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Wear properties of compact graphite cast iron with bionic units processed by deep laser cladding WC
Applied Surface Science 256 (2010) 6413–6419
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Wear properties of compact graphite cast iron with bionic units processed by deep laser cladding WC Hong Zhou a,∗ , Peng Zhang a , Na Sun a , Cheng-tao Wang a,c , Peng-yu Lin a , Lu-quan Ren b a b c
Key Lab of Automobile Materials, The Ministry of Education, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China Key Lab of Terrain Machinery Bionics Engineering, The Ministry of Education, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China Faw-Volkswagen Automotive Company Ltd., Changchun 130011, P.R. China
a r t i c l e
i n f o
Article history: Received 1 October 2009 Received in revised form 11 April 2010 Accepted 11 April 2010 Available online 24 April 2010 Keywords: Cast iron Bionic Laser cladding Wear properties
a b s t r a c t By simulating the cuticles of some soil animals, the wear resistance of compact graphite cast iron (CGI) processed by laser remelting gets a conspicuous improvement. In order to get a further anti-wear enhancement of CGI, a new method of deep laser cladding was used to process bionic units. By preplacing grooves then filling with WC powders and laser cladding, the bionic units had a larger dimension in depth and higher microhardness. Fe powder with different proportions from 30% (wt.) to 60% (wt.) was added into WC before laser processing for a good incorporation with CGI substrate. The improved laser cladding units turned out to induce higher wear resistance in comparison with laser remelting ones. The depth of the layer reached up to 1 mm. The results of dry sliding wear tests indicated that the specimen processed by laser cladding has a remarkable improvement than the ones processed by laser remelting. It should be noted that the wear mass loss was essentially dependent on the increase in WC proportion. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Wear and thermal fatigue are two main failure mechanics of brake discs. Since the development of railway transportation tends to be of high-speed, the service lives of brake discs decrease significantly. Compact graphite cast iron (CGI) is one kind of materials that are widely used in vehicle brakes [1] for its high friction coefficient, low wear rate and low cost. Many researches thereby have been carried out for improving the wear resistance, to meet higher braking requirements, e.g., adding alloying elements and using aluminum matrix composites. These methods have made significant progress to improve the wear resistance but would change the production process widely used nowadays. Animals and plants have been adapting the environment they live for tens of thousands of years. They often offer some breakthrough ideas to engineering. After studying the cuticle morphologies of some soil animals such as dung beetles and pangolins, Ren et al. [2] found that five simple structures on cuticles called ‘non-smooth construction units’ could provide excellent abrasion resistant against soil. They are convex, concave, stria, bristle and squama. By mimicking the cuticles of soil animals (Fig. 1), bionic units were obtained on the surfaces of some components by different processing methods such as cast-in process [4], laser remelting [5] and laser cladding. Generally bionic unit has different prop-
∗ Corresponding author. E-mail address: [email protected] (H. Zhou). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.04.027
erties from the substrate which includes hardness and strength. It presents a particular function like the non-smooth construction units. More particularly, the specimens with bionic units got various promotions by simulating the creatures, because animals and plants adapt to a variety of environmental conditions. Previous researches [5–7] prove that the bionic units processed by laser on CGI can provide a higher wear resistance and better thermal fatigue resistance. Laser processing bionic unit was proved to be an efficacious strengthening method, which adds only one step in the production process used today. However, the bionic units without additions during processing provided only limited improvement for the properties. On the one hand, improvement resulting from laser-induced phase transition is dependent on many original conditions, such as the chemical compositions (e.g. C and alloy contents) of materials. On the other hand, the thickness of the hardened layer (units) by laser processing is limited, with the average depth of 0.5 mm. Laser cladding is an alternative method, extensively used to improve wear resistance [8–11]. What makes it to be a flexible way to enhance the wear resistance is that different powders can be used for cladding. Bionic unit processed by this method is expected to have a better wear resistance. In this paper, a new ripple-shaped bionic unit made by deep laser cladding WC powders was obtained. The unit was provided with both high hardness and a thickness about 1 mm. Microstructures of the units were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS) analyses. Dry sliding wear tests were carried out on a block-on-ring tester with a GCr15 steel ring.
