- Email: [email protected]
substrate laser clad
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 549–555
journal homepage: www.elsevier.com/locate/jmatprotec
Effect of Y2 O3 addition on microstructure of Ni-based alloy + Y2 O3 /substrate laser clad Peiquan Xu a,∗ , Xinhua Tang b , Shun Yao b , Jianping He a , Guoxiang Xu a a b
College of Materials Engineering, Shanghai University of Engineering Science, 201620 Shanghai, China School of Materials Science and Engineering, Shanghai Jiaotong University, 200030 Shanghai, China
a r t i c l e
i n f o
Article history:
a b s t r a c t To improve hardness and corrosion resistance of aluminum substrate, using laser cladding
Received 29 June 2007
technique, metal composite with Ni-based alloy, refining and dispersion strengthening Y2 O3 ,
Received in revised form
particle hardening W, Cr was obtained. The microstructure and morphology, phase identi-
16 December 2007
fication, element diffusion and composition analysis of the Ni-based alloy + Y2 O3 /substrate
Accepted 16 January 2008
laser clad and laser clad/aluminum substrate interface were examined using scanning electron microscopy (SEM), Electron Probe Microanalyzer (EPMA), energy-dispersive spectrometer (EDS) analysis. The corrosion resistance was investigated also. The results showed,
Keywords:
with the addition of Y2 O3 , more fine microstructures consisted of isometric crystal, white
Laser clad
acicular crystal and fringe crystal; primary phases were mainly Ni3 Al, NiAl, NiAl3 , W, ␣-Al
Yttrium oxide
and Crx C. The corrosion rate of aluminum substrate was nearly two times more than that
Ni-based alloy + Y2 O3
of laser clad metals with addition of Y2 O3 . © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
Strengthening phase structures were often introduced into alloys for surface modification purposes by different surface modification technique. An obvious example is the extremely fine second-phase particles used for precipitation and dispersion hardening (Qi et al., 2006; Manna et al., 2006). While using spraying technique, thicker coating could be obtained, but the bonding between coating and substrate belong to mechanical bonding with weak and such drawbacks as gas porosity. Therefore, it is of interest to develop a surface modification technique, e.g., by using laser cladding with a new material deposited metal (DM) system design that provides good combination of desirable mechanical properties, in particular, both high strength and good corrosion resistance properties. Laser cladding technique is steadily becoming a tool for synthesis of materials. Recently, a large number of new clad materials and composites have been developed (Navas et
∗
Corresponding author. Tel.: +86 21 67791202. E-mail address: [email protected] (P. Xu). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.01.026
al., 2006; Duraiselvam et al., 2005; Shangping Li et al., 2007; Xu et al., 2006a, 2006b). Clad material included ceramic phases, strengthening wearing phases or special crystalline materials, e.g., Al2 O3 , NiTi, in-situ synthesis TiC coating (Liu et al., 2006; Yue et al., 2006), WC/Ni coating (Ibrahim et al., 1991; Lloyd, 1994), quasicrystal and amorphous mixtures (Lavernia et al., 1995; Taya and Lucay, 1991; Lebrat et al., 1992; Munir and Umberto, 1989; Lalas et al., 2007; Salehi and Brandt, 2006), in which the hardening phases itself has a coarse scale structure. While such materials offer high strength, they usually have to suffer from porosity and contamination problems in laser cladding process. To achieve high strength clad with high corrosion resistance and high wear-resisting property, we desire a Ni-based alloys + Y2 O3 deposited metal that can be obtained on the surface of aluminum alloys by using laser cladding technique, properly choosing the deposited metals’ composition and controlling the laser cladding parameters.Such the
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 549–555
solidification products will produce the desired microstructure as discussed in the preceding paragraph. The multiple elements added also render the crystal growth more sluggish, favoring the refinement of coarse scales to fine scales.
HNO3 , 95% H2 O and Reagent (20% KOH + 20% K3 [Fe(CN)6 ] + 60% H2 O: vol%) at room temperature.
3. 2.
6061 aluminum alloy (Al–Mg–Si) was selected as base materials, the details were: dimension: 60 mm × 50 mm × 30 mm; surface roughness: 0.2 m, and its composite was shown in Table 1. The deposited alloys were mixture of Ni-based alloys and yttrium oxide after milling. The purity was 99.99%. Molecular formula: Y2 O3 ; molecular weight: 225.82.
