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The thermal fatigue resistance of H13 steel repaired by a biomimetic laser remelting process
Materials and Design 55 (2014) 597–604
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
The thermal fatigue resistance of H13 steel repaired by a biomimetic laser remelting process Dalong Cong a, Hong Zhou a,⇑, Zhenan Ren a, Zhihui Zhang b, Haifeng Zhang a, Chao Meng a, Chuanwei Wang a 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
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Article history: Received 19 May 2013 Accepted 25 September 2013 Available online 23 October 2013 Keywords: Thermal fatigue Crack repair Laser surface remelting AISI H13 steel
a b s t r a c t In this study, the biomimetic laser remelting process was adopted to repair thermal fatigue cracks on an annealed hot work die AISI H13 steel. Several treated morphologies: spot, striation and lattice were processed by a pulsed Nd:YAG laser. The ultrafine microstructure within the unit was comprised of martensite, austenite and carbides. The average microhardness of the unit is much higher than that of the original surface, even after thermal cycles. Thermal fatigue results show that both crack density and crack length are reduced due to the blocking effect of strengthening units. And the partial laser surface remelting with lattice morphology is the most effective for repairing cracks and improving the thermal fatigue resistance of all. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Die casting dies are increasingly applied to manufacture metal components due to their high series production process, high surface quality and short lead time in engineering fields. The primary causes of failure in die casting are surface heat checks, which are induced by thermal fatigue of die material serving in alternatively heating and cooling conditions [1]. Recently, many studies have been developed in the field of heat checks repair on the mould or tool steel surface seeking to maximize the lifetime before failure occurs, including pulsed electron beam irradiation [2], laser processing [3–5], surface cold-spray method [6] and so forth. Laser processing such as laser welding, laser direct metal deposition and laser surface remelting (LSR) has been widely used mainly because of relatively small but highly focused energy, accuracy and flexibility of laser beam considering the fine shallow characteristics of thermal cracks [7,8]. LSR characterized by rapid melting and solidification on the base metal without additional filler materials, is one of the most effective methods to modify surface properties and repair damaged iron or steel materials through removing the cracks or defects [9–11]. For example, Cabeza et al. [9] repaired the damaged surface of 14 Ni maraging steel by multiple LSR tracks with 25% overlap. In Ref. [10], YAG laser processing was applied to repair cracks on the surface layer of tool steels with the results indicating that laser processing is applicable to remove ⇑ Corresponding author. Tel.: +86 431 85094427; fax: +86 431 85095592. E-mail address: [email protected] (H. Zhou). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.09.076
cracks on surface layer. Nevertheless, these methods are mainly aimed at treating the overall surface of materials consuming a large amount of working time and energy. Animals and plants in nature provide a whole host of superior multifunctional structures on their body surfaces that can be attributed to the evolution and optimization to adapt environments for millions of years as shown in Fig. 1. Actually, creatures not only can adapt to the living environment, but also can satisfy their survival with the lowest energy cost. Many studies have paid attention to the application of biomimetic principles to solve engineering problems. For instance, Ren et al. have developed biomimetic non-smooth theory and found that non-smooth surfaces can exhibit reduced sliding resistance against soil [12,13]. During past a few years, many researchers have studied the thermal fatigue resistance of cast iron and tool steel with selective surface remelting processed by a pulsed solid state Nd:YAG laser [14–17]. Tong et al. [14,15] modified the surface of a low alloy gray cast iron by LSR with various surface morphologies to improve the thermal fatigue resistance. In another Ref. [16], selective laser surface alloying was employed to improve the thermal fatigue behavior of medium carbon hot work die steel. The results indicated that when applied selective laser surface processing there had a considerable improvement in the thermal fatigue resistance by inhibiting the initiation and propagation of thermal fatigue cracks. It should be noted that, a few studies on this theory have been carried out elsewhere [18–20]. Wang et al. produced strengthening units with high hardness by laser processing to form the non-smooth surface on GCr15 steel and thus to improve its wear resistance [18].
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Fig. 1. Various surface structures of animals and plants [12,13]. (a) The pronotum of dung beetle; (b) ridged surface on the abdomen of a ground beetle; (c) surface of the tree leaf; and (d) lattice shape structure on the dragonfly wing.
Moreover, another study [19] reported effects of strengthening striations and grids on the surface of H13 steel and showed that biomimetic surfaces exhibited observably enhanced ultimate tensile strength and yield strength, while corresponding ductility was even heightened after thermal fatigue loaded. In Ref. [20], Shan et al. studied adhesion resistance behavior of the sample with striated non-smooth surface processed by laser technique and found that the biomimetic moulds have a beneficial effect on decreasing the adhesion to eject polymer parts. Note that the excellent properties of biomimetic surface are directly associated with the reasonable arrangement of strengthening units inspired by the structure of creatures. In this study, the selective LSR was adopted to repair the thermal fatigue cracks on the surface of medium carbon hot work die steel with different morphologies by imitating the heterogeneous surface structures of creatures in Fig. 1. In addition, microstructural evolution, microhardness and the thermal fatigue behavior of repaired samples were investigated. Related mechanism was also discussed. 2. Materials and methods 2.1. Materials Annealed medium carbon hot work die steel, AISI H13, was adopted as the base metal, of which chemical composition in wt.% is: 0.41 C, 0.97 Si, 0.38 Mn, 4.82 Cr, 1.19 Mo, 0.92 V, 0.07 Ni, 0.007 P, 0.002 S and balanced Fe. 2.2. Sample preparation As shown in Fig. 2, rectangular thermal testing samples were cut in the dimension of 40 20 5 mm3 using an electrodischarge machine. A 3 mm hole in diameter was mechanically drilled at one side of each sample so that it could be fixed onto the thermal fatigue test machine. To avoid the effects of surface machining marks on the fatigue life, samples were mechanically
