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Cracking behavior in the transitional region of laser-clad coatings on Al–Si alloy under multiple impact loading
Materials Characterization 49 (2003) 247 – 254
Cracking behavior in the transitional region of laser-clad coatings on Al–Si alloy under multiple impact loading A.H. Wanga,*, C.S. Xiea, W.Y. Wangb a State Key Laboratory of Plastic Forming Simulation and Die and Mould Technology, Faculty of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China b Institute of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, PR China
Received 4 June 2002; accepted 15 January 2003
Abstract An experiment was designed to investigate the cracking behavior in the transitional region of laser-clad iron – base alloy and bronze coatings on an Al – Si alloy under multiple impact loading. The concept of transitional crack ratio (TCR) was introduced to evaluate the crack resistance of the transitional region to multiple impact loading (simply called impact resistance). The results showed that the microstructure of the transitional regions and the impact resistances of both types of coating were significantly influenced by laser cladding process parameters (substrate temperature for the iron – base alloy and scanning velocity for the bronze). The laser-clad iron – base alloy coatings with the highest impact resistance were obtained at substrate temperatures between 275 and 320 C, while the equivalent laser-clad bronze coatings were produced using the scanning velocities in the range 10 – 12 mm s 1. The study also included analysis of the cracking mechanism in the transitional regions of the two kinds of coating. D 2003 Elsevier Science Inc. All rights reserved. Keywords: Laser cladding; Iron – base alloy coating; Bronze coating; Impact resistance; Transitional region
1. Introduction Aluminum alloys have been widely used in the manufacture of some important components in the aerospace and automobile industries because of their low density, high thermal expansion coefficient, large thermal conductivity, and excellent formability. However, their poor wear resistance is a major obstacle to a more widespread application of these alloys. An effective way of improving the wear resistance of * Corresponding author. Tel.: +86-27-8754-4364; fax: +86-27-8754-5438. E-mail address: [email protected] (A.H. Wang).
aluminum alloys has been found in laser cladding treatments [1 – 3]. Among the variety of metallic alloys investigated as coating materials, the Cu – base alloys have been identified as suitable for laser cladding onto aluminum alloys. This is because of their ability to control brittleness in the transitional region between the clad and the substrate, hitherto considered the main difficulty in laser cladding of aluminum alloys [4,5]. In earlier investigations, highpower lasers (with an output over 5 kW) had to be used to produce sound coatings because of the effect of powder-carrying gases and the low absorbability of aluminum substrates to the laser beam. For instance, Liu et al. [1,4] have reported that the laser cladding of an Al – Si alloy with Ni – Al bronze using a 10-kW
1044-5803/03/$ – see front matter D 2003 Elsevier Science Inc. All rights reserved. doi:10.1016/S1044-5803(03)00014-7
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continuous wave CO2 laser can significantly improve the wear resistance of the aluminum alloy substrate. Kawasaki et al. [2] and Tanaka et al. [3] have also investigated laser cladding a Cu – base alloy (Cu – 14.9Ni – 2.7Si – 1.4B) on an Al alloy by using a 5-kW CO2 laser. The present authors, in previous investigations [6 – 8], have used a lower-power CO2 laser with an output of only 2 kW to produce sound iron – base alloy and bronze coatings on the Al – Si alloy substrate, thereby improving the processing method and the development of the coating material. It is well known that aluminum displays a strong ability to react with most common elements (such as Fe, Ni, Co, Cr, Ti, and Cu) that are added into the alloyed layer. The result is the formation of a variety of intermetallic compounds and the creation of a rather complicated transitional region between the coating and the substrate [4,7,8]. In our previous reports [7,8], we examined the complex structures formed by laser-clad coatings of an iron – base alloy and bronze. Laser cladding using an iron – base alloy coating resulted in a nonuniform transitional region that could be identified as comprising an Al-rich zone, a Fe-rich zone, and a transitional zone, according to the variations in its composition. The phases found in these respective zones are a-Al + NiAl3, gFe + NiAl + Fe2Al5, and FeAl3 + Fe2Al5 + NiAl3. By comparison, the transitional region of the laser-clad bronze coating consists of only two layers. The layer close to the clad region has a polygonal crystalline structure while the layer close to the substrate region displays a needle-like structure. The polygonal crystalline layer and the needle-like structure layer contain, respectively, Cu3Al + Cu9Al4 and CuAl2 + a-Al. Furthermore, the bond strength of the two kinds of coatings was significantly influenced by their corresponding structures in the transitional regions [9,10]. In practical applications, the surfaces of aluminum components often suffer from impact loading (e.g., aluminum engine valve seats [2,3]) and the existence of brittle aluminides in the transitional region of laserclad coatings may diminish their properties. Unfortunately, to the best of the authors’ knowledge, there are few reports concerning this important problem. Therefore, the purpose of the present study was to investigate the cracking behavior in the transitional regions of laser-clad Fe – base alloy and Al – Fe bronze on the Al – Si alloy when multiple impact loading was applied to those brittle aluminide-containing regions.