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Fig. 1. Two kinds of soil animals surfaces [3] and bionic units: (a) surface of the dung beetle head, (b) surface of the elytrum of dung beetle elytrum, (c) convex-shaped units, and (d) ripple-shaped bionic units.
2. Experimental 2.1. Materials All CGI specimens were cut from high-speed train brake discs with the size of 14 mm × 10 mm ×10 mm. The chemical compositions of CGI are listed in Table 1. 2.2. Preparation of bionic specimens Before laser processing, grooves were cut on the specimens with the size of 1 mm × 0.8 mm (width × depth). The distance between each groove was 2 mm. In order to have a better combination with the substrate, Fe powders were added to mix with WC powders. The average size of powders was 45–75 m. Four kinds of powders with different WC proportions were prepared: 70%, 60%, 50% and 40%. Water glass (sodium silicate) was chosen as the bond to prevent powder from being splashed while laser processing. Powders were overlaid in the grooves and then compacted. After being dried for 20 h in the shade, powders were processed by a Nd:YAG laser. To compare the result of this experiment, an untreated specimen and a specimen processed by bionic laser remelting [5] were prepared. The parameters of the laser used in processing two kinds of bionic units are shown in Table 2. Microstructures on the samples were examined using X-ray diffraction (XRD, D/Max 2500PC Rigaku, Japan) and JSM-5600LV scanning electron microscope. Microhardness of the surface of units was measured using Vickers microhardness tester (model 5104, Buehler Co. Ltd., USA). 2.3. Wear tests Wear resistance of the specimens at room temperature was evaluated on an MM-200 block-on-ring dry sliding wear tester. The rotating ring was made of CGr15 steel which was austenitized
at 840 ◦ C, and then oil quenched and tempered for 2 h at 170 ◦ C obtaining an average hardness of 61–63 HRC. The load used in this test was 100 N and rotating speed was 400 rpm. Before testing, the ring was polished to a roughness of about 0.1 mm. The specimens were cleaned in alcohol, dried and then the weight was measured on an electronic balance with an accuracy of 0.0001 g. Wear tests were carried out twice. After first 15 min the mass loss was measured to estimate the effect of different component on the bionic unit. Then the specimens were tested for another 30 min to examine the effect of unit depth on the wear property. 3. Results and discussion 3.1. Microstructure and microhardness Fig. 2 shows the cross sections of the units with different contents of WC powders. Three zones were obtained which were distinguished by different phase characteristics. The cross sections of the units were semicircle shape after processing. The top of the unit is wider than the bottom. Because the diameter of laser is larger than the grooves, some substrate along the grooves was melt and became a part of the proceeded area. Similar phenomenon is not found in the bottom. But in this area the heat was transferred from melt powder to substrate which led to the boundary transformation from a straight line into a curve. The alteration of boundaries provides that powders and some substrate are mixed after melting and then solidified. Fig. 3 is the high magnified image in I position of every specimen. With the increase in the content of WC the white part extended and grew in the shape of herringbone (␣ in Fig. 3). The XRD analysis (Fig. 4) manifests that the main phases in the cross sections are martensite, Fe3 W3 C, and Fe3 C. Through further analysis of composition, EDS analysis (Table 3) of the white herringbone part indicates
H. Zhou et al. / Applied Surface Science 256 (2010) 6413–6419
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Table 1 Chemical compositions of compact graphite cast iron. Elements
C
Si
Mn
P
S
Re
Mg
Fe
Composition (wt.%)
3.56
2.56
0.71
0.03
0.03
0.02
0.02
Bal.
Table 2 Laser parameters used in laser remelting and deep laser cladding.
Laser remelting Deep laser cladding
Energy density (J/cm2 )
Pulse duration (ms)
Frequency (Hz)
Scanning speed (mm/s)
125 76
5 6
14 7
0.71 0.42
Fig. 2. Cross sections of units with different contents of WC. (a) 70%, (b) 60%, (c) 50%, and (d) 40%.
Fig. 3. Microstructures of up layer (zone I) with different contents of WC. (a) 70%, (b) 60%, (c) 50%, and (d) 40%.
