.
Fig. 19. Wear mass loss of reticulate shape with different unit space.
References [1] B.H. Ji, H.J. Gao, A study of fracture mechanisms in biological nano-composites via the virtual internal bond model, Mater. Sci. Eng. A Struct. Mater. Prop. Microstruct. Process. 366 (2004) 96–103. [2] J. Lomakin, Y. Arakane, K.J. Kramer, R.W. Beeman, M.R. Kanost, S.H. Gehrke, Mechanical properties of elytra from Tribolium castaneum wild-type and body color mutant strains, J. Insect Physiol. 56 (2010) 1901–1906. [3] X.S. Wang, Y. Li, Y.F. Shi, Effects of sandwich microstructures on mechanical behaviors of dragon fly wing vein, Compos. Sci. Technol. 68 (2008) 186–192. [4] A.W. Pederson, J.W. Ruberti, P.B. Messersmith, Thermal assembly of a biomimetic mineral/collagen composite, Biomaterials 24 (2003) 4881–4890. [5] D. Cong, H. Zhou, M.Q. Yang, The mechanical properties of H13 die steel repaired by a biomimetic laser technique, Opt. Laser Technol. 53 (2013) 1–8. [6] Z.K. Chen, S.C. Lu, X.B. Song, Effects of bionic units on the fatigue wear of gray cast iron surface with different shapes and distributions, Opt. Laser Technol. 66 (2015) 166–174. [7] Z.B. Pang, H. Zhou, P. Zhang, Study on quantitative relation between characteristics of striature bionic coupling unit and wear resistance of gray cast iron, Appl. Surf. Sci. 331 (2015) 179–184. [8] Z.B. Pang, H. Zhou, G.F. Xie, Effect of bionic coupling units ׳forms on wear resistance of gray cast iron under dry linear reciprocating sliding condition, Opt. Laser Technol. 70 (2015) 89–93. [9] Z.K. Chen, T. Zhou, P. Zhang, Influences of single laser tracks' space on the rolling fatigue contact of gray cast iron, Opt. Laser Technol. 72 (2015) 15–24. [10] H.H. Ding, Z.K. Fu, W.J. Wang, Investigation on the effect of rotational speed on rolling wear and damage behaviors of wheel/rail materials, Wear 330 (2015) 563–570. [11] K.Y. Benyounis, O.M.A. Fakron, J.H. Abboud, A.G. Olabi, M.J.S. M. Hashmi, Surfacemelting of nodular cast iron by Nd-YAG laser and TIG, J. Mater. Process. Technol. 170 (2005) 127–132. [12] C.Y. Cui, Z.X. Guo, H.Y. Wang, J.D. Hu, In situ TiC particles reinforced grey castiron composite fabricated by laser cladding of Ni–Ti–C system, J. Mater. Process. Technol. 183 (2007) 380–385. [13] X.B. Liu, G. Yu, J. Guo, Q.Y. Shang, Z.G. Zhang, Y.J. Gu, Analysis of laser surface hardened layers of automobile engine cylinder liner, J. Iron Steel Res. Int. 14 (2007) 6–42.
Fig. 20. Wear mass loss with different area ratio value of treated to contacted.