ted as the substrate material. An iron – base alloy and an Al – Fe bronze with the compositions, respectively, of Fe – 21.1Cr – 12.5Ni – 4.3Si – 1.4Cu – 1.4Mo – 1.5B – 0.2C and Cu – 8.49Al – 4.73Fe – 0.3Si (wt.%) were used as the clad powders to produce the two types of coating. The laser cladding treatments were carried out using a 2-kW CW CO2 laser. The alloy powder was delivered into the laser-generated zone by a home-made powder feeding system, in which the powder was allowed to flow into the pool under the action of gravity and without the assistance of a carrying gas. The optimal laser parameters for the cladding of the iron – base alloy coating were: laser power P = 2.0 kW; scanning velocity Vs = 4 mm s 1; beam diameter d = 4.0 mm; and relative powder feeding mass G = Vp/Vs = 20 mg mm 1 (Vp is the powder feeding rate, mg s 1). Different substrate temperatures were generated by cooling or preheating substrate to 0, 200, or 400 C before the laser cladding treatment. The substrate temperature was determined using Pt – Rh thermocouples during the laser cladding treatment [6]. The samples of the laser-clad bronze coating were prepared using the following parameters: laser power P = 2 kW; beam diameter d = 3 mm; relative powder flow rate G = 20 mg mm 1; and scanning velocity ranging from 6 to 16 mm s 1. Laser-clad single tracks with dimensions of 2 mm width, 8 mm length, and 10 mm height were taken out by wire cutting in the directions shown in Fig. 1. The arch part of the clad was ground to a flat surface such that the coating thickness was about 1 mm. The final total sample thickness was measured using a vernier caliper. Cross-sections were taken of the samples, then polished and etched—the aim being to observe the cracking morphology during the impact test. The etching reagents were: for the iron – base coating, 70 vol.% HNO3 + 30 vol.% HF, and for the bronze coating, 50 vol.% HNO3 + 50 vol.% CH3COOH. The impact loading experiments were carried out using an apparatus shown in Fig. 2. The impact head,
2. Experimental procedure A eutectic Al – Si alloy with the composition of Al – 13.2Si – 2.1Mg – 1.3Ni – 1.3Cu (wt.%) was adop-
Fig. 1. Schematic drawing showing the sectioned portion of the samples used for the impact test.
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of impact cycles. Therefore, an impact energy of 2.646 J was selected for the investigation of the crack formation mechanism and the cracking law. The value of the transitional crack ratio (TCR) described in our previous report [6] represents the extent of cracking in the transitional region, and thus can be used to express the impact resistance of the transitional region during the impact test. The TCR is calculated from: TCR ¼
N i¼1 Li
W
100%
ð2Þ
Fig. 2. Experimental setup of the impact test.
made of quenched steel with a hardness of HRC 62, has a cross-sectional dimension of 8 8 mm fixed by the holder. The total weight of the impact head and the impact head holder is 10 kg. The head can be moved up to a specific height through a motor-driven mechanical system, after which it is dropped onto the flat-clad surface driven only by its own gravity. The frequency of impact cycles is 30 cycles/min. The value of impact energy is expressed by the following formula: E ¼ mgh ¼ 98h
ðif g is 9:8Þ
ð1Þ
where E is the impact energy [in J], h is the moving distance of the impact head m, and g is acceleration of gravity [m s 2]. Thus, different impact energy values could be obtained by adjusting the height from which the impact head could be dropped.
3. Results and discussion In order to investigate the crack formation mechanism and the crack propagation law in the transitional region when an impact load was applied to the coating surface, a reasonable impact energy should be chosen. After a sample had undergone only five impact cycles under an impact energy of 3.43 J (h = 35 mm), a significant reduction of the sample thickness (including the total thickness of the coating and the substrate) could be measured by the vernier caliper. This indicated that there was significant substrate distortion at this impact energy. A series of tests was then conducted involving a gradual reduction in the impact energy, down to 2.646 J (h = 27 mm). The results indicated that the substrate distortion phenomenon on the vernier-measurable scale could be avoided at this last energy level. In addition, observations of the cross-sections showed that cracks in the transitional region could be observed after the completion of several hundreds
where W is the width of the coating (2 mm), N is the number of cracks, and Li is the length of each separate crack projecting on the originally unmolten surface of the substrate since a majority of cracks found in the transitional region were nearly parallel to the originally unmolten surface of the substrate. Thus, the larger the TCR value, the more inferior is the resistance of the transitional region to cracking under the impact load. Our previous investigations [6 – 10] indicated that the microstructure and the bond strength in the transitional regions of the two kinds of coatings were chiefly influenced by the substrate temperature for the laser-clad iron – base alloy coating and by the laser scanning velocity for the laser-clad bronze coating. Therefore, the present investigation focused on the effects of these parameters on the resistance of the respective coatings to impact loading. 3.1. Impact resistance of laser-clad iron – base alloy coating Fig. 3 shows the influence of the substrate temperature on the impact resistance (as represented by the TCR) of the transitional region in the laser-clad iron – base alloy coating. The curves can be categorized into three types in accordance with the substrate temperature present during the laser cladding process. The transitional regions with the substrate temperature in the range 275 – 320 C exhibit the best impact resistance. Decreasing the substrate temperature to a range of about 200 – 220 C caused some reduction in the impact resistance. However, when the substrate temperature was increased to 365 – 375 C, the impact resistance deteriorated significantly. A similar correlation was also found in the results of the bond strength tests of the laser-clad iron – base alloy coating [9]. It was reported in our previous work on the laserclad iron – base alloy coatings that the interface between the transitional region and the substrate region was rough and uneven. Some portions were concave (i.e., the substrate extended into the trans-
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Fig. 3. The effect of substrate temperature on the impact resistance of the laser-clad iron – base alloy coating.