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Fig. 4. XRD analysis of specimen with different bionic units. (a) 70%, (b) 60%, (c) 50%, and (d) 40%.
that the proportion is 23:75 (Fe:W in wt.%). The molar ratio is 1:1 approximately. According to the XRD, the herringbone shaped part is Fe3 W3 C. There are other white parts ( in Fig. 3) in the 60% and 50% units. The content of W in these parts is lower than that in the fish bone part. Laser processing makes the surface layer get heat in a very short period of time. WC is dissolved in the surface layer. While temperature decreased primary crystal M6 C precipitated from the liquid phase and the proportion of W and C decreased in the liquid. Herringbone shaped Fe3 W3 C was found in high WC content specimens (70% and 60%). With the WC content decreasing Fe3 W3 C gradually disappeared. In the units with 60% and 50% WC, martensite with saturated WC ( in Fig. 3) was obtained due to the lower WC proportion. In the unit with 40% WC, the carbides exist only in the crystal boundary. The black matrix was the product transformed from over-saturation, which contains W and C. Through further analysis of EDS and XRD, they are ␣-Fe Fe3 C. Small quantity of retained austenite may also exist. Table 3 EDS analysis of the bionic units. Weight percentage (wt%)
Atomic percent (at%)
Elements
W
Fe
C
W
Fe
C
Block part Herringbone Black matrix
94 75 28
– 23 72
6 2 –
49 43 10
– 43 89
49 14 –
Fig. 5 shows the highly magnified images in bottom layer (zone II) of every specimen. Compared with the up layer, the carbides decrease. A new kind of white part in the shape of block appears in the units with 70% and 60% WC while herringbone Fe3 W3 C decreases. There are also some small block shaped parts in the unit with 50% WC but they are only a few. In the 40% WC unit, less carbides are distributed along the grain boundaries compared with the up layer. According to the EDS analysis (Table 3) the block white parts are WC. The differences between up and bottom layers are mainly due to the temperature and heat transmission. The up layer is irradiated by laser directly. Heat is difficult to transmit to the bottom layer because of the interval among ceramic particles. The situation is different in the bottom. This layer gets less heat from the up layer and it is easier to transmit to CGI substrate. In this condition, WC particles do not have enough time to dissolve completely and remain in this layer. Other carbides decrease because less WC dissolved. Fig. 6 is the highly magnified images of WC particles in II position. The size of these particles is smaller than the powder used in this experiment because the boundary of it was dissolved (area B in Fig. 6 (b)). The dispersion strengthening of WC will give rise to microhardness of the units. The interface between unit and substrate is a position which needs attention. In comparison with laser remelting unit, laser cladding one has a different composition from the substrate. Metallurgical bonding is required to prevent shedding of the unit from
H. Zhou et al. / Applied Surface Science 256 (2010) 6413–6419
Fig. 5. Microstructure of bottom layer (zone II) of the units with different contents of WC. (a) 70%, (b) 60%, (c) 50%, and (d) 40%.
Fig. 6. The undissolved WC grains in zone II of the unit.
Fig. 7. Interfaces between different units and substrate (a) 70%, (b) 60%, (c) 50%, and (d) 40%.
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Fig. 10. Mass loss of different specimens after wear for 45 min.
Fig. 8. The microhardness distributing on the cross sections of different units.
those during the surface tests. While the wear mass losses of laser remelting specimens are more than 4.5 times. This is because after long-time wear, the laser remelting unit is almost worn out. Without bionic units, the wear on laser remelting specimen was similar to the untreated one.
3.3. Wear process and mechanism
Fig. 9. Mass loss of different specimens after wear for 15 min.
