The thermal fatigue resistance of cast iron with biomimetic non-smooth surface processed by laser with different parameters

The thermal fatigue resistance of cast iron with biomimetic non-smooth surface processed by laser with different parameters

Materials Science and Engineering A 428 (2006) 141–147 The thermal fatigue resistance of cast iron with biomimetic non-smooth surface processed by la...

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Materials Science and Engineering A 428 (2006) 141–147

The thermal fatigue resistance of cast iron with biomimetic non-smooth surface processed by laser with different parameters Hong Zhou a,∗ , Xin Tong a , Zhihui Zhang a , Xianzhou Li a , Luquan Ren b a

b

The Key Lab of Automobile Materials, The Ministry of Education, Jilin University, Changchun 130025, PR China The Key Lab of Terrain Machinery Bionics Engineering, The Ministry of Education, Jilin University, Changchun 130025, PR China Received 21 November 2005; accepted 27 April 2006

Abstract Thermal fatigue cracks occur frequently on the brake drums’ surface, and this phenomenon leads to their premature failure. In order to improve their thermal fatigue resistance, according to the principles of biomimetic non-smooth surface, through processing striate units on the surface of cast iron samples by laser with different parameters, the thermal fatigue performances of smooth and non-smooth samples were investigated, respectively. The rules which laser parameters effected thermal fatigue resistance of non-smooth samples were analyzed. The results indicated that non-smooth surface could decrease thermal crack quantity and their propagation rates; thermal fatigue resistance was improved by processing non-smooth units on sample’s surface. Different sizes of non-smooth units were obtained by adjusting laser parameters. The bigger non-smooth units resulted in better thermal fatigue resistance. Furthermore, the mechanisms of enhanced thermal fatigue resistance were studied. © 2006 Elsevier B.V. All rights reserved. Keywords: Thermal fatigue; Biomimetic; Non-smooth surface; Cast iron; Laser parameters

1. Introduction Brake drum is the most important component in the brake system of trucks. To brake a vehicle, a brake block is pressed against a rotating drum to convert the kinetic energy into the thermal energy, before releasing it into the atmosphere. The friction on brake drum surface is known to generate temperature as high as 900 ◦ C. Then, the thermal stresses that occur in braking process may cause initiation and propagation of cracks, which is considered as the reason for brake drum failure. The predominant trend in the development of large vehicle is toward higher speeds and heavier loads, which increase considerably the energy required to be dissipated by the brake drum [1–5]. Therefore, it is important and urgent to investigate and enhance thermal fatigue resistance of brake drum materials. A lot of methods that could improve thermal fatigue resistance of materials were reported, in which laser processing is an effective technique [6,7]. Bionics is the science that imitates the principles of bio-systems to build technological systems of useful components. Based on the researches of the non-smooth



Corresponding author. Tel.: +86 431 5094427; fax: +86 431 5095592. E-mail address: [email protected] (H. Zhou).

0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.04.101

surface principles of soil-burrowing animals such as dung beetles, black ants, etc., the biomimetic non-smooth surface method was developed by using laser to process some kinds of nonsmooth units, such as convex domes, dimpled concavities and stripes with same or different dimensions and chemical composition, on materials’ surface [8–10]. Biomimetic non-smooth surface method was used for medium-carbon steel and alloy steel, the experimental results indicated that it not only could enhance thermal fatigue resistance but also could enhance wear and adhesion resistance of these materials [11–13]. Grey cast irons are importantly applied on the brake drums [14]. For this work, the cast iron samples were processed biomimetic non-smooth units by laser with different parameters. The thermal fatigue resistances of smooth and non-smooth samples were investigated, and the results were reported. 2. Experimental 2.1. Experimental materials For this work a common grey cast iron, codenamed HT250 was used. Its chemical composition, similar to the widely used ones for brake drums, is illustrated in Table 1.

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Table 1 The composition of HT250 grey cast iron (wt.%)

Table 2 The laser processing parameters of corresponding non-smooth samples

Composition

Content

No.

Electric current (A)

Pulse duration (ms)

Frequency (Hz)

C Si Mn P S Cu Cr Fe

3.250 1.570 0.920 0.060 0.059 0.500 0.270 Bal.