2.1. Preparation method of Ni-based alloy/Y2 O3 deposited metal The grain size of mixture powers was 30 m. After milling, four pressed compacts were obtained with 1 mm thickness and dried in 120 ◦ C temperature. The pressed compacts were preset onto the surface of aluminum, and Ni-based alloy-Y2 O3 /aluminum alloy laser clad could be obtained by laser cladding using continuous wave 3 kW Nd:YAG laser. Parameters for laser clad and serial number of specimens were illustrated in Table 2. The overlap ratio was 30%, the welding speed was 10 mm s−1 , and the gas-flow rate is 1.4 L min−1 .
2.2.
Analyzing position and testing methods
Specimens for aluminum alloy matrix composite were obtained by laser cladding of the deposited metals (Ni-based alloys + Y2 O3 ) on 6061 aluminum alloy in argon atmosphere. The specimens were then investigated along the position of transverse cross section (clad–interface–Al matrix) direction and longitudinal direction (1st pass– 2nd pass–3rd pass), respectively. The specimen was dried, weighed and immerged in solution of 10% H2 SO4 until 4, 8, 12, 16 and 20 h for static corrosion, respectively. After corrosion, specimen was cleaned, dried and weighed in TG328B analytical balance with 0.1 mg precision. The corrosion was carried out at 50 ◦ C in HH-2 type digitalconstant-temperature water bath. The corrosion rate could be calculated as following equation: vloss =
Results and discussion
Experimental procedure
(m0 − m1 ) St
(1)
where, vloss is corrosion rate; m0 is mass before corrosion; m1 is mass after corrosion; S is surface area of specimen; t is corrosion time. The microstructures of the specimens were characterized by means of optical observation, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM). A Hitachi S-2700 SEM and Electron Probe Microanalyzer EPMA-8705 equipped with an energy-dispersive spectrometer (EDS) were used for microstructure observations and composition analyses. The samples for optical and SEM observations were etched in a solution of mixed acid consisted of 1% HF, 1.5% HCl, 2.5%
The whole morphology distribution of laser clad specimen along transverse cross section direction and laser scanning longitudinal direction was shown in Fig. 1, the macrostructure analysis results showed weld bead of deposited metal/Al matrix with well-metallurgical bonding was obtained with acicular structure on the surface, free from faults such as porosity (air hole), fine hair crack and slags etc.; SEM image showed that interface reaction had occurred near the fusion line with two significant transition zones.
3.1.
Microstructures
Microstructure of specimen 2# using Ni-based alloy and 2% Y2 O3 as clad metal on 6061 aluminum matrix was illustrated in Fig. 2(a). From the image, along the transverse cross section, upper microstructure of deposited metal was consisted of gray isometric crystal, white acicular crystal and fringe crystal at interface. The average diameter of isometric crystal was 1.6 m. The microstructure was mainly fine crystal under the deposited near deposited metals/6061 aluminum matrix interface, a significant transition belt with maximum size of 30 m was formed. Typical microstructure of deposited metals/6061 aluminum matrix interface near 6061 aluminum side was isometric crystal and arborescent crystal. Some agglomerated particle with finer crystals was formed. The arborescent crystals near interface were against to deposited metals/6061 aluminum matrix interface and priority grown diffusion along this direction. The transition belt was 150 m at the most. Contrast to different laser scanning passes, analysis along longitudinal direction, in the 1st pass, penetration, width and heat affected zone (HAZ) were smallest. In the 2nd pass, width and heat affected zone were smaller, in the 3rd pass; width and HAZ were small with coarse microstructure. Microstructure of specimen 4# using Ni-based alloy and 8% Y2 O3 as clad metal on 6061 aluminum matrix was illustrated in Fig. 2(b). From the image, we could see, along the transverse cross section, upper microstructure of deposited metal was consisted of more fine gray isometric crystal, white acicular crystal and fringe crystal at interface than specimen 2# . The average diameter of isometric crystal was less than 1 m. The microstructure was mainly fine crystal under the deposited near deposited metals/6061 aluminum matrix interface, a significant transition belt with maximum size of 20 m was formed. Typical microstructure of deposited metals/6061 aluminum matrix interface near 6061 almuinum side was isometric crystal and arborescent crystal. Some agglomerated particle with finer crystals was formed. The arborescent crystals near interface were against to deposited metals/6061 aluminum matrix interface and priority grown diffusion along this direction. The transition belt was 30 m at the most. The morphology of composite for specimen 4# was illustrated in Fig. 3, from it we could see, besides above microstructure, acicular crystal, bulk and ellipse reinforced crystals were formed.