the substrate in wear tests. As shown in Fig. 7, there is a good combination between the unit and the substrate. Fig. 8 shows the microhardness of the bionic units along the direction from surface to bottom. The last points of all deep laser cladding specimens and the last four points of laser remelting specimen are the data tested on CGI substrate. The microhardness of bionic units is improved by cladding WC powders. The average hardness increases with the increase of the content of WC. The curves of the units with 60% and 70% WC fluctuate in the bottom part of these units. This is relative to the dispersed WC particles as discussed above. 3.2. Wear tests The short-time wear test result (Fig. 9) indicates that all bionic specimens have a high wear resistance. In this test, only surface layers are lost and every bionic unit remains more or less. The deep laser cladding specimens get a better wear resistance as WC content increases. After long-time wear test, the wear mass loss increases significantly (Fig. 10). The wear mass losses of untreated and deep laser cladding specimens are nearly 3 times higher than
As studied in the previous research [6], there are two states in the wear process of bionic specimen. At first, units and substrate are in the same plane (Fig. 11a). With wear test proceeding, substrate gets severe wear while hard unit is difficult to be ground off and gradually rises from the substrate. Frictional pairs will be supported only by the remaining bionic units (Fig. 11b). In this period only these salient units bear the abrasion until they are levelled with the substrate again. This circulation repeats until bionic unit is worn out. Harder unit will lengthen the period that only salient units bear the wear and improve the wear resistance of the sample. Among the bionic units, cladding units got strengthened by WC adjunction. The unit with strengthening phase like WC and Fe3 W3 C has a better wear resistance than that processed by laser remelting. Fe3 W3 C in high WC content units (60% and 70%) is recognized as a high hardness phase and one of the main red-hardness carbides in high-speed steel. Low WC content specimens (40% and 50%) have less carbide, but they still have better wear resistance than the specimen processed by laser remelting. The wear mass losses of bionic specimens decrease with the increase of WC contents. Increasing concentration of the carbides is favorable for the wear resistance [12]. Deeper unit brings further improvement to the sample. It increases the cycle index and then extends the effective time of strengthening by bionic unit. From the perspective of bionics, deep unit processed of different materials is more closely follows the creature surface. Compared with laser remelting ones, units processed by deep laser cladding WC bring longer duration of single-cycle time and more recycling. Both these methods improved the wear resistance of specimen.
Fig. 11. Wear mechanism of bionic specimen: (a) early period of abrasion cycle and (b) later period of abrasion cycle.
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4. Conclusions
Acknowledgements
By imitating the cuticles of soil animals, bionic units were made by laser cladding. After analyzing the microstructure, microhardness, and the results of wear tests, the conclusions can be obtained as follows.
The authors would like to acknowledge the Project 985Automotive Engineering of Jilin University, the National Natural Science Fund of China (No. 50635030) and the Science and Development Foundation of Jilin (No. 20060196) for financial support.
1. By the method of cutting pre-grooves and then filling with ceramic powders, a new kind of deep laser cladding unit was processed. Properties and depths of the deep laser cladding units can be adjusted by composition of the adjunction and dimension of the pre-groove. Experiment results indicate that good metallurgical bonding can be obtained between units and substrate and the additives are well-distributed. Deep laser cladding provides a new method to fabricate bionic units. 2. The laser processed WC and Fe powder units have a complicated phase and composition. In the up layer of high WC proportion units, WC was dissolved and Fe3 W3 C was the main product. In the bottom layer WC particles remained, and became the dispersion strengthening phase. 3. All deep laser cladding units have a batter performance than laser remelting unit. And among deep laser cladding units, microhardness and wear resistance increased with the increase of WC proportion because the harder unit extends the period which only salient bionic unit bears abrasion. Deep laser cladding units increase the thickness of wear resisting layer. It ameliorates wear resistance of the expendable parts working in long-time, like brake disc.