1 2 3 4

300 200 200 200

5.0 5.0 2.5 5.0

14 14 14 7

2.2. Sample preparation Samples of 30 mm long, 20 mm wide and 6 mm thick were cut by electric spark machine, and then were drilled 3 mm diameter round hole at one side of every sample, so that they could be fixed on the plate of thermal fatigue experimental machine. There are five samples. One with polished surface is called smooth sample. The others’ surface was processed stripes by laser with different parameters. The stripes are to form a continuously geometrical non-smooth surface to imitate the rough surface of the soil animals. Therefore, the stripes are named as non-smooth units, and the processed samples are called biomimetic non-smooth samples in this experiment. Fig. 1 shows the sketch of the nonsmooth sample. A solid state Nd-YAG laser of 1.06 ␮m wavelength and maximum power 100 W was used to process non-smooth samples. The working-bench moved at 0.88 mm/s. Through controlling the displacement of working-bench, the units were parallel to the length direction of sample, and the units distance between adjacent stripes centerlines was 2 mm. The processing was carried out by using different electric current, pulse duration and frequency parameters. Table 2 shows the samples’ number and their corresponding laser processing parameters. 2.3. Experimental method After laser processing, transverse sections were obtained from each unit, and standard method of metallography was followed for microstructure, dimensions and micro-hardness

Fig. 2. The actual temperature profile of one thermal cycle.

analysis. Optical microscope, micro-hardness instrument and scanning electron microscope of type (JSM-5500LV, Japan) were used for these investigations. Thermal fatigue tests were carried out using self-restrain thermal fatigue testing machine. The samples were heated by electric resistance furnace, and cooled by water flow. This machine could record thermal cycle times automatically. Fig. 2 shows an actual temperature profile of one thermal cycle. The samples were heated for 60 s up to 700 ◦ C, and then cooled for 3 s to 25 ◦ C during one cycle. At the beginning, the samples were taken out to observe the initiation of cracks every 100 cycles. After cracks appeared, their propagations were observed every 200 cycles until 1300 cycles altogether. 3. Results and discussion 3.1. The dimensions of non-smooth units

Fig. 1. The sketch of the non-smooth sample.

Fig. 3 shows the units’ transverse sections optical micrograph of Nos. 1–4 biomimetic non-smooth samples. The measurements were made on polished and etched transverse crosssections of each non-smooth unit, and it presented white and bright crescent. The unit depth, width, area, average microhardness and the energy input of single pulse laser beam are illustrated in Table 3. The unit area was calculated by the depth and width which were measured from each sample using optical microscope. The reported micro-hardness values in Table 3 are the average of five measurements taken at different locations in the non-smooth unit zone of each sample.

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Fig. 3. Optical micrographs of units’ transverse sections from Nos. 1 to 4 non-smooth samples (a–d).

When process a non-smooth unit, the light and heat energy exchanged between laser and cast iron. The light energy was absorbed, and in this process, the surface of cast iron was melted, and then solidified by conduction. The formation of non-smooth units was accomplished. When the laser electric current, pulse duration and frequency varied, the energy input was different. The energy input of single pulse laser used in Nos. 1–4 non-smooth sample could be measured by energy measurement instrument. They were No. 1 (4.84 J), No. 2 (3.34 J), No. 3 (1.95 J) and No. 4 (3.34 J). The more energy the cast iron absorbed, the bigger volume the unit zone was. Because of the same electric current and pulse duration used for Nos. 2 and 4 sample, the energy of single pulse was equivalent. However, the frequency used for No. 2 sample was twice than No. 4, and the higher frequency made the adjoining laser beam lapping more adequately. The energy absorbed by No. 2 sample was more than No. 4 in practice. So, it can be seen that the dimensions of units sort No. 1 > No. 2 > No. 3 > No. 4 by size.

Table 3 The characteristic differences of non-smooth samples No.