itional region), while in other areas, the interface was convex with the transitional region stretching into the substrate [7]. Observations of the crack morphology in the transitional region indicated that cracks preferred to form in the concave portions rather than at the convex portions, as shown in Fig. 4. It could be that, in the concave portions, the action of the impact load caused significant microdeformation of the substrate but less deformation of the transitional region because the microhardness transitional region was much harder than that of the substrate region (see Fig. 5). The irregular shape of the interface between the transitional region and the substrate region thus could have given rise to the development of stress concentrations. It is known that the transitional region consists of some brittle aluminides, such as FeAl3, Fe2Al5, and NiAl3, of which the Fe2Al5 phase has a very lower tensile strength [7,11]. Therefore, it is
reasonable to postulate that the formation of cracks took place preferably in the concave portions of the transitional region. For the laser-clad iron – base alloy coating with the substrate temperature as high as 365 C, cracking in the transitional region was more severe and even some networks of cracks were found in this region (see Fig. 6). When the substrate temperature was in a lower range (200 – 220 C), the transitional region contained some residual cracks after the laser cladding treatment because of the large stress induced by the higher cooling rate of the pool when compared to the situation at higher substrate temperatures. These original (residual) cracks became the initiation points for further cracking to develop under the impact loading but they were eliminated when the substrate temperature was increased to 275 C. Consequently, the impact resistance of the samples produced when the substrate temperature was
Fig. 4. Cracks formed during impact testing in the transitional region of the laser-clad iron – base alloy coating produced using a substrate temperature of 275 – 286 C.
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Fig. 5. Microhardness profile along the depth direction of the laser-clad iron – base alloy coating.
200 – 220 C was much lower than that of those produced with the substrate temperature in the range 275 – 320 C. As reported in our previous work [9], the substrate temperature has a significant influence on the microstructure of the transitional region. Since two elements, Fe and Al, have significant differences in their chemical and physical properties, the transitional region of the laser-clad iron – base alloy coating displayed an inhomogeneous composition and an inhomogeneous microstructure consisting of an Alrich zone, a Fe-rich zone, and the transitional zone between them. It has been noted above (from Ref. [9]) that the transitional zone contained some brittle
and low-strength aluminides such as FeAl3 and Fe2Al5. Therefore, cracks can still initiate in the transitional zone even though residual cracking can be eliminated at substrate temperatures greater than 275 C. However, both the Al-rich zone and the Ferich zones provide a degree of resistance to crack propagation, the former because it contains a ductile phase (a-Al) and the latter because it contains a tough phase (g-Fe with a microcrystalline structure). Therefore, an inhomogeneous transitional structure should make a positive contribution to the impact resistance. Furthermore, an increase in the substrate temperature could increase the temperature of the laser-generated melt pool and reduce the cooling rate of the pool
Fig. 6. Network cracks formed during impact testing in the transitional region of the laser-clad iron – base alloy coating produced with a substrate temperature of 365 – 375 C.
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during the laser cladding process, prolonging the interaction time between the clad region and the substrate region when they interdiffuse in the liquid state. Therefore, it is to be expected that the higher the substrate temperature is, the more homogenous the composition and the microstructure in the transitional region will be. When the substrate temperature exceeded 325 C, a homogeneous transitional region was generated (see Ref. [9]) and a larger volume fraction of aluminides formed in the transitional region. Consequently, brittle aluminides, especially the lower-strength phases, such as FeAl3 and Fe2Al5 provided more locations for the crack initiation under these circumstances. As a result, the samples produced at substrate temperatures greater than 365 C demonstrated an extremely low impact resistance. 3.2. Impact resistance of laser-clad bronze coating Fig. 7 shows the influence of scanning velocity on the impact resistance of the transitional region of the laser-clad bronze coating. The samples produced using scanning velocities of 10 and 12 mm s 1 exhibited the most impact resistance at the highest numbers of cycles. The impact resistances displayed by samples produced using scanning velocities of 6 and 8 mm s 1 were similar to these at low cycles but significantly worse at high cycles. Those produced with a scanning velocity of 16 mm s 1 showed the worst resistance of all at both low and high cycles. Fig. 8 shows the typical crack morphology in the transitional region of laser-clad bronze coating after the impact test. It was evident that the main cracks
were chiefly distributed in the needle-like structure zone and were parallel to the original substrate surface, although some subcracks were perpendicular to it. Our previous investigations [8,9] showed that the transitional region consisted of two layers. One layer, close to the clad region, had a polygonal crystalline structure, while the other layer, which lays close to the substrate region, displayed a needle-like structure. X-ray diffraction analyses showed that the main phases in the polygonal crystalline layer were Cu3Al + Cu9Al4, while those in the needle-like structure layer were CuAl2 + a-Al. Among the three aluminides existing in the transitional region, the Cu3Al and Cu9Al4 phases have a cubic lattice structure but the CuAl2 phase has a tetragonal lattice structure, which means that this phase possesses a lower symmetry [12]. The CuAl2 phase also has the lowest strength (about 5 MPa) and the weakest intercrystalline bond among the three aluminides [11]. Therefore, it can be understood that cracks preferred to originate and to propagate in the CuAl2-rich zone under the impact loading. The interaction time (dwell time) between the laser beam and the irradiated substrate can be expressed as d/Vs, and the heat penetration depth into the substrate is proportional to the dwell time. Thus, an increase in Vs, the scanning velocity, can reduce the melted depth of the substrate during the laser cladding process and reduce the depth of the transitional region, as shown in Table 1. During the rapid solidification process, the transitional region comprising a-Al, Cu9Al4, Cu3Al, and CuAl2 was generated
Fig. 7. The effect of the scanning velocity on the impact resistance of the laser-clad bonze coating.