References [1] G. Cueva, A. Sinatora, W.L. Guesser, A.P. Tschiptschin, Wear 255 (2003) 1256–1260. [2] L.Q. Ren, J. Tong, J.Q. Li, B.C. Chen, J. Agric. Eng. Res. 79 (2001) 239–263. [3] X. Jia, L.Q. Ren, B.C. Chen, Chin. J. Mater. Res. 10 (1996) 556. [4] H. Zhou, Q. Guo, P. Lin, W. Zhang, X. Zhang, L. Ren, Appl. Surf. Sci. 255 (2008) 3394–3399. [5] H. Zhou, N. Sun, H. Shan, D. Ma, X. Tong, L. Ren, Appl. Surf. Sci. 253 (2007) 9513–9520. [6] Q. Guo, H. Zhou, C. Wang, W. Zhang, P. Lin, N. Sun, L. Ren, Appl. Surf. Sci. 255 (2009) 6266–6273. [7] X. Tong, H. Zhou, W. Zhang, Z. Zhang, L. Ren, J. Mater. Process Technol. 206 (2008) 473–480. ˜ [8] F. Lusquinos, J. Pou, M. Boutinguiza, F. Quintero, R. Soto, B. León, M. Pérez-Amor, Appl. Surf. Sci. 247 (2005) 486–492. ˜ ˜ F. Quintero, J. Pou, Appl. Surf. Sci. 254 [9] A. Riveiro, F. Lusquinos, R. Comesana, (2007) 926–929. ˜ [10] M.J. Tobar, C. Álvarez, J.M. Amado, G. Rodríguez, A. Yánez, Surf. Coat. Technol. 200 (2006) 6313–6317. ˜ [11] J.M. Amado, M.J. Tobar, J.C. Alvarez, J. Lamas, A. Yánez, Appl. Surf. Sci. 255 (2009) 5553–5556. [12] K. Van Acker, D. Vanhoyweghen, R. Persoons, J. Vangrunderbeek, Wear 258 (2005) 194–202.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Wear properties of compact graphite cast iron with bionic units processed by deep laser cladding WC Hong Zhou a,∗ , Peng Zhang a , Na Sun a , Cheng-tao Wang a,c , Peng-yu Lin a , Lu-quan Ren b a b c
Key Lab of Automobile Materials, The Ministry of Education, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China Key Lab of Terrain Machinery Bionics Engineering, The Ministry of Education, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China Faw-Volkswagen Automotive Company Ltd., Changchun 130011, P.R. China
a r t i c l e
i n f o
Article history: Received 1 October 2009 Received in revised form 11 April 2010 Accepted 11 April 2010 Available online 24 April 2010 Keywords: Cast iron Bionic Laser cladding Wear properties
a b s t r a c t By simulating the cuticles of some soil animals, the wear resistance of compact graphite cast iron (CGI) processed by laser remelting gets a conspicuous improvement. In order to get a further anti-wear enhancement of CGI, a new method of deep laser cladding was used to process bionic units. By preplacing grooves then filling with WC powders and laser cladding, the bionic units had a larger dimension in depth and higher microhardness. Fe powder with different proportions from 30% (wt.) to 60% (wt.) was added into WC before laser processing for a good incorporation with CGI substrate. The improved laser cladding units turned out to induce higher wear resistance in comparison with laser remelting ones. The depth of the layer reached up to 1 mm. The results of dry sliding wear tests indicated that the specimen processed by laser cladding has a remarkable improvement than the ones processed by laser remelting. It should be noted that the wear mass loss was essentially dependent on the increase in WC proportion. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Wear and thermal fatigue are two main failure mechanics of brake discs. Since the development of railway transportation tends to be of high-speed, the service lives of brake discs decrease significantly. Compact graphite cast iron (CGI) is one kind of materials that are widely used in vehicle brakes [1] for its high friction coefficient, low wear rate and low cost. Many researches thereby have been carried out for improving the wear resistance, to meet higher braking requirements, e.g., adding alloying elements and using aluminum matrix composites. These methods have made significant progress to improve the wear resistance but would change the production process widely used nowadays. Animals and plants have been adapting the environment they live for tens of thousands of years. They often offer some breakthrough ideas to engineering. After studying the cuticle morphologies of some soil animals such as dung beetles and pangolins, Ren et al. [2] found that five simple structures on cuticles called ‘non-smooth construction units’ could provide excellent abrasion resistant against soil. They are convex, concave, stria, bristle and squama. By mimicking the cuticles of soil animals (Fig. 1), bionic units were obtained on the surfaces of some components by different processing methods such as cast-in process [4], laser remelting [5] and laser cladding. Generally bionic unit has different prop-
∗ Corresponding author. E-mail address: [email protected] (H. Zhou). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.04.027
erties from the substrate which includes hardness and strength. It presents a particular function like the non-smooth construction units. More particularly, the specimens with bionic units got various promotions by simulating the creatures, because animals and plants adapt to a variety of environmental conditions. Previous researches [5–7] prove that the bionic units processed by laser on CGI can provide a higher wear resistance and better thermal fatigue resistance. Laser processing bionic unit was proved to be an efficacious strengthening method, which adds only one step in the production process used today. However, the bionic units without additions during processing provided only limited improvement for the properties. On the one hand, improvement resulting from laser-induced phase transition is dependent on many original conditions, such as the chemical compositions (e.g. C and alloy contents) of materials. On the other hand, the thickness of the hardened layer (units) by laser processing is limited, with the average depth of 0.5 mm. Laser cladding is an alternative method, extensively used to improve wear resistance [8–11]. What makes it to be a flexible way to enhance the wear resistance is that different powders can be used for cladding. Bionic unit processed by this method is expected to have a better wear resistance. In this paper, a new ripple-shaped bionic unit made by deep laser cladding WC powders was obtained. The unit was provided with both high hardness and a thickness about 1 mm. Microstructures of the units were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS) analyses. Dry sliding wear tests were carried out on a block-on-ring tester with a GCr15 steel ring.