Energy (J)

Depth (mm)

Width (mm)

Area (mm2 )

Hardness (HV)

1 2 3 4

4.84 3.34 1.95 3.34

0.275 0.125 0.117 0.100

0.625 0.350 0.287 0.280

0.1558 0.0438 0.0288 0.0240

627 583 563 548

3.2. The microstructure of non-smooth units Fig. 4 shows the SEM micrograph of biomimetic non-smooth units from Nos. 1 to 4 sample. There is a complete dissolution of the graphite and re-solidification of the dendritic structure. The dendritic structure is proeutectic austenite and the reticulate white parts are cementite. The SEM micrograph shows a ledeburite structure through out the units. This structure was finer and more compact. It can be seen from Fig. 4 that the size of microstructure becomes much finer from Nos. 1 to 4. The differences in size are due to different cooling rates. The higher cooling rate is associated with smaller volume of non-smooth unit, and it can make the microstructure much finer. The smaller interdendritic spacing resulted in the higher micro-hardness and the better mechanical properties theoretically. 3.3. The thermal fatigue tests 3.3.1. The contrast of thermal fatigue resistance between non-smooth and smooth samples During the thermal tests, the sample was heated, and subsequently cooled down. When it was heated, due to the long heating time (60 s), there was nearly no temperature gradient between sample surface and interior. But, when the sample was subsequently cooled down in a short cooling time (3 s), because of the sample surface contacted with cool water, temperature in the portion was suddenly cooled down, but the interior temperature

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Fig. 4. SEM micrographs of biomimetic non-smooth units from Nos. 1 to 4 sample (a–d).

was still high. At this process, the surface materials shrank, but the interior materials expand. The shrinkage of surface materials was limited by interior materials, so the tensile residual tresses were produced on the surface and the interior portion suffered compressive stresses [15]. Accordingly, the sample suffered repeated thermal stresses with increasing of thermal cycles. This causes cracks initiation and propagation, and that results in thermal fatigue failure. Fig. 5 shows L–N curve which is drawn with the number of cycles (N) taken as horizontal coordinate and the length of the thermal crack on each sample surface (L) taken as vertical coordinate. It can be seen from Fig. 5 that cracks on smooth sample surface initiate at 200 cycles, while on Nos. 1–4 nonsmooth sample surface initiate at 400 cycles. When tests reach 1300 cycles, the length of the longest crack on smooth sample surface was 4.1 mm, while on non-smooth sample surface vary from 1.5 to 3 mm. Fig. 6 shows Q–N curve which is drawn with the number of cycles (N) taken as horizontal coordinate and the quantities of thermal cracks on each sample surface (Q) taken as vertical coordinate. It also can be seen from Fig. 6 that the quantities of thermal cracks whose length are more than 0.5 mm on smooth sample are 50, but the ones on non-smooth samples surface vary from 18 to 40. The results indicate that the thermal fatigue resistance of bionic non-smooth samples is better than that of smooth sample.

3.3.2. The contrast of thermal fatigue resistance of non-smooth samples The cracks propagation rates of non-smooth samples are shown by different slope of their curves in Fig. 5. It becomes clear from these results that the cracks propagation rates sort No. 1 < No. 2 < No. 3 < No. 4. The cracks quantities of non-smooth

Fig. 5. The L–N curve.

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Fig. 6. The Q–N curve.

Fig. 7. The original microstructure of HT250 grey cast iron.

samples are shown in Fig. 6, and they sort No. 1 < No. 2 < No. 3 < No. 4. Non-smooth sample No. 1 (unit transverse section area (hereinafter referred to as a “UTSA”): 0.1558 mm2 ), sample No. 2 (UTSA: 0.0438 mm2 ), sample No. 3 (UTSA: 0.0288 mm2 ), sample No. 4 (UTSA: 0.0240 mm2 ) are chosen for tests. The testing conditions are under the same thermal cycles (1300). It is obvious that the cracks quantity (hereinafter referred to as a “Q”) of No. 4 sample is the most, and the crack length (hereinafter referred to as a “L”) of it is the longest. Q of No. 3 sample is 10% fewer than that of No. 4 sample, and L is equivalent with that of No. 4 sample. Q of No. 2 sample is 42.5% fewer than that of No. 4 sample, and L is 23.3% shorter than that of No. 4 sample. Q of No. 1 sample is 52.5% fewer than that of No. 4 sample, and L is 50% shorter than that of No. 4 sample. The thermal fatigue performance was depending on the Q and N of sample. It is found that the thermal fatigue resistance sort No. 1 < No. 2 < No. 3 < No. 4, and they increase with the unit transverse section area augment.