obstruct the propagation of cracks. Therefore, compared with the untreated one, the addition of bionic structure make the treated samples possess better wear resistance. According to the research of Zhou et al. [17], the laser cladding WC powder samples have a less wear mass loss than the laser remelting ones. Besides, with the increasing WC content from 50% to 70%, the wear mass losses decrease gradually from 80% to 55%, indicating that WC is beneficial to improvement of wear resistance. Though the laser treated unit is benefit for the wear resistance, it cannot suggest that the RWR is in direct proportion to the treated area. Due to the phase transformation, the treated portions with high hardness exhibit excellent deformation resistance and meanwhile the substrate acts as a soft phase to cushion shear stress coming from rollers, reducing the possibility of stress concentrations in the process of suffering high alternating load. It makes the proportion of laser treated area become a crucial factor to RWR of LRG. Various shapes with different unit spaces result in dissimilar stress situation and further affect the area of stress concentration. As contact condition between cylindrical roller and sample surface is the narrow rectangle, it makes striate shape samples reduce the area of stress concentration ranges effectively so as to improve rolling contact wear resistance. All in all, when the proportion of laser treated area is less than 0.15, the laser treated area is the dominant factor, while the area of stress
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hardness and residual stress distributions in multitrack laser surface melting roller steel, Opt. Lasers Eng. 48 (2010) 1224. [17] Z.J. Jing, H. Zhou, P. Zhang, C.W. Wang, C. Meng, D.L. Cong, Effect of thermal fatigue on the wear resistance of graphite cast iron with bionic units processed by laser cladding WC, Appl. Surf. Sci. (2013) 329–336.
[14] J.Z. Ma, The Discussion of Several Problems About Improving the Quality of CNC Machine’s Castings, The technical seminar about machine tool’s castings, 2012. [15] J.J. Candel, V. Amigo, J.A. Ramos, D. Busquets, Sliding wear resistance of TiCp reinforced titanium composite coating produced by laser cladding, Surf. Coat. Technol. (2010) 3161–3166. [16] C.W. Li, Y. Wang, Z. Zhang, B. Han, T. Han, Influence of overlapping ratio on
62
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Wear resistance of machine tools' bionic linear rolling guides by laser cladding
MARK
⁎
Yiqiang Wanga, Botao Liub, , Zhengcai Guoc a b c
College of Mechanical and Energy Engineering, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, China School of Mechanical Engineering, Zhejiang University, Hangzhou 310000, China School of Mechanical Engineering, Taiyuan University of Science and Technology, Taiyuan 030024,China
A R T I C L E I N F O
A BS T RAC T
Keywords: Rolling wear resistance Bionic linear rolling guide Laser cladding
In order to improve the rolling wear resistance (RWR) of linear rolling guides (LRG) as well as prolong the life of machine tools, various shape samples with different units spaces ranged from 1 to 5 mm are designed through the observation of animals in the desert and manufactured by laser cladding. Wear resistance tests reproducing closely the real operational condition are conducted by using a homemade linear reciprocating wear test machine, and wear resistance is evaluated by means of weight loss measurement. Results indicate that the samples with bionic units have better RWR than the untreated one, of which the reticulate treated sample with unit space 3 mm present the best RWR. More specifically, among the punctuate treated samples, the mass loss increases with the increase of unit space; among the striate treated samples, the mass loss changes slightly with the increase of unit space, attaining a minimum at the unit space of 4 mm; among the reticulate treated samples, with the increase of unit space, the mass loss initially decreases, but turns to increase after reaching a minimum at the unit space of 3 mm. Additionally, the samples with striate shape perform better wear resistance than the other shape groups on the whole. From the ratio value of laser treated area to contacted area perspective, that the samples with ratio value between 0.15 and 0.3 possess better wear resistance is concluded.