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Fig. 1. Two kinds of soil animals surfaces [3] and bionic units: (a) surface of the dung beetle head, (b) surface of the elytrum of dung beetle elytrum, (c) convex-shaped units, and (d) ripple-shaped bionic units.
2. Experimental 2.1. Materials All CGI specimens were cut from high-speed train brake discs with the size of 14 mm × 10 mm ×10 mm. The chemical compositions of CGI are listed in Table 1. 2.2. Preparation of bionic specimens Before laser processing, grooves were cut on the specimens with the size of 1 mm × 0.8 mm (width × depth). The distance between each groove was 2 mm. In order to have a better combination with the substrate, Fe powders were added to mix with WC powders. The average size of powders was 45–75 m. Four kinds of powders with different WC proportions were prepared: 70%, 60%, 50% and 40%. Water glass (sodium silicate) was chosen as the bond to prevent powder from being splashed while laser processing. Powders were overlaid in the grooves and then compacted. After being dried for 20 h in the shade, powders were processed by a Nd:YAG laser. To compare the result of this experiment, an untreated specimen and a specimen processed by bionic laser remelting [5] were prepared. The parameters of the laser used in processing two kinds of bionic units are shown in Table 2. Microstructures on the samples were examined using X-ray diffraction (XRD, D/Max 2500PC Rigaku, Japan) and JSM-5600LV scanning electron microscope. Microhardness of the surface of units was measured using Vickers microhardness tester (model 5104, Buehler Co. Ltd., USA). 2.3. Wear tests Wear resistance of the specimens at room temperature was evaluated on an MM-200 block-on-ring dry sliding wear tester. The rotating ring was made of CGr15 steel which was austenitized
at 840 ◦ C, and then oil quenched and tempered for 2 h at 170 ◦ C obtaining an average hardness of 61–63 HRC. The load used in this test was 100 N and rotating speed was 400 rpm. Before testing, the ring was polished to a roughness of about 0.1 mm. The specimens were cleaned in alcohol, dried and then the weight was measured on an electronic balance with an accuracy of 0.0001 g. Wear tests were carried out twice. After first 15 min the mass loss was measured to estimate the effect of different component on the bionic unit. Then the specimens were tested for another 30 min to examine the effect of unit depth on the wear property. 3. Results and discussion 3.1. Microstructure and microhardness Fig. 2 shows the cross sections of the units with different contents of WC powders. Three zones were obtained which were distinguished by different phase characteristics. The cross sections of the units were semicircle shape after processing. The top of the unit is wider than the bottom. Because the diameter of laser is larger than the grooves, some substrate along the grooves was melt and became a part of the proceeded area. Similar phenomenon is not found in the bottom. But in this area the heat was transferred from melt powder to substrate which led to the boundary transformation from a straight line into a curve. The alteration of boundaries provides that powders and some substrate are mixed after melting and then solidified. Fig. 3 is the high magnified image in I position of every specimen. With the increase in the content of WC the white part extended and grew in the shape of herringbone (␣ in Fig. 3). The XRD analysis (Fig. 4) manifests that the main phases in the cross sections are martensite, Fe3 W3 C, and Fe3 C. Through further analysis of composition, EDS analysis (Table 3) of the white herringbone part indicates
H. Zhou et al. / Applied Surface Science 256 (2010) 6413–6419
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Table 1 Chemical compositions of compact graphite cast iron. Elements
C
Si
Mn
P
S
Re
Mg
Fe
Composition (wt.%)
3.56
2.56
0.71
0.03
0.03
0.02
0.02
Bal.