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Fig. 8. Typical crack morphology observed in the transitional region of the laser-clad bronze coating during impact testing.
first. The phases were determined by the composition of the melt, of which the Cu-rich phases (such as Cu9Al4 and Cu3Al) were found near the clad region while Cu-deficient phases (such as a-Al and CuAl2) were generated adjacent to the substrate region. As the liquid/solid interface propagated forward, the release of the latent heat of solidification and the heat of the exothermic reactions caused remelting of both the substrate and the low-melting-point phases in the transitional region (such as a-Al and CuAl2). The liquid interdiffusion between the remelted substrate and the remelted transitional region, and the corresponding solidification, resulted in the formation of a new transitional region (i.e., transitional region II). At the same time, the portion of the transitional region close to the clad region remained unmelted due to its high melting point (Cu9Al4—1030 C, Cu3Al—1048 C), corresponding to the high Cu content portion (called transitional region I). From the phase diagram of the Al – Cu alloy system, we know that Cu9Al4 and Cu3Al form in a very narrow compositional range. Therefore, the thickening of the transitional region that resulted from the decrease of scanning velocity only increased the thickness of the needle-like layer, which could result in an increase in the volume fraction of the needle-like CuAl2 phase. Consequently, the impact resistance of laser-clad bronze coatings is significantly increased when the scanning velocity is raised from 6 to 12 mm s 1. Table 1 Variation with scanning velocity of the transitional region depth of the laser-clad bronze coating Scanning velocity (mm s 1) Average transitional region depth (mm)
6 0.6
10 0.42
14 0.23
However, a further increase in scanning velocity, to 16 mm s 1, could induce some nonbonded areas due to insufficient laser energy, thus apparently reducing the impact resistance.
4. Conclusions The cracking resistances of the transitional regions in the laser-clad iron – base alloy and bronze coatings under multiple impact loading have been successfully investigated through a specially designed method. The laser-clad iron – base coating with the best impact resistance was produced with the substrate temperature in the range 275 – 320 C. Substrate temperatures in the ranges 200 – 220 and 365 – 375 C resulted in significant deterioration in the impact resistance. The lower impact resistance of the coatings obtained with the substrate temperature in the range 200 – 220 C was caused by the existence of residual stresses in the transitional region, which act as crack initiation points during the impact test. Cracking could also initiate in the transitional zone when the transitional cracks are eliminated at the substrate temperature more than 275 C. The highest impact resistance may result from the benefits of an inhomogeneous transitional structure. Raising the substrate temperature up to 365 C could cause the homogenization of the transitional region, resulting in a larger fraction of the transitional zone and presenting an extremely low impact resistance. The laser-clad bronze coating produced using a scanning velocity range of 10 – 12 mm s 1 showed the best impact resistance. Reducing the scanning velocity range down to 6 – 8 mm s 1 or increasing it to more than 16 mm s 1 caused the impact resistance to drop. Cracks initiated and propagated in the
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needle-like layer (i.e., CuAl2-rich layer) because of its low strength. An increase in scanning velocity from 6 to 12 mm s 1 reduced the depth of the needle-like layer gradually and thus made a positive contribution to the impact resistance. The lowest impact resistance of all, produced at the scanning velocity of more than 16 mm s 1, was attributed to insufficient diffusion between the transitional region and the substrate region.
Acknowledgements The authors would like to thank the Open Research Projects for the financial support provided, supported by the Fund of the Hubei Province Key Laboratory of Ceramic and Refractories, Wuhan University of Science and Technology.
References [1] Liu Y, Koch J, Mazumder J, Shibata K. Microstructure and properties of laser-clad Ni alloy FP-5 on Al alloy AA333. Metall Mater Trans, B, Proc Metall Mater Proc Sci 1994;25B:425 – 34. [2] Kawasaki M, Takase K, Kato S, Nakagawa M, Mori K, Nemoto M, et al. Development of engine valve seats directly deposited onto aluminum cylinder head by laser cladding process. Proceedings of the 1992 SAE International Congress and Exposition. SAE technical paper series, vol. 920571. Detroit (MI): Society of Automotive Engineers; 1992, February: 1 – 15.