1. Introduction High precision, efficiency and automation are the features that machine tools face with the rapid development of modern industry, which make a higher requirement on the precision and service life of machine tools. As LRG have many advantages such as high stiffness, smooth motion and low friction coefficient compared with sliding contact guides, they have been widely used for precise positioning devices to transport machine parts through a linear path in machine tools and X-Y tables etc. As the key part of machine tool, LRG's wear resistance level determines the precision level of machine tools to great extent. Due to the excellent properties such as cast-ability, low melting point, low cost and toughness, grey cast iron (GCI) has been utilized as an engineering material for applications where the applied loads are extremely high and cyclic. Additionally, GCI is better as a shockabsorber owing to the amount of flake graphite spread in the iron matrix. But as the presence of graphite, component such as rolling guide made by GCI, which is typically suffered high alternating stress, will be easy to fail if it's used directly without any treatment. Based on survival of the fittest, nature provides species a multi-
⁎
functional structure so that they can survive in the complex environment. The function adapt to living environment is the result of coupling factors on morphologies, structures and materials of living things to the optimization of the biological function. During the past many years, great progress has been made on applied sciences by imitating the biological structures or habits of plants and animals in nature [1–3]. Evidence is provided to demonstrate that the biomimetic structure possesses excellent properties on mechanical behavior [4,5]. Moreover, scientists have carried on many experiments concerning the effect of bionic structure on wear resistance, and the experiment results manifest that the wear resistance of bionic coupling samples is better than that of untreated sample [6–10]. Previous theoretical and experimental study [11–13] suggests that GCI performs satisfactorily under rolling contact fatigue after laser remelting. However, a considerable drawback to laser remelting is the formation of heterogeneous microstructures due to the non-uniform distribution of carbon, which limits wear resistance improvement. Laser cladding, as a new laser surface modification technology, sample surface covered with WC is coupled by high-energy laser beam, has a better performance in many applications compared with laser remelting. It is thus signifi-
Corresponding author. E-mail address: [email protected] (B. Liu).
http://dx.doi.org/10.1016/j.optlastec.2016.12.015 Received 11 May 2016; Received in revised form 15 August 2016; Accepted 17 December 2016 0030-3992/ © 2016 Elsevier Ltd. All rights reserved.
Optics & Laser Technology 91 (2017) 55–62
Y. Wang et al.
compared with other materials shown in Fig. 1 [14], GCI is chosen as the raw material. The used GCI is cut from the rail of a machine tool by electric spark machine. The nominal chemical compositions are given in Table 1. Samples are cut with dimensions of 60×50×8 mm (L×W×H) by electric spark machine which is equipped with a computer numerical control system, then removed cutting trace and oil on the surface of the sample by sand papers to obtain a smooth and clean surface. 2.2. Bionic shape design By studying the cuticle morphologies of some animals in desert such as the snake, lizard and camel's sole, the authors find that these structures on their surfaces are different. Enlightened by them, samples with different bionic shapes are concluded and designed. The bionic shapes are divided into punctuate, striate and reticulate as shown in Fig. 2. In order to study the effect of unit space on the wear resistance of treated samples, each kind of shape is divided into five samples ranged from 1 to 5 mm. Among the classification, as shown in Figs. 3– 5, the punctuate shape with units space ranged from 1 to 5 mm are marked Nos. 1–5, the striate shape with units space ranged from 1 to 5 mm are marked Nos. 6–10, the reticulate shape with units space ranged from 1 to 5 mm are marked Nos. 11–15, the untreated sample used as the reference is marked No. 16, respectively.
Fig. 1. Vibration attenuation of different materials. Table 1 The nominal chemical compositions of HT300 (wt%). Elements
C
Si
Mn
P
S
Cu
Cr
Fe
Content (%)
3.200
1.690
0.940
0.130
0.096
0.500
0.270
Bal
2.3. Laser cladding The laser cladding experiments are performed with a 300 W Nd:YAG laser, which is equipped with CNC and cooling system as shown in Fig. 6. The laser head is vertically mounted in the Z-direction, while the workbench is able to move along X, Y and Z axes, or rotate on a given X-Y plane. The WC powders are pre-coated on the substrate to form a pre-layer with the thickness of 0.3 mm. During the laser cladding, the pre-coated samples are placed on the workbench. Movement along X and Y axes is used to process the bionic units. Due to the variety of processing parameters set, accordingly, the characteristics of bionic unit present diversity, which increase the difficulty of quality control in the manufacturing process. The criteria for determining the optimum quality for the coating were based on a compromise of hardness, volume, homogeneity and pores [15]. On the base of these criteria, the best laser cladding processing parameters are listed in Table 2.