polished using progressively finer grades of silicon carbide impregnated emery paper prior to the thermal fatigue test. The preliminary experiment of fabricating thermal fatigue cracks on the smooth samples was conducted on a self-restrain thermal fatigue testing machine as shown in Fig. 3, which could record times of thermal cycles automatically. In order to simulate the thermal condition in the actual die casting process, a complete thermal cycle including heating for 75 s up to 750 ± 5 °C, and then cooling for 5 s to 25 ± 5 °C by running water was used. The thermal fatigue test was carried out according to the GB/T 15824-2008 standard [21]. In the preliminary experiment, the thermal fatigue test was carried out 1000 cycles. Fig. 4 shows a schematic drawing of the laser and experimental set-up used in this work. A pulsed solid-state Nd:YAG laser system (Han’s Laser, WF300, China) with the wavelength of 1.06 lm and mean power of 300 W was used as the laser source. The circular spot size of Gaussian laser beam on the sample surface was 1.6 ± 0.1 mm. The displacement of laser beam was controlled by a multi-purpose robot (Motoman, MH6, Japan) with the precision of ±0.08 mm. Pure argon gas was used for shielding with a flow rate of 5 l/min through a side nozzle. Before LSR, the fatigue samples were cleaned using NaOH solution and then alcohol to remove oxide scales on the surface. In this study, only part of thermal fatigue cracks was repaired in the forms of different surface morphologies (see Fig. 2). Accordingly, the remelting zones were named spot unit (No. 1), striation unit (No. 2) and lattice unit (No. 3), respectively, with a designed spacing of 2 mm (S2) between every two adjacent units. The sample used for analyzing the phase structures of unit was processed by striation units with the same parameters as Nos. 2 and 3; every two adjacent units have no spacing. Table 1 shows the laser processing parameters. Finally, thermal fatigue test was carried out in order to study the effects of laser-repaired areas to overall steel surface on thermal fatigue resistance with the same conditions to the preliminary experiment. There were a total of four samples in this test, i.e., No. 1, No. 2, No. 3 and No. 4. No. 4 is the fatigue sample without repair used to compare with the other three ones. The individual
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Fig. 2. The schematic diagram of laser surface remelting morphologies and the sample dimension.
Table 1 Laser parameters for remelting process.
Fig. 3. The schematic diagram of the thermal fatigue test machine.
Sample
Laser energy density (J/mm2)
Pulse duration (ms)
Frequency (Hz)
Scanning speed (mm/s)
Defocusing amount (mm)
No. 1 Nos. 2 and 3
3.23 3.23
7 7
1 6
4 0.5
7 7
Hardness Tester (Buehler, 5104, USA) with a 200 gf applied load. Each microhardness test was carried out three times based on the ISO 22826: 2005 standard [22]. Values of hardness reported thus represent the average of the three readings. The phase structures were identified by X-ray diffraction (XRD) instrument (D/Max, 2500PC, Japan) with Cu Ka radiation operated at a voltage of 40 kV, a current of 40 mA and a scanning rate of 4 deg/min. A stereomicroscope (XTL-2400) was used to observe the propagation of thermal cracks. 3. Results and discussion 3.1. Phase identification and microstructure of unit
Fig. 4. The schematic diagram of the experimental setup for laser processing.
measurements of thermal fatigue results were conducted three times for mean values of results. 2.3. Experimental methods Optical microscopy (OLYMPUS, PMG3, Japan) was applied to investigate the cross-sectional morphologies of samples. The samples for microstructure observation were evaluated by Scanning Electron Microscopy (SEM) (Zeiss, Evo18, Germany) after standard methods of metallography. The microhardness profiles along the depth direction of cross-section were obtained on Vickers
The microstructure of annealed AISI H13 samples before and after preliminary thermal fatigue test was shown in Fig. 5. Compared with that of original annealed AISI H13 steel (Fig. 5(a)), the microstructure of fatigue samples got coarser due to the preliminary thermal fatigue, and mixed with pearlite and carbides (Fig. 5(b)). Fig. 6 shows the cross-section and SEM morphologies of laser remelting samples. Typical cross-section morphology of the unit is shown in Fig. 6(a), which includes laser melted zone (LMZ) and heat affected zone (HAZ). The unit width was about 1850 lm, while the depth was 430 lm. Note that no pores and cracks were found in the unit. The microstructures of units at higher magnification were given in Fig. 6(b–d), respectively. The microstructure in the LMZ is mainly characterized by a very fine cellular structure (Fig. 6(b)) in the center and dendritic structure (Fig. 6(d)) in the boundary. At the grain boundaries ultrafine carbide network or the eutectic crossed. The super-refined carbides inside the crystal grain of these cellular and dendritic structures were found, which were much finer and more uniform than those in the base material. These results are consistent with the cases reported by
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there is a notable change of microhardness distribution between the unit zone and the substrate. It can be noted that the hardness decreases abruptly from the unit to HAZ and then substrate. After remelting, the hardness of unit varies from 600 to 700 HV, and the HAZ varies from 350 to 450 HV while the substrate hardness is around 270 HV (curve of 0 cycle). This could be attributed to the formation of martensite, ultrafine carbides and microstructure refinement in the remelting process. A notable decline in the microhardness of the unit zone was caused by thermal fatigue process. In the early stage (62000 cycles), the microhardness within unit was considerably decreased, while in the late stage, that was slightly decreased. Fig. 9 shows the modification of microstructure within the LMZ of unit during thermal fatigue. As can be seen in Fig. 9(b and c), both grains growth and precipitation of large size carbides occurred obviously in the early stage of thermal fatigue test. These changes induced notable decline in the microhardness of the LMZ of the units. In the later stage, the modification was minor and a stable microstructure formed gradually as shown in Fig. 9(d). The relevant microhardness in this stage changed slightly and continued to be significantly higher than that of the substrate. The similar evolution of laser remelting microstructure during the thermal test has been found in the H21 steel by Zhang et al. [29]. 3.3. Thermal fatigue resistance During thermal fatigue test, the number of thermal cracks (Nc) and total length of the longest crack (L) on each sample surface were measured every 1000 cycles. For crack initiation studies, failure was defined when a crack on the specimen surface was 0.05–0.125 mm in length [30]. To decrease the testing error, if a crack is longer than 0.1 mm, it was defined as one crack in this study. Additionally, the mean crack density (D) was calculated to evaluate the thermal fatigue resistance. The calculation formula: Fig. 5. Microstructure of annealed AISI H13 steel (a) before and (b) after preliminary thermal fatigue test.