Table 2 Laser parameters used in laser remelting and deep laser cladding.
Laser remelting Deep laser cladding
Energy density (J/cm2 )
Pulse duration (ms)
Frequency (Hz)
Scanning speed (mm/s)
125 76
5 6
14 7
0.71 0.42
Fig. 2. Cross sections of units with different contents of WC. (a) 70%, (b) 60%, (c) 50%, and (d) 40%.
Fig. 3. Microstructures of up layer (zone I) with different contents of WC. (a) 70%, (b) 60%, (c) 50%, and (d) 40%.
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Fig. 4. XRD analysis of specimen with different bionic units. (a) 70%, (b) 60%, (c) 50%, and (d) 40%.
that the proportion is 23:75 (Fe:W in wt.%). The molar ratio is 1:1 approximately. According to the XRD, the herringbone shaped part is Fe3 W3 C. There are other white parts ( in Fig. 3) in the 60% and 50% units. The content of W in these parts is lower than that in the fish bone part. Laser processing makes the surface layer get heat in a very short period of time. WC is dissolved in the surface layer. While temperature decreased primary crystal M6 C precipitated from the liquid phase and the proportion of W and C decreased in the liquid. Herringbone shaped Fe3 W3 C was found in high WC content specimens (70% and 60%). With the WC content decreasing Fe3 W3 C gradually disappeared. In the units with 60% and 50% WC, martensite with saturated WC ( in Fig. 3) was obtained due to the lower WC proportion. In the unit with 40% WC, the carbides exist only in the crystal boundary. The black matrix was the product transformed from over-saturation, which contains W and C. Through further analysis of EDS and XRD, they are ␣-Fe Fe3 C. Small quantity of retained austenite may also exist. Table 3 EDS analysis of the bionic units. Weight percentage (wt%)
Atomic percent (at%)
Elements
W
Fe
C
W
Fe
C
Block part Herringbone Black matrix
94 75 28
– 23 72
6 2 –
49 43 10
– 43 89
49 14 –
Fig. 5 shows the highly magnified images in bottom layer (zone II) of every specimen. Compared with the up layer, the carbides decrease. A new kind of white part in the shape of block appears in the units with 70% and 60% WC while herringbone Fe3 W3 C decreases. There are also some small block shaped parts in the unit with 50% WC but they are only a few. In the 40% WC unit, less carbides are distributed along the grain boundaries compared with the up layer. According to the EDS analysis (Table 3) the block white parts are WC. The differences between up and bottom layers are mainly due to the temperature and heat transmission. The up layer is irradiated by laser directly. Heat is difficult to transmit to the bottom layer because of the interval among ceramic particles. The situation is different in the bottom. This layer gets less heat from the up layer and it is easier to transmit to CGI substrate. In this condition, WC particles do not have enough time to dissolve completely and remain in this layer. Other carbides decrease because less WC dissolved. Fig. 6 is the highly magnified images of WC particles in II position. The size of these particles is smaller than the powder used in this experiment because the boundary of it was dissolved (area B in Fig. 6 (b)). The dispersion strengthening of WC will give rise to microhardness of the units. The interface between unit and substrate is a position which needs attention. In comparison with laser remelting unit, laser cladding one has a different composition from the substrate. Metallurgical bonding is required to prevent shedding of the unit from
H. Zhou et al. / Applied Surface Science 256 (2010) 6413–6419
Fig. 5. Microstructure of bottom layer (zone II) of the units with different contents of WC. (a) 70%, (b) 60%, (c) 50%, and (d) 40%.
Fig. 6. The undissolved WC grains in zone II of the unit.
Fig. 7. Interfaces between different units and substrate (a) 70%, (b) 60%, (c) 50%, and (d) 40%.
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Fig. 10. Mass loss of different specimens after wear for 45 min.