cance to examine the wear resistance of machine tools' bionic LRG by laser cladding. Through the observation of some sand creatures such as the snake, lizard and camel's sole, whose cuticles moving against sands possess high favorable wear resistance, the authors find that the apparently common characteristic of those creatures’ cuticles is soft-hard alternately. Enlightened by it, samples with different bionic coupling units are designed and manufactured by laser cladding. As we know, wear resistance is not an intrinsic property of the materials, but a tribosystem property where the materials in contact, type of reciprocal movement, relative speed, load levels, environmental conditions, presence of abrasive particles and lubricants can play crucial roles in the wear behavior of a component. Based on the truth, in order to improve the RWR of LRG as well as prolong the life of machine tools, it is advisable test the wear resistance of LRG reproducing closely the real operational condition from an engineering point of view. Subsequently, a linear reciprocating wear test machine made by the ball screw feed drive system is designed and manufactured. Both the wear pressure and speed are adjustable.
2.4. Wear tests The rolling contact wear tests are performed by using a rolling wear facility at room temperature. An illustration of the wear tester is shown in Fig. 7. The samples are fixed on the base by the screws passing through the transition pieces as shown in Fig. 8. Rolling slider whose 9 rollers (φ10×38) are always supported by samples, is mounted under the load platform as shown in Fig. 9. The photo of IKO SR rolling slider composed by 26 rollers is given as shown in Fig. 10. The space between rollers is 0.2 mm. In the accelerated wear test, a constant load of 250 N
2. Experimental 2.1. Preparation of original samples As GCI presents excellent performance on the vibration attenuation
Fig. 2. Three kinds of bionic shapes: punctuate shape, striate shape, reticulate shape.
56
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Fig. 3. Samples of punctuate shape with unit space ranged from 1 mm to 5 mm.
Fig. 4. Samples of the striate shape with unit space ranged from 1 mm to 5 mm.
Fig. 5. Samples of reticulate shape with unit space ranged from 1 mm to 5 mm.
actuated on the samples’ surface back and forth. The rotating speed of electric motor is kept to be an average value of 1200 rpm that the equivalent horizontal velocity of the roller is 10 cm/s. During the accelerated lifetime test, the operational conditions (static load, temperature, speed) are kept constant. Prior to the test, the weight of each sample is measured by electronic balance with a precision of 0.1 mg. Similarly, the test is interrupted to measure the mass of sample after 60 h, as well. Particularly, samples are ultrasonically cleaned with anhydrous alcohol and blow-dried before mass measurement. The Dvalues of the mass of samples before and after the test are taken as the data to assess the RWR. 3. Result and discussion 3.1. The microstructure characteristics of bionic unit and X-ray diffraction
Fig. 6. Equipment of making bionic samples.
According to Li et al. [16] who reports the temperature distribution during laser processing by finite element, the isotherms present an inverted parabola shape and the temperature gradually decreases outward from the heat source. After the irradiation of high-energy laser beam, GCI is heated and subsequently resulted in phase transformation. In light of this, the cross-section of created unit shows inverted parabola shape, where the top of the unit is wider than its bottom, implying the temperature field feature. According to the temperature field, the region under temperature higher than the melting point is defined as laser cladding zone (LCZ) or laser remelting zone (LRZ), while the region under temperature lower than the melting point but higher than 727 °C is defined as heat-affected zone (HAZ). Additionally, the tempering effect is ignored in the zone under
Table 2 Laser cladding processing parameters. Pulse output power (W)
Scanning speed (mm/s)
Diameter of spot size (mm)
Frequency (Hz)
Current (A)
Pulse duration (ms)
233
0.3
2
9
100
5
is transformed to samples’ surface through a narrow contact area under the non-lube condition. The load platform and nut block are connected by screws. By the driving of electric motor, the rolling sliders are 57
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Fig. 7. Homemade linear reciprocating wear test machine.