D ¼ Nc =S
ð1Þ 3
Zhang et al. [17] and Chiang and Chen [23], and can be explained by a very high cooling rate in laser remelting process. As indicated in Fig. 6(c), HAZ is quite narrow with the width of about 30 lm; the particles of MAC carbides became larger and many more formed as a result of tempering [23]. As shown in Fig. 7, X-ray diffraction was used for comparing the phase compositions of the LMZ of units and the substrate. It can be deduced from Fig. 7(a) that constitutional phase in the substrate is ferrite. However, carbide peaks were absent probably due to the low carbide volume fraction (Fig. 7(a)); this case is also reported previously by Aqida et al. [24] and Rafi et al. [25].The intensity distribution of the peaks was similar to the ferrite phase (06-0696) peaks listed in the Joint Committee on Powder Diffraction Standards (JCPDS) database. Due to laser remelting, microstructure in the LMZ of the unit is different from that in the substrate, which induces martensite, retained austenite and MAC carbides (M@Cr, Fe). It can be seen that the peak of martensite/ferrite (1 1 0) in the LMZ of units broadened compared with that in the substrate. In general, the XRD line broadening is caused by two main effects: (1) refinement of the microstructure [26,27] and (2) the deformation (strain) broadening. The latter is often related to dislocations causing extended displacement fields in the crystal which indicates the formation of martensite in the LMZ [27,28]. 3.2. Microhardness The microhardness as a function of thermal fatigue cycle in the depth direction of sample cross-section is given in Fig. 8. Due to the formation of an inhomogeneous structure, as mentioned above,
2
where S is the area of the sample, here S is 0.8 10 mm (the hole area was ignored). Fig. 10(a and b) gives the D–N curve and L–N curve to show the variations of mean crack density and main crack length versus thermal cycles (N), respectively. It can be seen that both D and L sort No. 4 > No. 1 > No. 2 > No. 3, indicating that all repaired samples in the forms of strengthening spots, striations and lattices exhibit higher resistance to thermal cracks compared with the original fatigue sample. In the meanwhile, the sample with strengthening lattices has the best resistance. 3.4. Mechanisms of enhanced thermal fatigue resistance Thermal fatigue process includes three stages: nucleation, initial growth and proceeded crack growth [1,31]. Once a sample subjected to the thermo-mechanical loadings from steep temperature gradients in thickness, it may induce large thermal stresses on the surface layer [32]. When tensile stresses exceed the yield stress of material, nucleation and initial crack growth occurred. The crack growth is promoted by oxidation of the working surface and surface of the crack. Air oxygen in high temperature environment is causing the oxidation of the crack tip, which induces crack opening due to the brittleness of oxide layer [33]. Another factor facilitating the crack propagation is the thermal softening; in the contrary, the growth of thermal cracks is suppressed by better mechanical properties of surface material i.e. higher hardnesses [1]. On the other hand, it is clear that the crack tip is a main initiation source of microcracks. Many microcracks could connect gradually in the propagation and formed a large crack. Therefore the propagation of main crack was realized by bridge connections with microcracks
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Fig. 6. Cross-sectional morphologies of the unit: (a) optical micrograph of unit; (b) SEM microstructure in unit center; (c) SEM microstructure in HAZ; and (d) SEM microstructure in unit boundary.
Fig. 8. The distribution of microhardness in unit depth direction with increase of thermal cycles.
Fig. 7. XRD patterns of the unit and substrate.
[15]. This phenomenon was found in the original fatigue sample during thermal fatigue test as shown in Fig. 11. It can be seen that the original fatigue sample has poor resistance to thermal fatigue damages on which surface many random thermal cracks and oxidation corrosion occurred during thermal fatigue. The mechanical property of units is independent of laser remelting morphologies because it is an instinctive nature from the microstructure. As aforementioned, the laser remelting microstructure within the unit is composed of hard martensite, soft austenite and finer carbides. For the property of metal materials, the ‘‘hard’’ phase contributes to the strength and hardness, whereas the ‘‘soft’’ phase benefits ductility and toughness [34]. Thereby, the mixed microstructure of these two kinds of phases can provide
the unit with a proper combination of strength and toughness. Furthermore, this character was enhanced by the considerable grain refinement caused by rapid melting and solidification on the basis of Hall–Petch relationship. On the other hand, finer crystal grains have a beneficial effect on the improvement in material’s fatigue resistance [17,35–37]. Therefore, the units have a superior thermal fatigue strength compared to the base material. Though changes of microstructure within the unit were produced due to the thermal heat treatments of thermal fatigue testing and the microhardness also declined, it was still higher than that of the substrate that is, the units can still act as hard structure to restrain the propagation of cracks. Due to the intense influences of body structures on the surface properties in animals and plants (Fig. 1), it is essential to discuss the thermal fatigue resistance of samples with different surface remelting morphologies. Typical propagation patterns of thermal fatigue cracks on each sample surface during thermal fatigue test were shown in
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Fig. 9. The microstructural evolution in LMZ of units during thermal fatigue test: (a) the microstructure after 0 cycle (before the thermal fatigue test); (b) after 2000 cycles; (c) after 3000 cycles and (d) after 5000 cycles.
Fig. 10. The thermal fatigue resistance of samples: (a) the D–N curve and (b) the L–N curve.