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 549–555
Table 1 – Chemical composition of 6061 Al alloy and Ni-based alloy powder (wt%) Materials 6061 Al Ni-based alloy
Si
Fe
Ni
Cu
0.4–0.8 –
0.7 6
– 48
0.15–0.4 –
Co
Mn
Mg
Cr
Mo
C
Zn
W
Al
– 8
0.15 –
0.8–1.2 –
0.04–0.35 14
0.15 5
– 1
0.25 –
– 12
Bal. 6
Table 2 – The numbering and processing parameters of laser ladding layer Specimen 2# 4#
Compositions of clad layer Ni-Based alloy + 2% Y2 O3 Ni-Based alloy + 8% Y2 O3
Output power (kW) 1.688 1.688
Scanning speed (mm s−1 ) 10 10
Laser-beam Shape Narrow band (diameter: 1 mm) Narrow band (diameter: 1 mm)
Gas-flow rate (L min−1 ) 1.4 1.4
Fig. 1 – SEM observation for morphology of three passes laser clad.
Therefore, for specimen 2# or 4#, microstructure distributions abided by aluminum matrix → dendritic crystal + fringe crystal → fine dendritic, disperse reinforced particles and acicular crystal → coarse dendritic crystal. Contrast to different laser scanning passes, analysis along longitudinal direction, in the 1st pass, penetration, width and HAZ were smallest. In the 2nd pass, width and HAZ were smaller, in the 3rd pass; width and HAZ were large with coarse microstructure.
SEM observation of typical dendritic crystals near deposited metals/6061 aluminum matrix interface was shown in Fig. 3. Dendritic crystals grew from fusion line and bulk cell coalescence (see Fig. 3(a)), prior grew along perpendicular to interface. White in black microstructure could be formed while cell growth. Form the high resolution SEM image (see Fig. 3(b)), we could see that there was black bulk microstructure and abscission zone besides dendritic crystals and matrix.
Fig. 2 – Microstructure of deposited metal and interface with Ni-based alloy and 2-wt% Y2 O3 or 8-wt%Y2 O3 . (a) Microstructure in deposited metal with Ni-based alloy + 2%Y2 O3 ; (b) microstructure in deposited metal with Ni-based alloy + 8%Y2 O3 ; (c) microstructure near interface with Ni-based alloy + 2%Y2 O3 ; (d) microstructure near interface Ni-based alloy + 8%Y2 O3 .
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Fig. 3 – SEM observation of the Ni-based alloy + 2% Y2 O3 /Al matrix interface, taken the upper of the interface of the transverse cross section, showing the isometric dendrite crystal and matrix morphology. (a) Several isometric dendrite crystals distribution; (b) black–white morphology of isometric dendrite crystal.
3.2.
Composition and element diffusion
Composition analysis and element diffusion analysis were investigated using EPMA-8705 analyzer. The distribution of elements Ni, Cr, Al, Y and W near deposited metals/6061 aluminum matrix interface was studied in 500 m distances between 6061 aluminum matrix and deposited metals. Taking specimen 2# for example, the analysis spectrum 1 was illustrated in Fig. 4. The results showed: average composition in white bulk crystal was 57.65% Al, 35.01% Ni, 2.14% Cr and 1.38% Fe (at.%); average composition in acicular crystal was 64.63%
Al, 21.94% Ni, 1.10% Cr, 8.11% Si and 3.74% Fe (at.%). According to theory calculation, the microstructure might be Nix Al, primary phase near deposited metals/6061 aluminum matrix interface was Nix Al, Crx C was discovered also in this region. According to literature (Shangping Li et al., 2007) (see Table 3), the matrix is composed of NiAl, Ni3 Al, NiAl3 etc. From the results, white bulk crystal might be tungsten, and the particles were CrC, which led to dispersion strengthening. Dendritic crystal near deposited metals/6061 aluminum matrix interface was made up of NiAl3 , and the clad matrix was NiAl, which led to corrosion resistance. The distribution of elements Ni, Cr, Al, Y and W near deposited metals/6061 aluminum matrix interface in 255 and 500 m distance was investigated for specimen 2# and 4# , the results were illustrated in Fig. 5. From the profiles, gradient of Al, Cr, Ni is much larger than those of specimen 2# and specimen 4# . Significant Y and W peak could be found in specimen 2# or 4# .
4.
Mechanical properties
Micro-hardness distribution on near deposited metals/6061 aluminum matrix interface of 2# and 4# specimen and their corrosion resistance were carried out, respectively.
4.1.