[3] Tanaka K, Saito T, Shimura Y, Mori K, Kawasaki M, Koyama M, et al. New copper based composite for engine valve seat directly deposited onto aluminum alloy by laser cladding process. J Jpn Inst Metals 1993;57:1114 – 22. [4] Liu Y, Mazumder J, Shibata K. Microstructural study of the interface in laser-clad Ni – Al bronze on alloy AA333 and its relation to cracking. Metall Mater Trans, A, Phys Metall Mater Sci 1995;26A:1519 – 33. [5] Poire L, Blank E. Laser cladding of copper base alloys onto Al Si7 Mg0.3. J Phys IV 1994;4:C4-81 – 4. [6] Wang A, Fan C, Xie C, Huang W, Cui K. Laser cladding of iron base alloy on Al – Si alloy and its relation to cracking at the interface. J Mater Eng Perform 1996; 5:775 – 83. [7] Wang AH, Xie CS, Nie JH. Microstructure characteristics of iron based alloy laser clad on Al – Si alloy. Mater Sci Technol 1999;15:957 – 64. [8] Wang AH, Xie CS. Microstructural study of laser-clad Fe – Al bronze on Al – Si alloy. J Mater Sci 2001;36: 1975 – 9. [9] Wang AH, Xie CS, Nie JH. Bond strength of a laserclad iron – base alloy coating on Al – Si alloy substrate and its fracture behavior. Mater Charact 2001;47(1): 1 – 7. [10] Wang AH, Xie CS, Nie JH. Fractural behavior in the transitional region of laser-clad Al – Fe bronze on Al – Si alloy under tensile loading. Mater Lett 2002;56: 822 – 6. [11] Babob BP. Welding of aluminum and its alloys with other metals. Beijing: Yuhang Press; 1990. p. 20. [12] Hu JD, Li YL, Lian JS. Twin structure in laser-melted Al – Si alloy containing rare earths. J Mater Sci Lett 1993;12:578 – 80.
Cracking behavior in the transitional region of laser-clad coatings on Al–Si alloy under multiple impact loading A.H. Wanga,*, C.S. Xiea, W.Y. Wangb a State Key Laboratory of Plastic Forming Simulation and Die and Mould Technology, Faculty of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China b Institute of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, PR China
Received 4 June 2002; accepted 15 January 2003
Abstract An experiment was designed to investigate the cracking behavior in the transitional region of laser-clad iron – base alloy and bronze coatings on an Al – Si alloy under multiple impact loading. The concept of transitional crack ratio (TCR) was introduced to evaluate the crack resistance of the transitional region to multiple impact loading (simply called impact resistance). The results showed that the microstructure of the transitional regions and the impact resistances of both types of coating were significantly influenced by laser cladding process parameters (substrate temperature for the iron – base alloy and scanning velocity for the bronze). The laser-clad iron – base alloy coatings with the highest impact resistance were obtained at substrate temperatures between 275 and 320 C, while the equivalent laser-clad bronze coatings were produced using the scanning velocities in the range 10 – 12 mm s 1. The study also included analysis of the cracking mechanism in the transitional regions of the two kinds of coating. D 2003 Elsevier Science Inc. All rights reserved. Keywords: Laser cladding; Iron – base alloy coating; Bronze coating; Impact resistance; Transitional region
1. Introduction Aluminum alloys have been widely used in the manufacture of some important components in the aerospace and automobile industries because of their low density, high thermal expansion coefficient, large thermal conductivity, and excellent formability. However, their poor wear resistance is a major obstacle to a more widespread application of these alloys. An effective way of improving the wear resistance of * Corresponding author. Tel.: +86-27-8754-4364; fax: +86-27-8754-5438. E-mail address: [email protected] (A.H. Wang).
aluminum alloys has been found in laser cladding treatments [1 – 3]. Among the variety of metallic alloys investigated as coating materials, the Cu – base alloys have been identified as suitable for laser cladding onto aluminum alloys. This is because of their ability to control brittleness in the transitional region between the clad and the substrate, hitherto considered the main difficulty in laser cladding of aluminum alloys [4,5]. In earlier investigations, highpower lasers (with an output over 5 kW) had to be used to produce sound coatings because of the effect of powder-carrying gases and the low absorbability of aluminum substrates to the laser beam. For instance, Liu et al. [1,4] have reported that the laser cladding of an Al – Si alloy with Ni – Al bronze using a 10-kW
1044-5803/03/$ – see front matter D 2003 Elsevier Science Inc. All rights reserved. doi:10.1016/S1044-5803(03)00014-7
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A.H. Wang et al. / Materials Characterization 49 (2003) 247–254
continuous wave CO2 laser can significantly improve the wear resistance of the aluminum alloy substrate. Kawasaki et al. [2] and Tanaka et al. [3] have also investigated laser cladding a Cu – base alloy (Cu – 14.9Ni – 2.7Si – 1.4B) on an Al alloy by using a 5-kW CO2 laser. The present authors, in previous investigations [6 – 8], have used a lower-power CO2 laser with an output of only 2 kW to produce sound iron – base alloy and bronze coatings on the Al – Si alloy substrate, thereby improving the processing method and the development of the coating material. It is well known that aluminum displays a strong ability to react with most common elements (such as Fe, Ni, Co, Cr, Ti, and Cu) that are added into the alloyed layer. The result is the formation of a variety of intermetallic compounds and the creation of a rather complicated transitional region between the coating and the substrate [4,7,8]. In our previous reports [7,8], we examined the complex structures formed by laser-clad coatings of an iron – base alloy and bronze. Laser cladding using an iron – base alloy coating resulted in a nonuniform transitional region that could be identified as comprising an Al-rich zone, a Fe-rich zone, and a transitional zone, according to the variations in its composition. The phases found in these respective zones are a-Al + NiAl3, gFe + NiAl + Fe2Al5, and FeAl3 + Fe2Al5 + NiAl3. By comparison, the transitional region of the laser-clad bronze coating consists of only two layers. The layer close to the clad region has a polygonal crystalline structure while the layer close to the substrate region displays a needle-like structure. The polygonal crystalline layer and the needle-like structure layer contain, respectively, Cu3Al + Cu9Al4 and CuAl2 + a-Al. Furthermore, the bond strength of the two kinds of coatings was significantly influenced by their corresponding structures in the transitional regions [9,10]. In practical applications, the surfaces of aluminum components often suffer from impact loading (e.g., aluminum engine valve seats [2,3]) and the existence of brittle aluminides in the transitional region of laserclad coatings may diminish their properties. Unfortunately, to the best of the authors’ knowledge, there are few reports concerning this important problem. Therefore, the purpose of the present study was to investigate the cracking behavior in the transitional regions of laser-clad Fe – base alloy and Al – Fe bronze on the Al – Si alloy when multiple impact loading was applied to those brittle aluminide-containing regions.