Fig. 12. As is known to us, residual stress can be induced by rapid solidification of laser treatment. In general, tensile stresses promote the fatigue crack growth, in contrast, residual compressive stresses enhance the closure of the crack tip resulting in decreased fatigue crack growth [38]. The laser energy density used in this study was 3.23 J/mm2, at such low energy residual tensile stress formed in the units and residual compressive stress formed in adjoining material [5,39]. Consequently, the distribution of residual stresses on laser treated samples is in favor of decreasing fatigue crack growth on the original fatigue surface. However, the cracking of unit may occur due to the tensile stress in it. Therefore, it is necessary to check the surface bug of units before thermal fatigue test. Fig. 12 indicates that no cracks formed in the units even after thermal fatigue test. Original thermal cracks and oxidation corrosion pits in unit zones have been eliminated. The residual cracks on the samples are helpful for propagation energy release, thus reducing the initiation of cracks. Because in the process of thermal cycling, propagation energy was accumulated at the crack tip, due to the action of thermal stresses; with the growth of
cracks, the accumulated energy would be released [40]. On the other hand, due to stress concentration, the crack tip is a main initiation source of microcracks which can be promoted by oxidation. The local formation and dropping of the oxidation film result in oxidation corrosion pits which in return further facilitate crack initiation. It is conceivable that the samples with reduced initiation source of thermal cracks and oxidation corrosion pits possess improved thermal fatigue resistance. Fig. 12(a4, b4 and c4) indicates that few oxidation corrosion pits formed on units during thermal fatigue compared with that on original fatigue surface. Fig. 12(a1–a4) shows the hindering characteristic of a sample with spot units. When a crack encountered the unit, it would be deflected to round the strengthening spot. In other words, the spot units can only hamper the crack towards them. Fig. 12(b1–b4) shows the hindering characteristic of a sample with striation units. The cracks propagating to the strengthening striations were unable to round them but deflected with a larger angle. As similar with spot units, striation units have less effect on resisting the propagation of cracks parallel with them; these cracks can form a large
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Fig. 11. The typical propagation patterns of cracks on untreated samples: (a) precracks, (b) after 3000 cycles and (c) after 5000 cycles.
Fig. 12. Typical propagation patterns of cracks on laser treated samples after various thermal fatigue cycles: (a1) after repair, (a2) after 1000 times, (a3) after 3000 times and (a4) after 5000 times of No. 1 sample; (b1) after repair, (b2) after 1000 times, (b3) after 3000 times and (b4) after 5000 times of No. 2 sample; (c1) after repair, (c2) after 1000 times, (c3) after 3000 times and (c4) after 5000 times of No. 3 sample.
crack by connection with each other as shown by the rectangles. Fig. 12(c1–c4) shows the hindering characteristic of a sample with lattice units. The cracks were limited in closed strengthening
lattices, and that led to the circuit of crack in a lattice. Since the lattice units can block cracks in all directions, the growth of the crack was controlled more effectively. These results are agreement
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well with those of Tong et al. and Zhang et al. [14,17]. On the other hand, as shown in Fig. 12(a1, b1 and c1), the latticed sample possessed the largest remelting area, that is, the minimum amount of residual cracks and oxidation corrosion pits were left on original fatigue surface which reduced the initiation of thermal cracks. Moreover, thermal cracks and oxidation corrosion pits were surrounded by the lattice units; the connection of thermal cracks was blocked effectively. As a result, the samples with lattice units achieved the highest thermal fatigue resistance of all. 4. Conclusions Thermal fatigue cracks on a hot work die steel H13 were repaired by means of biomimetic laser surface remelting process in forms of spot, striation and lattice surface morphologies. Due to the ultrafine microstructure comprising of martensite, austenite and fine carbides formed in the rapid remelting process, units have a super thermal fatigue resistance. Moreover, the average microhardness of the unit is much higher than that of the original surface, even after 5000 thermal cycles. Thermal fatigue results show that both crack density and crack length are reduced due to the blocking effect of strengthening units. Strengthening spots and striations can only hamper the propagation of cracks towards them; nevertheless strengthening lattices are able to resist cracks in all directions. Furthermore, the lattice sample (No. 3) possesses the largest remelting area, which eliminated the largest number of cracks and oxidation corrosion pits, and blocked the connection of thermal cracks effectively. Hence, the partial laser surface remelting with lattice morphology is the most effective for repairing cracks and improving the thermal fatigue resistance of all. On the other hand, some cracks left on original surface are helpful for the propagation energy release, thus reducing the initiation of cracks. Acknowledgements This article was financially supported by Project 985-High Performance Materials of Jilin University, Project 985-Bionic Engineering Science and Technology Innovation and the National Natural Science Foundation of China (No. 51275200). References [1] Klobcˇar D, Kosec L, Kosec B, Tušek J. Thermo fatigue cracking of die casting dies. Eng Fail Anal 2012;20:43–53. [2] Murray JW, Clare AT. Repair of EDM induced surface cracks by pulsed electron beam irradiation. J Mater Process Technol 2012;212:2642–51. [3] Pleterski M, Tušek J, Kosec L, Muhicˇ M, Muhicˇ T. Laser repair welding of molds with various pulse shapes. Metalurgija 2010;49:41–4. [4] Nowotny S, Scharek S, Beyer E, Richter KH. Laser beam build-up welding: precision in repair, surface cladding, and direct 3D metal deposition. J Therm Spray Technol 2007;16:344–8. [5] Grum J, Slabe JM. Effect of laser-remelting of surface cracks on microstructure and residual stresses in 12Ni maraging steel. Appl Surf Sci 2006;252:4486–92. [6] Lee JC, Kang HJ, Chu WS, Ahn SH. Repair of damaged mold surface by coldspray method. CIRP Ann 2007;56:577–80. [7] Wang W, Pinkerton A, Wee L, Li L. Component repair using laser direct metal deposition. Springer; 2007. p. 345–50. [8] Song R, Hanaki S, Yamashita M, Uchida H. Reliability evaluation of a laser repaired die-casting die. Mater Sci Eng A 2008;483–484:343–5. [9] Cabeza M, Castro G, Merino P, Pena G, Román M. Laser surface melting: A suitable technique to repair damaged surfaces made in 14Ni (200 grade) maraging steel. Surf Coat Technol 2012;212:159–68. [10] Sun Y, Sunada H, Tsujii N. Crack repair of hot work tool steel by laser melt processing. ISIJ Int 2001;41:1006–9.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