Fig. 8. The microhardness distributing on the cross sections of different units.
those during the surface tests. While the wear mass losses of laser remelting specimens are more than 4.5 times. This is because after long-time wear, the laser remelting unit is almost worn out. Without bionic units, the wear on laser remelting specimen was similar to the untreated one.
3.3. Wear process and mechanism
Fig. 9. Mass loss of different specimens after wear for 15 min.
the substrate in wear tests. As shown in Fig. 7, there is a good combination between the unit and the substrate. Fig. 8 shows the microhardness of the bionic units along the direction from surface to bottom. The last points of all deep laser cladding specimens and the last four points of laser remelting specimen are the data tested on CGI substrate. The microhardness of bionic units is improved by cladding WC powders. The average hardness increases with the increase of the content of WC. The curves of the units with 60% and 70% WC fluctuate in the bottom part of these units. This is relative to the dispersed WC particles as discussed above. 3.2. Wear tests The short-time wear test result (Fig. 9) indicates that all bionic specimens have a high wear resistance. In this test, only surface layers are lost and every bionic unit remains more or less. The deep laser cladding specimens get a better wear resistance as WC content increases. After long-time wear test, the wear mass loss increases significantly (Fig. 10). The wear mass losses of untreated and deep laser cladding specimens are nearly 3 times higher than
As studied in the previous research [6], there are two states in the wear process of bionic specimen. At first, units and substrate are in the same plane (Fig. 11a). With wear test proceeding, substrate gets severe wear while hard unit is difficult to be ground off and gradually rises from the substrate. Frictional pairs will be supported only by the remaining bionic units (Fig. 11b). In this period only these salient units bear the abrasion until they are levelled with the substrate again. This circulation repeats until bionic unit is worn out. Harder unit will lengthen the period that only salient units bear the wear and improve the wear resistance of the sample. Among the bionic units, cladding units got strengthened by WC adjunction. The unit with strengthening phase like WC and Fe3 W3 C has a better wear resistance than that processed by laser remelting. Fe3 W3 C in high WC content units (60% and 70%) is recognized as a high hardness phase and one of the main red-hardness carbides in high-speed steel. Low WC content specimens (40% and 50%) have less carbide, but they still have better wear resistance than the specimen processed by laser remelting. The wear mass losses of bionic specimens decrease with the increase of WC contents. Increasing concentration of the carbides is favorable for the wear resistance [12]. Deeper unit brings further improvement to the sample. It increases the cycle index and then extends the effective time of strengthening by bionic unit. From the perspective of bionics, deep unit processed of different materials is more closely follows the creature surface. Compared with laser remelting ones, units processed by deep laser cladding WC bring longer duration of single-cycle time and more recycling. Both these methods improved the wear resistance of specimen.
Fig. 11. Wear mechanism of bionic specimen: (a) early period of abrasion cycle and (b) later period of abrasion cycle.
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4. Conclusions
Acknowledgements
By imitating the cuticles of soil animals, bionic units were made by laser cladding. After analyzing the microstructure, microhardness, and the results of wear tests, the conclusions can be obtained as follows.
The authors would like to acknowledge the Project 985Automotive Engineering of Jilin University, the National Natural Science Fund of China (No. 50635030) and the Science and Development Foundation of Jilin (No. 20060196) for financial support.
1. By the method of cutting pre-grooves and then filling with ceramic powders, a new kind of deep laser cladding unit was processed. Properties and depths of the deep laser cladding units can be adjusted by composition of the adjunction and dimension of the pre-groove. Experiment results indicate that good metallurgical bonding can be obtained between units and substrate and the additives are well-distributed. Deep laser cladding provides a new method to fabricate bionic units. 2. The laser processed WC and Fe powder units have a complicated phase and composition. In the up layer of high WC proportion units, WC was dissolved and Fe3 W3 C was the main product. In the bottom layer WC particles remained, and became the dispersion strengthening phase. 3. All deep laser cladding units have a batter performance than laser remelting unit. And among deep laser cladding units, microhardness and wear resistance increased with the increase of WC proportion because the harder unit extends the period which only salient bionic unit bears abrasion. Deep laser cladding units increase the thickness of wear resisting layer. It ameliorates wear resistance of the expendable parts working in long-time, like brake disc.
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