ted as the substrate material. An iron – base alloy and an Al – Fe bronze with the compositions, respectively, of Fe – 21.1Cr – 12.5Ni – 4.3Si – 1.4Cu – 1.4Mo – 1.5B – 0.2C and Cu – 8.49Al – 4.73Fe – 0.3Si (wt.%) were used as the clad powders to produce the two types of coating. The laser cladding treatments were carried out using a 2-kW CW CO2 laser. The alloy powder was delivered into the laser-generated zone by a home-made powder feeding system, in which the powder was allowed to flow into the pool under the action of gravity and without the assistance of a carrying gas. The optimal laser parameters for the cladding of the iron – base alloy coating were: laser power P = 2.0 kW; scanning velocity Vs = 4 mm s 1; beam diameter d = 4.0 mm; and relative powder feeding mass G = Vp/Vs = 20 mg mm 1 (Vp is the powder feeding rate, mg s 1). Different substrate temperatures were generated by cooling or preheating substrate to 0, 200, or 400 C before the laser cladding treatment. The substrate temperature was determined using Pt – Rh thermocouples during the laser cladding treatment [6]. The samples of the laser-clad bronze coating were prepared using the following parameters: laser power P = 2 kW; beam diameter d = 3 mm; relative powder flow rate G = 20 mg mm 1; and scanning velocity ranging from 6 to 16 mm s 1. Laser-clad single tracks with dimensions of 2 mm width, 8 mm length, and 10 mm height were taken out by wire cutting in the directions shown in Fig. 1. The arch part of the clad was ground to a flat surface such that the coating thickness was about 1 mm. The final total sample thickness was measured using a vernier caliper. Cross-sections were taken of the samples, then polished and etched—the aim being to observe the cracking morphology during the impact test. The etching reagents were: for the iron – base coating, 70 vol.% HNO3 + 30 vol.% HF, and for the bronze coating, 50 vol.% HNO3 + 50 vol.% CH3COOH. The impact loading experiments were carried out using an apparatus shown in Fig. 2. The impact head,
2. Experimental procedure A eutectic Al – Si alloy with the composition of Al – 13.2Si – 2.1Mg – 1.3Ni – 1.3Cu (wt.%) was adop-
Fig. 1. Schematic drawing showing the sectioned portion of the samples used for the impact test.
A.H. Wang et al. / Materials Characterization 49 (2003) 247–254
249
of impact cycles. Therefore, an impact energy of 2.646 J was selected for the investigation of the crack formation mechanism and the cracking law. The value of the transitional crack ratio (TCR) described in our previous report [6] represents the extent of cracking in the transitional region, and thus can be used to express the impact resistance of the transitional region during the impact test. The TCR is calculated from: TCR ¼
N i¼1 Li
W
100%
ð2Þ
Fig. 2. Experimental setup of the impact test.
made of quenched steel with a hardness of HRC 62, has a cross-sectional dimension of 8 8 mm fixed by the holder. The total weight of the impact head and the impact head holder is 10 kg. The head can be moved up to a specific height through a motor-driven mechanical system, after which it is dropped onto the flat-clad surface driven only by its own gravity. The frequency of impact cycles is 30 cycles/min. The value of impact energy is expressed by the following formula: E ¼ mgh ¼ 98h
ðif g is 9:8Þ
ð1Þ
where E is the impact energy [in J], h is the moving distance of the impact head m, and g is acceleration of gravity [m s 2]. Thus, different impact energy values could be obtained by adjusting the height from which the impact head could be dropped.