The thermal fatigue resistance of H13 steel repaired by a biomimetic laser remelting process Dalong Cong a, Hong Zhou a,⇑, Zhenan Ren a, Zhihui Zhang b, Haifeng Zhang a, Chao Meng a, Chuanwei Wang a 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
a r t i c l e
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Article history: Received 19 May 2013 Accepted 25 September 2013 Available online 23 October 2013 Keywords: Thermal fatigue Crack repair Laser surface remelting AISI H13 steel
a b s t r a c t In this study, the biomimetic laser remelting process was adopted to repair thermal fatigue cracks on an annealed hot work die AISI H13 steel. Several treated morphologies: spot, striation and lattice were processed by a pulsed Nd:YAG laser. The ultrafine microstructure within the unit was comprised of martensite, austenite and carbides. The average microhardness of the unit is much higher than that of the original surface, even after thermal cycles. Thermal fatigue results show that both crack density and crack length are reduced due to the blocking effect of strengthening units. And the partial laser surface remelting with lattice morphology is the most effective for repairing cracks and improving the thermal fatigue resistance of all. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Die casting dies are increasingly applied to manufacture metal components due to their high series production process, high surface quality and short lead time in engineering fields. The primary causes of failure in die casting are surface heat checks, which are induced by thermal fatigue of die material serving in alternatively heating and cooling conditions [1]. Recently, many studies have been developed in the field of heat checks repair on the mould or tool steel surface seeking to maximize the lifetime before failure occurs, including pulsed electron beam irradiation [2], laser processing [3–5], surface cold-spray method [6] and so forth. Laser processing such as laser welding, laser direct metal deposition and laser surface remelting (LSR) has been widely used mainly because of relatively small but highly focused energy, accuracy and flexibility of laser beam considering the fine shallow characteristics of thermal cracks [7,8]. LSR characterized by rapid melting and solidification on the base metal without additional filler materials, is one of the most effective methods to modify surface properties and repair damaged iron or steel materials through removing the cracks or defects [9–11]. For example, Cabeza et al. [9] repaired the damaged surface of 14 Ni maraging steel by multiple LSR tracks with 25% overlap. In Ref. [10], YAG laser processing was applied to repair cracks on the surface layer of tool steels with the results indicating that laser processing is applicable to remove ⇑ Corresponding author. Tel.: +86 431 85094427; fax: +86 431 85095592. E-mail address: [email protected] (H. Zhou). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.09.076
cracks on surface layer. Nevertheless, these methods are mainly aimed at treating the overall surface of materials consuming a large amount of working time and energy. Animals and plants in nature provide a whole host of superior multifunctional structures on their body surfaces that can be attributed to the evolution and optimization to adapt environments for millions of years as shown in Fig. 1. Actually, creatures not only can adapt to the living environment, but also can satisfy their survival with the lowest energy cost. Many studies have paid attention to the application of biomimetic principles to solve engineering problems. For instance, Ren et al. have developed biomimetic non-smooth theory and found that non-smooth surfaces can exhibit reduced sliding resistance against soil [12,13]. During past a few years, many researchers have studied the thermal fatigue resistance of cast iron and tool steel with selective surface remelting processed by a pulsed solid state Nd:YAG laser [14–17]. Tong et al. [14,15] modified the surface of a low alloy gray cast iron by LSR with various surface morphologies to improve the thermal fatigue resistance. In another Ref. [16], selective laser surface alloying was employed to improve the thermal fatigue behavior of medium carbon hot work die steel. The results indicated that when applied selective laser surface processing there had a considerable improvement in the thermal fatigue resistance by inhibiting the initiation and propagation of thermal fatigue cracks. It should be noted that, a few studies on this theory have been carried out elsewhere [18–20]. Wang et al. produced strengthening units with high hardness by laser processing to form the non-smooth surface on GCr15 steel and thus to improve its wear resistance [18].
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Fig. 1. Various surface structures of animals and plants [12,13]. (a) The pronotum of dung beetle; (b) ridged surface on the abdomen of a ground beetle; (c) surface of the tree leaf; and (d) lattice shape structure on the dragonfly wing.
Moreover, another study [19] reported effects of strengthening striations and grids on the surface of H13 steel and showed that biomimetic surfaces exhibited observably enhanced ultimate tensile strength and yield strength, while corresponding ductility was even heightened after thermal fatigue loaded. In Ref. [20], Shan et al. studied adhesion resistance behavior of the sample with striated non-smooth surface processed by laser technique and found that the biomimetic moulds have a beneficial effect on decreasing the adhesion to eject polymer parts. Note that the excellent properties of biomimetic surface are directly associated with the reasonable arrangement of strengthening units inspired by the structure of creatures. In this study, the selective LSR was adopted to repair the thermal fatigue cracks on the surface of medium carbon hot work die steel with different morphologies by imitating the heterogeneous surface structures of creatures in Fig. 1. In addition, microstructural evolution, microhardness and the thermal fatigue behavior of repaired samples were investigated. Related mechanism was also discussed. 2. Materials and methods 2.1. Materials Annealed medium carbon hot work die steel, AISI H13, was adopted as the base metal, of which chemical composition in wt.% is: 0.41 C, 0.97 Si, 0.38 Mn, 4.82 Cr, 1.19 Mo, 0.92 V, 0.07 Ni, 0.007 P, 0.002 S and balanced Fe. 2.2. Sample preparation As shown in Fig. 2, rectangular thermal testing samples were cut in the dimension of 40 20 5 mm3 using an electrodischarge machine. A 3 mm hole in diameter was mechanically drilled at one side of each sample so that it could be fixed onto the thermal fatigue test machine. To avoid the effects of surface machining marks on the fatigue life, samples were mechanically