related with the quantities and distributing of graphite in front of the main crack. The microstructure of non-smooth unit is ledeburite, and there is no graphite in unit zone. Due to a lot of units to form biomimetic non-smooth surface of sample, the quantity of graphite equivalent with crack initiation fountainhead decrease. When the crack moved to units, because of no graphite existed in, the original route of crack propagation was cut off. Therefore, cracks initiation and propagation can be resisted by non-smooth units. It was found that the more laser energy input, the bigger volume of unit; furthermore the decrement of graphite increase and units are traversed by crack difficultly, that all resulted in better thermal fatigue resistance. Though the smaller volume of unit can make finer microstructure and better property, they do nothing to the dominant factor of graphite. It can be

3.4. Mechanisms of enhanced thermal fatigue resistance 3.4.1. Cracks initiation and propagation resisted by non-smooth units Fig. 7 shows the original microstructure of HT250. It consists of graphite surrounded by some amount of pearlite. Just like a hole, the graphite among metallic matrix has no intensity. Graphite in grey cast iron is flaky, and there are stresses concentration at its edge, which makes the edge metallic matrix suffering bigger thermal stresses. When stresses are bigger than yield strength, micro-cracks initiate around the graphite [16]. Fig. 8 shows the optical micrograph of a polished sample after thermal tests. It can be seen from Fig. 8 that cracks propagation routes are along the graphite and the nearest distance between graphite. The propagation of main crack is realized by connecting of micro-cracks. So, the propagation rate and direction are

Fig. 8. The optical micrograph of a polished sample after thermal tests.

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Fig. 9. The hindered crack propagation pattern named “newborn” (a–b), * (b) is the model of (a).

Fig. 10. The hindered crack propagation pattern named “circuit” (a–b), * (b) is the model of (a).

said that the effects of difference in size of microstructure are very weak, when investigated their function to enhance thermal fatigue resistance. 3.4.2. The hindered crack propagation pattern 3.4.2.1. Newborn crack. Fig. 9 shows the hindered crack propagation pattern named “newborn”. When the crack propagated to unit, it stopped and could not traverse the unit. With the thermal cycles increasing, the propagation energy was accumulated in the front of crack, and crack became much wider and deeper at one side of non-smooth unit. After a few thermal cycles, the newborn cracks appeared at the other side of the unit, and they are finer and shallow. The crack propagation energy was absorbed by the newborn cracks. During this process, the crack propagated slowly. It resulted in longer thermal fatigue life. 3.4.2.2. Crack circuit. The other propagation pattern of the hindered cracks is shown in Fig. 10. The cracks initiated at the space between two adjacent non-smooth units. When the crack propagated to the unit, it changed the direction of propagation and turned to other directions. This phenomenon is called “crack circuit”, and it makes the routes of propagation much more complex. The cracks were limited between two adjacent units of 2 mm distance, and the longer cracks would not form. It also resulted in longer thermal fatigue life.

4. Conclusion (1) The designed biomimetic non-smooth surface can decrease thermal crack quantity and propagation rate; the thermal fatigue resistance is improved by processing non-smooth striate units on the surface of grey cast iron samples. (2) The dimension of unit is the key factor which affects the thermal fatigue resistance of biomimetic non-smooth samples. The bigger units can be obtained by increasing the laser energy input, and that resulted in better thermal fatigue resistance. (3) The microstructure of unit is ledeburite. The smaller volume of the unit, the finer microstructure is. But the effects of different microstructure dimensions are weak, when investigated their function to the thermal fatigue resistance of non-smooth samples. (4) Due to no graphite existing at non-smooth unit zone, when cracks propagate to the unit, they are hindered. The subsequent propagation pattern is “newborn” or “circuit”. Acknowledgements This article was supported by Project 985-Automotive Engineering of Jilin University, the Research Fund for the Doctoral Program of Higher Education (No. 20040183026), and the Sci-

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