Fig. 10. The photo of IKO SR rolling slider. Fig. 8. The schematic of assembly relationship among sample, transition piece and base.
trapping carbon, and subsequently transform to martensite or retained austenite. During the phase transition process, some α-Fe keeps its original phase in the absence of carbon diffusion. WC is retained in the melt pool because of its extra high melting point.
temperature lower than 727 °C. By the changed temperature in crosssection, the distinct regions in units are apparent in visual. Fig. 11 shows the cross-sectional SEM micrographs of laser remelting and laser cladding. More specifically, LRZ and LCZ are experienced rapidly melting and solidification causing phase transformation. Fig. 12 shows the microstructure of LRZ and LCZ. During the laser cladding process, WC is retained in the melt pool because of its extra high melting point. The typical WC particle microstructure can be found clearly and distributed evenly as shown in Fig. 12b. The boundary is clearly observed because of the distinct microstructure. LCZ, HAZ and substrate are combined by metallurgical bonding as shown in Fig. 13a. The microstructure of the GCI substrate contains pearlite and graphite as shown in Fig. 13b. The phases of substrate, laser remelting zone and laser cladding zone are detected by means of X-ray diffraction as shown in Fig. 14. The new phases in laser cladding units, such as martensite, retained austenite, Fe3C, and WC, performing the role of reinforced phase are validated. After laser treatment, most of α-Fe transform to γ-Fe by
3.2. Results of the RCF wear tests The results of the rolling wear tests are given in Table 3. Base on the wear test results, the histogram shown in Fig. 15 is the recorded mass losses for samples marked Nos. 1–16 at the end of test. According to the figure, it can be clearly seen that the mass loss of untreated sample is always higher than that of the treated samples. Moreover, the reticulate sample with unit space 3 mm possesses the least mass loss, a reduction of 86.38%. It intensively demonstrates laser cladding to be effective on the improvement of rolling contact wear resistance. The histogram shown in Fig. 16 is the wear mass loss of three shape groups with the unit space from 1 to 5 mm, respectively. From the perspective of average mass loss, that the samples with striate shape present better wear resistance than the other shape groups on the whole is concluded.
Fig. 9. The schematic of assembly relationship among sample, rolling slider and load platform.
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Fig. 11. The micrographs of sample cross-section: (a) the cross-sectional SEM micrograph of laser remelting, (b) the cross-sectional SEM micrograph of laser cladding.
Fig. 12. The microstructure of LRZ and LCZ.
Fig. 13. The boundary of LCZ, HAZ and Substrate, and the microstructure of the GCI substrate.
3.3. Effects of different laser treated area, shape and space
More specifically, among the punctuate treated samples, the mass loss increases with the increase of unit space as shown in Fig. 17; among the striate treated samples, the mass loss changes slightly with the increase of unit space, attaining a minimum at the unit space of 4 mm as shown in Fig. 18; among the reticulate treated samples, with the increase of unit space, the mass loss initially decreases, but turns to increase after reaching a minimum at the unit space of 3 mm as shown in Fig. 19. Additionally, from the ratio value of laser treated area to contacted area perspective, that the samples with ratio value between 0.15 and 0.3 has better wear resistance is concluded as shown in Fig. 20.
The average value of the hardness is got from six individual measurements. The results manifest that the hardness of the units processed by laser cladding WC powder is 1100 HV, the laser remelting one is 680 HV, and the substrate is 260 HV, respectively. It's obvious that the hardness of the units processed by laser cladding WC powder is greater than that of laser remelting ones and the substrate. Since the hardness of materials plays an important role in determining the wear characteristics. High hardness could restrain the surface plastic deformation and improve the wear properties. In addition, graphite is relatively softer than the substrate materials and it cyclically softened
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Fig. 14. X-ray diffraction of untreated sample and treated sample by laser cladding.
Fig. 16. Wear mass loss of three kinds of shape groups with different unit space.
Table 3 The wear tests results of samples marked Nos. 1–16. No.