polished using progressively finer grades of silicon carbide impregnated emery paper prior to the thermal fatigue test. The preliminary experiment of fabricating thermal fatigue cracks on the smooth samples was conducted on a self-restrain thermal fatigue testing machine as shown in Fig. 3, which could record times of thermal cycles automatically. In order to simulate the thermal condition in the actual die casting process, a complete thermal cycle including heating for 75 s up to 750 ± 5 °C, and then cooling for 5 s to 25 ± 5 °C by running water was used. The thermal fatigue test was carried out according to the GB/T 15824-2008 standard [21]. In the preliminary experiment, the thermal fatigue test was carried out 1000 cycles. Fig. 4 shows a schematic drawing of the laser and experimental set-up used in this work. A pulsed solid-state Nd:YAG laser system (Han’s Laser, WF300, China) with the wavelength of 1.06 lm and mean power of 300 W was used as the laser source. The circular spot size of Gaussian laser beam on the sample surface was 1.6 ± 0.1 mm. The displacement of laser beam was controlled by a multi-purpose robot (Motoman, MH6, Japan) with the precision of ±0.08 mm. Pure argon gas was used for shielding with a flow rate of 5 l/min through a side nozzle. Before LSR, the fatigue samples were cleaned using NaOH solution and then alcohol to remove oxide scales on the surface. In this study, only part of thermal fatigue cracks was repaired in the forms of different surface morphologies (see Fig. 2). Accordingly, the remelting zones were named spot unit (No. 1), striation unit (No. 2) and lattice unit (No. 3), respectively, with a designed spacing of 2 mm (S2) between every two adjacent units. The sample used for analyzing the phase structures of unit was processed by striation units with the same parameters as Nos. 2 and 3; every two adjacent units have no spacing. Table 1 shows the laser processing parameters. Finally, thermal fatigue test was carried out in order to study the effects of laser-repaired areas to overall steel surface on thermal fatigue resistance with the same conditions to the preliminary experiment. There were a total of four samples in this test, i.e., No. 1, No. 2, No. 3 and No. 4. No. 4 is the fatigue sample without repair used to compare with the other three ones. The individual
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Fig. 2. The schematic diagram of laser surface remelting morphologies and the sample dimension.
Table 1 Laser parameters for remelting process.
Fig. 3. The schematic diagram of the thermal fatigue test machine.
Sample
Laser energy density (J/mm2)
Pulse duration (ms)
Frequency (Hz)
Scanning speed (mm/s)
Defocusing amount (mm)
No. 1 Nos. 2 and 3
3.23 3.23
7 7
1 6
4 0.5
7 7
Hardness Tester (Buehler, 5104, USA) with a 200 gf applied load. Each microhardness test was carried out three times based on the ISO 22826: 2005 standard [22]. Values of hardness reported thus represent the average of the three readings. The phase structures were identified by X-ray diffraction (XRD) instrument (D/Max, 2500PC, Japan) with Cu Ka radiation operated at a voltage of 40 kV, a current of 40 mA and a scanning rate of 4 deg/min. A stereomicroscope (XTL-2400) was used to observe the propagation of thermal cracks. 3. Results and discussion 3.1. Phase identification and microstructure of unit
Fig. 4. The schematic diagram of the experimental setup for laser processing.
measurements of thermal fatigue results were conducted three times for mean values of results. 2.3. Experimental methods Optical microscopy (OLYMPUS, PMG3, Japan) was applied to investigate the cross-sectional morphologies of samples. The samples for microstructure observation were evaluated by Scanning Electron Microscopy (SEM) (Zeiss, Evo18, Germany) after standard methods of metallography. The microhardness profiles along the depth direction of cross-section were obtained on Vickers
The microstructure of annealed AISI H13 samples before and after preliminary thermal fatigue test was shown in Fig. 5. Compared with that of original annealed AISI H13 steel (Fig. 5(a)), the microstructure of fatigue samples got coarser due to the preliminary thermal fatigue, and mixed with pearlite and carbides (Fig. 5(b)). Fig. 6 shows the cross-section and SEM morphologies of laser remelting samples. Typical cross-section morphology of the unit is shown in Fig. 6(a), which includes laser melted zone (LMZ) and heat affected zone (HAZ). The unit width was about 1850 lm, while the depth was 430 lm. Note that no pores and cracks were found in the unit. The microstructures of units at higher magnification were given in Fig. 6(b–d), respectively. The microstructure in the LMZ is mainly characterized by a very fine cellular structure (Fig. 6(b)) in the center and dendritic structure (Fig. 6(d)) in the boundary. At the grain boundaries ultrafine carbide network or the eutectic crossed. The super-refined carbides inside the crystal grain of these cellular and dendritic structures were found, which were much finer and more uniform than those in the base material. These results are consistent with the cases reported by
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there is a notable change of microhardness distribution between the unit zone and the substrate. It can be noted that the hardness decreases abruptly from the unit to HAZ and then substrate. After remelting, the hardness of unit varies from 600 to 700 HV, and the HAZ varies from 350 to 450 HV while the substrate hardness is around 270 HV (curve of 0 cycle). This could be attributed to the formation of martensite, ultrafine carbides and microstructure refinement in the remelting process. A notable decline in the microhardness of the unit zone was caused by thermal fatigue process. In the early stage (62000 cycles), the microhardness within unit was considerably decreased, while in the late stage, that was slightly decreased. Fig. 9 shows the modification of microstructure within the LMZ of unit during thermal fatigue. As can be seen in Fig. 9(b and c), both grains growth and precipitation of large size carbides occurred obviously in the early stage of thermal fatigue test. These changes induced notable decline in the microhardness of the LMZ of the units. In the later stage, the modification was minor and a stable microstructure formed gradually as shown in Fig. 9(d). The relevant microhardness in this stage changed slightly and continued to be significantly higher than that of the substrate. The similar evolution of laser remelting microstructure during the thermal test has been found in the H21 steel by Zhang et al. [29]. 3.3. Thermal fatigue resistance During thermal fatigue test, the number of thermal cracks (Nc) and total length of the longest crack (L) on each sample surface were measured every 1000 cycles. For crack initiation studies, failure was defined when a crack on the specimen surface was 0.05–0.125 mm in length [30]. To decrease the testing error, if a crack is longer than 0.1 mm, it was defined as one crack in this study. Additionally, the mean crack density (D) was calculated to evaluate the thermal fatigue resistance. The calculation formula: Fig. 5. Microstructure of annealed AISI H13 steel (a) before and (b) after preliminary thermal fatigue test.