3. Results and discussion In order to investigate the crack formation mechanism and the crack propagation law in the transitional region when an impact load was applied to the coating surface, a reasonable impact energy should be chosen. After a sample had undergone only five impact cycles under an impact energy of 3.43 J (h = 35 mm), a significant reduction of the sample thickness (including the total thickness of the coating and the substrate) could be measured by the vernier caliper. This indicated that there was significant substrate distortion at this impact energy. A series of tests was then conducted involving a gradual reduction in the impact energy, down to 2.646 J (h = 27 mm). The results indicated that the substrate distortion phenomenon on the vernier-measurable scale could be avoided at this last energy level. In addition, observations of the cross-sections showed that cracks in the transitional region could be observed after the completion of several hundreds
where W is the width of the coating (2 mm), N is the number of cracks, and Li is the length of each separate crack projecting on the originally unmolten surface of the substrate since a majority of cracks found in the transitional region were nearly parallel to the originally unmolten surface of the substrate. Thus, the larger the TCR value, the more inferior is the resistance of the transitional region to cracking under the impact load. Our previous investigations [6 – 10] indicated that the microstructure and the bond strength in the transitional regions of the two kinds of coatings were chiefly influenced by the substrate temperature for the laser-clad iron – base alloy coating and by the laser scanning velocity for the laser-clad bronze coating. Therefore, the present investigation focused on the effects of these parameters on the resistance of the respective coatings to impact loading. 3.1. Impact resistance of laser-clad iron – base alloy coating Fig. 3 shows the influence of the substrate temperature on the impact resistance (as represented by the TCR) of the transitional region in the laser-clad iron – base alloy coating. The curves can be categorized into three types in accordance with the substrate temperature present during the laser cladding process. The transitional regions with the substrate temperature in the range 275 – 320 C exhibit the best impact resistance. Decreasing the substrate temperature to a range of about 200 – 220 C caused some reduction in the impact resistance. However, when the substrate temperature was increased to 365 – 375 C, the impact resistance deteriorated significantly. A similar correlation was also found in the results of the bond strength tests of the laser-clad iron – base alloy coating [9]. It was reported in our previous work on the laserclad iron – base alloy coatings that the interface between the transitional region and the substrate region was rough and uneven. Some portions were concave (i.e., the substrate extended into the trans-
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Fig. 3. The effect of substrate temperature on the impact resistance of the laser-clad iron – base alloy coating.
itional region), while in other areas, the interface was convex with the transitional region stretching into the substrate [7]. Observations of the crack morphology in the transitional region indicated that cracks preferred to form in the concave portions rather than at the convex portions, as shown in Fig. 4. It could be that, in the concave portions, the action of the impact load caused significant microdeformation of the substrate but less deformation of the transitional region because the microhardness transitional region was much harder than that of the substrate region (see Fig. 5). The irregular shape of the interface between the transitional region and the substrate region thus could have given rise to the development of stress concentrations. It is known that the transitional region consists of some brittle aluminides, such as FeAl3, Fe2Al5, and NiAl3, of which the Fe2Al5 phase has a very lower tensile strength [7,11]. Therefore, it is
reasonable to postulate that the formation of cracks took place preferably in the concave portions of the transitional region. For the laser-clad iron – base alloy coating with the substrate temperature as high as 365 C, cracking in the transitional region was more severe and even some networks of cracks were found in this region (see Fig. 6). When the substrate temperature was in a lower range (200 – 220 C), the transitional region contained some residual cracks after the laser cladding treatment because of the large stress induced by the higher cooling rate of the pool when compared to the situation at higher substrate temperatures. These original (residual) cracks became the initiation points for further cracking to develop under the impact loading but they were eliminated when the substrate temperature was increased to 275 C. Consequently, the impact resistance of the samples produced when the substrate temperature was
Fig. 4. Cracks formed during impact testing in the transitional region of the laser-clad iron – base alloy coating produced using a substrate temperature of 275 – 286 C.
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Fig. 5. Microhardness profile along the depth direction of the laser-clad iron – base alloy coating.
200 – 220 C was much lower than that of those produced with the substrate temperature in the range 275 – 320 C. As reported in our previous work [9], the substrate temperature has a significant influence on the microstructure of the transitional region. Since two elements, Fe and Al, have significant differences in their chemical and physical properties, the transitional region of the laser-clad iron – base alloy coating displayed an inhomogeneous composition and an inhomogeneous microstructure consisting of an Alrich zone, a Fe-rich zone, and the transitional zone between them. It has been noted above (from Ref. [9]) that the transitional zone contained some brittle
and low-strength aluminides such as FeAl3 and Fe2Al5. Therefore, cracks can still initiate in the transitional zone even though residual cracking can be eliminated at substrate temperatures greater than 275 C. However, both the Al-rich zone and the Ferich zones provide a degree of resistance to crack propagation, the former because it contains a ductile phase (a-Al) and the latter because it contains a tough phase (g-Fe with a microcrystalline structure). Therefore, an inhomogeneous transitional structure should make a positive contribution to the impact resistance. Furthermore, an increase in the substrate temperature could increase the temperature of the laser-generated melt pool and reduce the cooling rate of the pool
Fig. 6. Network cracks formed during impact testing in the transitional region of the laser-clad iron – base alloy coating produced with a substrate temperature of 365 – 375 C.