Shape
Unit space (mm)
Treated to contacted area ratio
Mass loss (mg)
Mass loss decrease percent
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Punctuate Punctuate Punctuate Punctuate Punctuate Striate Striate Striate Striate Striate Reticulate Reticulate Reticulate Reticulate Reticulate Untreated
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
0.22 0.13 0.08 0.05 0.04 0.32 0.23 0.18 0.15 0.13 0.44 0.38 0.31 0.28 0.25 0
4.47 6.60 7.87 10.20 15.97 5.80 4.60 7.80 2.77 9.30 10.23 13.13 2.27 7.30 7.57 16.67
73.19% 60.41% 52.79% 38.81% 4.12% 65.21% 72.41% 53.21% 83.38% 44.21% 38.63% 21.24% 86.38% 56.21% 54.59% Fig. 17. Wear mass loss of punctuate shape with different unit space.
Fig. 15. Wear mass loss of samples marked Nos. 1–16.
Fig. 18. Wear mass loss of striate shape with different unit space.
to the substrate resulting in a concentration of the plastic strain. It means that graphite is a potential threat to GCI component in RCF conditions. After laser cladding process, graphite in LCZ is completely dissolved. The LCZ is mainly characterized of compact and refinement
grains, such as retain austenite, martensite, Fe3C and WC, which are benefit for the wear resistance. For microscopic behavior, the bionic units not only delay the initiation of micro-cracks, but also significantly
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concentration seduced by different unit shapes becomes the prominent factor when the proportion of laser treated area is more than 0.15. 4. Conclusions After laser cladding and rolling wear test on different samples, the following conclusions can be drawn from this study: a. Graphite in LCZ is completely dissolved and the laser treated unit is mainly characterized of compact and refinement grains, such as retain austenite, martensite, Fe3C and WC, which are benefit for the wear resistance. Hence, samples with bionic units present better rolling wear resistance than the untreated, of which the reticulate sample with unit space 3 mm possesses the least mass loss, a reduction of 86.38%. b. The treated portions with high hardness exhibit excellent deformation resistance and meanwhile the substrate acts as a soft phase to cushion shear stress coming from rollers, reducing the possibility of stress concentrations in the process of suffering high alternating load. From the ratio value of laser treated area to contacted area perspective, the samples with ratio value between 0.15 and 0.3 present better wear resistance. c. Various shapes of bionic units result in dissimilar stress situation and further affect the area of stress concentration. The wear resistance of treated sample changes along with the variety of unit shapes. After data analysis, that the samples with striate shape perform better wear resistance than the other shape groups on the whole is concluded. d. The unit space has great effect on rolling wear resistance. Among the punctuate treated samples, the mass loss increases with the increase of unit space; among the striate treated samples, the mass loss changes slightly with the increase of unit space, attaining a minimum at the unit space of 4 mm; among the reticulate treated samples, with the increase of unit space, the mass loss initially decreases, but turns to increase after reaching a minimum at the unit space of 3 mm.
Fig. 19. Wear mass loss of reticulate shape with different unit space.
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Fig. 20. Wear mass loss with different area ratio value of treated to contacted.
obstruct the propagation of cracks. Therefore, compared with the untreated one, the addition of bionic structure make the treated samples possess better wear resistance. According to the research of Zhou et al. [17], the laser cladding WC powder samples have a less wear mass loss than the laser remelting ones. Besides, with the increasing WC content from 50% to 70%, the wear mass losses decrease gradually from 80% to 55%, indicating that WC is beneficial to improvement of wear resistance. Though the laser treated unit is benefit for the wear resistance, it cannot suggest that the RWR is in direct proportion to the treated area. Due to the phase transformation, the treated portions with high hardness exhibit excellent deformation resistance and meanwhile the substrate acts as a soft phase to cushion shear stress coming from rollers, reducing the possibility of stress concentrations in the process of suffering high alternating load. It makes the proportion of laser treated area become a crucial factor to RWR of LRG. Various shapes with different unit spaces result in dissimilar stress situation and further affect the area of stress concentration. As contact condition between cylindrical roller and sample surface is the narrow rectangle, it makes striate shape samples reduce the area of stress concentration ranges effectively so as to improve rolling contact wear resistance. All in all, when the proportion of laser treated area is less than 0.15, the laser treated area is the dominant factor, while the area of stress
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