D ¼ Nc =S
ð1Þ 3
Zhang et al. [17] and Chiang and Chen [23], and can be explained by a very high cooling rate in laser remelting process. As indicated in Fig. 6(c), HAZ is quite narrow with the width of about 30 lm; the particles of MAC carbides became larger and many more formed as a result of tempering [23]. As shown in Fig. 7, X-ray diffraction was used for comparing the phase compositions of the LMZ of units and the substrate. It can be deduced from Fig. 7(a) that constitutional phase in the substrate is ferrite. However, carbide peaks were absent probably due to the low carbide volume fraction (Fig. 7(a)); this case is also reported previously by Aqida et al. [24] and Rafi et al. [25].The intensity distribution of the peaks was similar to the ferrite phase (06-0696) peaks listed in the Joint Committee on Powder Diffraction Standards (JCPDS) database. Due to laser remelting, microstructure in the LMZ of the unit is different from that in the substrate, which induces martensite, retained austenite and MAC carbides (M@Cr, Fe). It can be seen that the peak of martensite/ferrite (1 1 0) in the LMZ of units broadened compared with that in the substrate. In general, the XRD line broadening is caused by two main effects: (1) refinement of the microstructure [26,27] and (2) the deformation (strain) broadening. The latter is often related to dislocations causing extended displacement fields in the crystal which indicates the formation of martensite in the LMZ [27,28]. 3.2. Microhardness The microhardness as a function of thermal fatigue cycle in the depth direction of sample cross-section is given in Fig. 8. Due to the formation of an inhomogeneous structure, as mentioned above,
2
where S is the area of the sample, here S is 0.8 10 mm (the hole area was ignored). Fig. 10(a and b) gives the D–N curve and L–N curve to show the variations of mean crack density and main crack length versus thermal cycles (N), respectively. It can be seen that both D and L sort No. 4 > No. 1 > No. 2 > No. 3, indicating that all repaired samples in the forms of strengthening spots, striations and lattices exhibit higher resistance to thermal cracks compared with the original fatigue sample. In the meanwhile, the sample with strengthening lattices has the best resistance. 3.4. Mechanisms of enhanced thermal fatigue resistance Thermal fatigue process includes three stages: nucleation, initial growth and proceeded crack growth [1,31]. Once a sample subjected to the thermo-mechanical loadings from steep temperature gradients in thickness, it may induce large thermal stresses on the surface layer [32]. When tensile stresses exceed the yield stress of material, nucleation and initial crack growth occurred. The crack growth is promoted by oxidation of the working surface and surface of the crack. Air oxygen in high temperature environment is causing the oxidation of the crack tip, which induces crack opening due to the brittleness of oxide layer [33]. Another factor facilitating the crack propagation is the thermal softening; in the contrary, the growth of thermal cracks is suppressed by better mechanical properties of surface material i.e. higher hardnesses [1]. On the other hand, it is clear that the crack tip is a main initiation source of microcracks. Many microcracks could connect gradually in the propagation and formed a large crack. Therefore the propagation of main crack was realized by bridge connections with microcracks
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Fig. 6. Cross-sectional morphologies of the unit: (a) optical micrograph of unit; (b) SEM microstructure in unit center; (c) SEM microstructure in HAZ; and (d) SEM microstructure in unit boundary.
Fig. 8. The distribution of microhardness in unit depth direction with increase of thermal cycles.
Fig. 7. XRD patterns of the unit and substrate.
[15]. This phenomenon was found in the original fatigue sample during thermal fatigue test as shown in Fig. 11. It can be seen that the original fatigue sample has poor resistance to thermal fatigue damages on which surface many random thermal cracks and oxidation corrosion occurred during thermal fatigue. The mechanical property of units is independent of laser remelting morphologies because it is an instinctive nature from the microstructure. As aforementioned, the laser remelting microstructure within the unit is composed of hard martensite, soft austenite and finer carbides. For the property of metal materials, the ‘‘hard’’ phase contributes to the strength and hardness, whereas the ‘‘soft’’ phase benefits ductility and toughness [34]. Thereby, the mixed microstructure of these two kinds of phases can provide
the unit with a proper combination of strength and toughness. Furthermore, this character was enhanced by the considerable grain refinement caused by rapid melting and solidification on the basis of Hall–Petch relationship. On the other hand, finer crystal grains have a beneficial effect on the improvement in material’s fatigue resistance [17,35–37]. Therefore, the units have a superior thermal fatigue strength compared to the base material. Though changes of microstructure within the unit were produced due to the thermal heat treatments of thermal fatigue testing and the microhardness also declined, it was still higher than that of the substrate that is, the units can still act as hard structure to restrain the propagation of cracks. Due to the intense influences of body structures on the surface properties in animals and plants (Fig. 1), it is essential to discuss the thermal fatigue resistance of samples with different surface remelting morphologies. Typical propagation patterns of thermal fatigue cracks on each sample surface during thermal fatigue test were shown in
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Fig. 9. The microstructural evolution in LMZ of units during thermal fatigue test: (a) the microstructure after 0 cycle (before the thermal fatigue test); (b) after 2000 cycles; (c) after 3000 cycles and (d) after 5000 cycles.
Fig. 10. The thermal fatigue resistance of samples: (a) the D–N curve and (b) the L–N curve.