Micro-hardness test
After polishing treatment, HXD-1000 type micro-hardness tester was used to investigate the distribution on near deposited metal/6061 Al matrix interface and the results were showed in Fig. 6. From the profile, micro-hardness of clad metal (780–1100 HV0.2 ) was much higher than that of aluminum matrix (62 HV0.2 ) significantly. In details, for specimen 2# , much larger width transition zones was achieved from aluminum matrix to deposited metal than those of specimen 4# . This agreed with former microstructure analysis. Fig. 4 – Composition analysis position of deposited metals/6061 aluminum matrix interface. (a) Position of spectrum 1 composition analysis; (b) composition analysis result.
4.2.
Corrosion resisting property
Corrosion resistance experimental results were illustrated in Table 4. From it we could see that the corrosion rate of alu-
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 549–555
Table 3 – Heats of formation, aluminum compositions, and melting points of nickel aluminides (Shangping Li et al., 2007) Intermetallics
Heat of formation Hf 298 (kcal/mol) −36.6 −28.3 −67.5 −36.6
Ni3 Al NiAl Ni2 Al3 NiAl3
± ± ± ±
Weight percent of aluminum
1.2 1.2 4.0 2.0
13.28 31.49 40.81 57.96
Melting point (◦ C) 1395 1639 1133 854
Fig. 5 – Line-scanning analysis for element Ni, Cr, Al, Y and W diffusion near interface. (a) EDS analysis of specimen 2# ; (b) Y in specimen; (c) W in specimen.
Table 4 – The numbering and processing parameters of laser ladding layer Specimen
Al alloy 2# 4#
Mass before corrosion, m0 (g) 0.76685 0.02184 0.00631
Mass after corrosion, m1 (g) 0.67213 0.02145 0.00621
Surface area S (m2 )
Corrosion time, t (h)
285.4926 × 10–6 2.4687 × 10–6 0.7831 × 10–6
minum matrix is nearly the double than that of deposited metal. After corrosion, the specimen was dried and analyzed using S-2700 SEM Analyzer. The results were shown in Fig. 7. From it we could see that the main erosion-resistance phase was Nix Al, Ni matrix and remainder Crx C reinforced particles. While the aluminum matrix is consist of porous ␣-Al.
5.
20 20 20
Corrosion rate, vloss (g m−2 h−1 ) 16.5889 7.8465 6.3669
Discussion
According to the above experimental results, the microstructure and mechanical properties changed with quantity of addition Y2 O3 . The details were as followed. The addition of Y2 O3 not only improved the weldability of deposited metal on aluminum matrix, but altered
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 549–555
Fig. 6 – Micro-hardness distribution near interface of different composites. (a) Specimen 2# ; (b) specimen 4# .
Fig. 7 – Erosion-resistance test results of 6061 aluminum matrix and deposited metals in specimen 4 (a), (b) morphology of Al matrix after erosion; (c), (d) morphology of specimen 4# after erosion.
the microstructure and mechanical properties. From Fig. 2, contrast to 2% Y2 O3 addition, microstructure of deposited metals with 8% Y2 O3 manifested more fine crystals, this is because added Y2 O3 played a role of grain refinement in microstructure systems. However, there is Y element detected during composition analysis using EMPA 8705 Microanalyzer while too less Y2 O3 was added to deposited metals. This was might be the Y2 O3 burning loss. Contrast Fig. 2(a) with Fig. 2(b), we could see, with the increase of Y2 O3 , the crystals near deposited metals/6061 aluminum matrix interface became shorter and the transition
zone became smaller, this agreed with element diffusion analysis results in Fig. 5. From Table 4, with the increase of Y2 O3 , the better corrosion resistance could be obtained. This was might be the formation of Ni3 Al, NiAl and NiAl3 , the dispersion strengthening W and Crx C are the main strengthening model.
6.
Conclusions
(1) The addition of Y2 O3 improved the weldability of deposited metal on aluminum matrix, altering the
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 549–555
microstructure from coarse scales to fine scales and shortening the transition zone near deposited metals/6061aluminum matrix interface. (2) Well corrosion property of deposited metal was contributed to Nix Al matrix, and with the increase of addition of Y2 O3 , the corrosion rate became slower with Crx C and W dispersion strengthening.