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during the laser cladding process, prolonging the interaction time between the clad region and the substrate region when they interdiffuse in the liquid state. Therefore, it is to be expected that the higher the substrate temperature is, the more homogenous the composition and the microstructure in the transitional region will be. When the substrate temperature exceeded 325 C, a homogeneous transitional region was generated (see Ref. [9]) and a larger volume fraction of aluminides formed in the transitional region. Consequently, brittle aluminides, especially the lower-strength phases, such as FeAl3 and Fe2Al5 provided more locations for the crack initiation under these circumstances. As a result, the samples produced at substrate temperatures greater than 365 C demonstrated an extremely low impact resistance. 3.2. Impact resistance of laser-clad bronze coating Fig. 7 shows the influence of scanning velocity on the impact resistance of the transitional region of the laser-clad bronze coating. The samples produced using scanning velocities of 10 and 12 mm s 1 exhibited the most impact resistance at the highest numbers of cycles. The impact resistances displayed by samples produced using scanning velocities of 6 and 8 mm s 1 were similar to these at low cycles but significantly worse at high cycles. Those produced with a scanning velocity of 16 mm s 1 showed the worst resistance of all at both low and high cycles. Fig. 8 shows the typical crack morphology in the transitional region of laser-clad bronze coating after the impact test. It was evident that the main cracks
were chiefly distributed in the needle-like structure zone and were parallel to the original substrate surface, although some subcracks were perpendicular to it. Our previous investigations [8,9] showed that the transitional region consisted of two layers. One layer, close to the clad region, had a polygonal crystalline structure, while the other layer, which lays close to the substrate region, displayed a needle-like structure. X-ray diffraction analyses showed that the main phases in the polygonal crystalline layer were Cu3Al + Cu9Al4, while those in the needle-like structure layer were CuAl2 + a-Al. Among the three aluminides existing in the transitional region, the Cu3Al and Cu9Al4 phases have a cubic lattice structure but the CuAl2 phase has a tetragonal lattice structure, which means that this phase possesses a lower symmetry [12]. The CuAl2 phase also has the lowest strength (about 5 MPa) and the weakest intercrystalline bond among the three aluminides [11]. Therefore, it can be understood that cracks preferred to originate and to propagate in the CuAl2-rich zone under the impact loading. The interaction time (dwell time) between the laser beam and the irradiated substrate can be expressed as d/Vs, and the heat penetration depth into the substrate is proportional to the dwell time. Thus, an increase in Vs, the scanning velocity, can reduce the melted depth of the substrate during the laser cladding process and reduce the depth of the transitional region, as shown in Table 1. During the rapid solidification process, the transitional region comprising a-Al, Cu9Al4, Cu3Al, and CuAl2 was generated
Fig. 7. The effect of the scanning velocity on the impact resistance of the laser-clad bonze coating.
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Fig. 8. Typical crack morphology observed in the transitional region of the laser-clad bronze coating during impact testing.
first. The phases were determined by the composition of the melt, of which the Cu-rich phases (such as Cu9Al4 and Cu3Al) were found near the clad region while Cu-deficient phases (such as a-Al and CuAl2) were generated adjacent to the substrate region. As the liquid/solid interface propagated forward, the release of the latent heat of solidification and the heat of the exothermic reactions caused remelting of both the substrate and the low-melting-point phases in the transitional region (such as a-Al and CuAl2). The liquid interdiffusion between the remelted substrate and the remelted transitional region, and the corresponding solidification, resulted in the formation of a new transitional region (i.e., transitional region II). At the same time, the portion of the transitional region close to the clad region remained unmelted due to its high melting point (Cu9Al4—1030 C, Cu3Al—1048 C), corresponding to the high Cu content portion (called transitional region I). From the phase diagram of the Al – Cu alloy system, we know that Cu9Al4 and Cu3Al form in a very narrow compositional range. Therefore, the thickening of the transitional region that resulted from the decrease of scanning velocity only increased the thickness of the needle-like layer, which could result in an increase in the volume fraction of the needle-like CuAl2 phase. Consequently, the impact resistance of laser-clad bronze coatings is significantly increased when the scanning velocity is raised from 6 to 12 mm s 1. Table 1 Variation with scanning velocity of the transitional region depth of the laser-clad bronze coating Scanning velocity (mm s 1) Average transitional region depth (mm)
6 0.6
10 0.42
14 0.23
However, a further increase in scanning velocity, to 16 mm s 1, could induce some nonbonded areas due to insufficient laser energy, thus apparently reducing the impact resistance.
4. Conclusions The cracking resistances of the transitional regions in the laser-clad iron – base alloy and bronze coatings under multiple impact loading have been successfully investigated through a specially designed method. The laser-clad iron – base coating with the best impact resistance was produced with the substrate temperature in the range 275 – 320 C. Substrate temperatures in the ranges 200 – 220 and 365 – 375 C resulted in significant deterioration in the impact resistance. The lower impact resistance of the coatings obtained with the substrate temperature in the range 200 – 220 C was caused by the existence of residual stresses in the transitional region, which act as crack initiation points during the impact test. Cracking could also initiate in the transitional zone when the transitional cracks are eliminated at the substrate temperature more than 275 C. The highest impact resistance may result from the benefits of an inhomogeneous transitional structure. Raising the substrate temperature up to 365 C could cause the homogenization of the transitional region, resulting in a larger fraction of the transitional zone and presenting an extremely low impact resistance. The laser-clad bronze coating produced using a scanning velocity range of 10 – 12 mm s 1 showed the best impact resistance. Reducing the scanning velocity range down to 6 – 8 mm s 1 or increasing it to more than 16 mm s 1 caused the impact resistance to drop. Cracks initiated and propagated in the
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needle-like layer (i.e., CuAl2-rich layer) because of its low strength. An increase in scanning velocity from 6 to 12 mm s 1 reduced the depth of the needle-like layer gradually and thus made a positive contribution to the impact resistance. The lowest impact resistance of all, produced at the scanning velocity of more than 16 mm s 1, was attributed to insufficient diffusion between the transitional region and the substrate region.
Acknowledgements The authors would like to thank the Open Research Projects for the financial support provided, supported by the Fund of the Hubei Province Key Laboratory of Ceramic and Refractories, Wuhan University of Science and Technology.
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