Fig. 12. As is known to us, residual stress can be induced by rapid solidification of laser treatment. In general, tensile stresses promote the fatigue crack growth, in contrast, residual compressive stresses enhance the closure of the crack tip resulting in decreased fatigue crack growth [38]. The laser energy density used in this study was 3.23 J/mm2, at such low energy residual tensile stress formed in the units and residual compressive stress formed in adjoining material [5,39]. Consequently, the distribution of residual stresses on laser treated samples is in favor of decreasing fatigue crack growth on the original fatigue surface. However, the cracking of unit may occur due to the tensile stress in it. Therefore, it is necessary to check the surface bug of units before thermal fatigue test. Fig. 12 indicates that no cracks formed in the units even after thermal fatigue test. Original thermal cracks and oxidation corrosion pits in unit zones have been eliminated. The residual cracks on the samples are helpful for propagation energy release, thus reducing the initiation of cracks. Because in the process of thermal cycling, propagation energy was accumulated at the crack tip, due to the action of thermal stresses; with the growth of
cracks, the accumulated energy would be released [40]. On the other hand, due to stress concentration, the crack tip is a main initiation source of microcracks which can be promoted by oxidation. The local formation and dropping of the oxidation film result in oxidation corrosion pits which in return further facilitate crack initiation. It is conceivable that the samples with reduced initiation source of thermal cracks and oxidation corrosion pits possess improved thermal fatigue resistance. Fig. 12(a4, b4 and c4) indicates that few oxidation corrosion pits formed on units during thermal fatigue compared with that on original fatigue surface. Fig. 12(a1–a4) shows the hindering characteristic of a sample with spot units. When a crack encountered the unit, it would be deflected to round the strengthening spot. In other words, the spot units can only hamper the crack towards them. Fig. 12(b1–b4) shows the hindering characteristic of a sample with striation units. The cracks propagating to the strengthening striations were unable to round them but deflected with a larger angle. As similar with spot units, striation units have less effect on resisting the propagation of cracks parallel with them; these cracks can form a large
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Fig. 11. The typical propagation patterns of cracks on untreated samples: (a) precracks, (b) after 3000 cycles and (c) after 5000 cycles.
Fig. 12. Typical propagation patterns of cracks on laser treated samples after various thermal fatigue cycles: (a1) after repair, (a2) after 1000 times, (a3) after 3000 times and (a4) after 5000 times of No. 1 sample; (b1) after repair, (b2) after 1000 times, (b3) after 3000 times and (b4) after 5000 times of No. 2 sample; (c1) after repair, (c2) after 1000 times, (c3) after 3000 times and (c4) after 5000 times of No. 3 sample.
crack by connection with each other as shown by the rectangles. Fig. 12(c1–c4) shows the hindering characteristic of a sample with lattice units. The cracks were limited in closed strengthening
lattices, and that led to the circuit of crack in a lattice. Since the lattice units can block cracks in all directions, the growth of the crack was controlled more effectively. These results are agreement
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well with those of Tong et al. and Zhang et al. [14,17]. On the other hand, as shown in Fig. 12(a1, b1 and c1), the latticed sample possessed the largest remelting area, that is, the minimum amount of residual cracks and oxidation corrosion pits were left on original fatigue surface which reduced the initiation of thermal cracks. Moreover, thermal cracks and oxidation corrosion pits were surrounded by the lattice units; the connection of thermal cracks was blocked effectively. As a result, the samples with lattice units achieved the highest thermal fatigue resistance of all. 4. Conclusions Thermal fatigue cracks on a hot work die steel H13 were repaired by means of biomimetic laser surface remelting process in forms of spot, striation and lattice surface morphologies. Due to the ultrafine microstructure comprising of martensite, austenite and fine carbides formed in the rapid remelting process, units have a super thermal fatigue resistance. Moreover, the average microhardness of the unit is much higher than that of the original surface, even after 5000 thermal cycles. Thermal fatigue results show that both crack density and crack length are reduced due to the blocking effect of strengthening units. Strengthening spots and striations can only hamper the propagation of cracks towards them; nevertheless strengthening lattices are able to resist cracks in all directions. Furthermore, the lattice sample (No. 3) possesses the largest remelting area, which eliminated the largest number of cracks and oxidation corrosion pits, and blocked the connection of thermal cracks effectively. Hence, the partial laser surface remelting with lattice morphology is the most effective for repairing cracks and improving the thermal fatigue resistance of all. On the other hand, some cracks left on original surface are helpful for the propagation energy release, thus reducing the initiation of cracks. Acknowledgements This article was financially supported by Project 985-High Performance Materials of Jilin University, Project 985-Bionic Engineering Science and Technology Innovation and the National Natural Science Foundation of China (No. 51275200). References [1] Klobcˇar D, Kosec L, Kosec B, Tušek J. Thermo fatigue cracking of die casting dies. Eng Fail Anal 2012;20:43–53. [2] Murray JW, Clare AT. Repair of EDM induced surface cracks by pulsed electron beam irradiation. J Mater Process Technol 2012;212:2642–51. [3] Pleterski M, Tušek J, Kosec L, Muhicˇ M, Muhicˇ T. Laser repair welding of molds with various pulse shapes. Metalurgija 2010;49:41–4. [4] Nowotny S, Scharek S, Beyer E, Richter KH. Laser beam build-up welding: precision in repair, surface cladding, and direct 3D metal deposition. J Therm Spray Technol 2007;16:344–8. [5] Grum J, Slabe JM. Effect of laser-remelting of surface cracks on microstructure and residual stresses in 12Ni maraging steel. Appl Surf Sci 2006;252:4486–92. [6] Lee JC, Kang HJ, Chu WS, Ahn SH. Repair of damaged mold surface by coldspray method. CIRP Ann 2007;56:577–80. [7] Wang W, Pinkerton A, Wee L, Li L. Component repair using laser direct metal deposition. Springer; 2007. p. 345–50. [8] Song R, Hanaki S, Yamashita M, Uchida H. Reliability evaluation of a laser repaired die-casting die. Mater Sci Eng A 2008;483–484:343–5. [9] Cabeza M, Castro G, Merino P, Pena G, Román M. Laser surface melting: A suitable technique to repair damaged surfaces made in 14Ni (200 grade) maraging steel. Surf Coat Technol 2012;212:159–68. [10] Sun Y, Sunada H, Tsujii N. Crack repair of hot work tool steel by laser melt processing. ISIJ Int 2001;41:1006–9.
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