Acknowledgments The authors acknowledge the support of the Scientific Program of Science & Technology department of Shanghai, People’s Republic of China (Grant No. 061111034), Specialized Research Fund for Excellent Young Teachers of Shanghai, People’s Republic of China (Grant No. 06xpyq17), Shanghai Leading Academic Discipline Project (No.J51402) and, many thanks would be given to Prof. Shun Yao, Dr. Xinhua Tang, Fenggui Lu, Hailiang Yu, J.J. Ding from Welding Engineering Institute, Shanghai Jiaotong University.
references
Duraiselvam, M., Galun, R., Siegmann, S., Wesling, V., Mordike, B.L., 2005. Particle-laden liquid impact erosion characteristics of laser clad Ni-based intermetallic matrix composites with TiC and WC reinforcements. Lasers Eng. 15 (5–6), 355–373. Ibrahim, I.A., Mohamed, F.A., LAVERNIA.F E.J., 1991. Particulate reinforced metal matrix composites-a review. J. Mater. Sci. 26, 1156–1187. Lalas, C., Tsirbas, K., Salonitis, K., Chryssolouris, G., 2007. An analytical model of the laser clad geometry 32 (1–2), 34–41. Lavernia, E.J., Perez, R.J., Zhang, J., 1995. Damping behavior of discontinuously reinforced Al alloy metal-matrix composites. Metall. Mater. Trans. A 26A (11), 2803–2818. Lebrat, J.P., Varma, A., Miller, A.E., 1992. Combustion synthesis of Ni3Al and Ni3Al-matrix composites. Metall. Trans. A 23A (1), 69–76.
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Liu, X.-B., Wang, H.-M., 2006. Microstructure, wear and high-temperature oxidation resistance of laser clad Ti5Si3/␥/TiSi composite coatings on ␥-TiAl intermetallic alloy. Surface and Coatings Technology 200 (14–15), 4462–4470. Lloyd, D.J., 1994. Particle reinforced aluminum and magnesium matrix composites. Int. Mater. Rev. 39 (1), 1–23. Manna, I., Dutta Majumdar, J., Ramesh Chandra, B., Nayak, S., Dahotre, Narendra B, 2006. Laser surface cladding of Fe-B-C, Fe-B-Si and Fe-BC-Si-Al-C on plain carbon steel. Surface Coatings Technol. 201 (1–2), 434–440. Munir, Z.A., Umberto, A.T., 1989. Self-propagating exothermic reactions: the synthesis of high-temperature materials by combustion. Mater. Sci. R. 3 (7–8), 277–365. Navas, C., Conde, A., Cadenas, M., De Damborenea, J., 2006. Tribological properties of laser clad Stellite 6 coatings on steel substrates. Surface Eng. 22 (1), 26–34. Qi, Huan, Mazumder, J., Ki, Hyungson, 2006. Numerical simulation of heat transfer and fluid flow in coaxial laser cladding process for direct metal deposition. J. Appl. Phys. 100 (2), 024903. Salehi, D., Brandt, M., 2006. Melt pool temperature control using LabVIEW in Nd:YAG laser blown powder cladding process. The International Journal of Advanced Manufacturing Technology 29 (3–4), 273–278. Shangping, Li, Di, F., Heli, L., 2007. Microstructure and abrasive wear performance of chromium carbide reinforced Ni3Al matrix composite coating. Surface Coatings Technol. 201, 4542–4546. Taya, M., Lucay, K.E., 1991. Strengthening of a particulate metal matrix composite by quenching. Acta Metall. Mater. 39 (1), 73–87. Xu, J., Liu, W., Kan, Y., Zhong, M., 2006a. Microstructure and wear properties of laser cladding Ti-Al-Fe-B coatings on AA2024 aluminum alloy. Mater Des. 27 (5), 405–410. Xu, Jiang, Liu, Wenjin, 2006b. Wear characteristic of in situ synthetic TiB2 particulate-reinforced Al matrix composite formed by laser cladding. Wear 260 (4–5), 486–492. Yue, T.M., Huang, K.J., Man, H.C., 2006. In-situ laser cladding of Al2O3 reaction coating on aluminium alloy 7075 for corrosion resistance improvement Materials Transactions 47 (3), 948–951.
journal homepage: www.elsevier.com/locate/jmatprotec
Effect of Y2 O3 addition on microstructure of Ni-based alloy + Y2 O3 /substrate laser clad Peiquan Xu a,∗ , Xinhua Tang b , Shun Yao b , Jianping He a , Guoxiang Xu a a b
College of Materials Engineering, Shanghai University of Engineering Science, 201620 Shanghai, China School of Materials Science and Engineering, Shanghai Jiaotong University, 200030 Shanghai, China
a r t i c l e
i n f o
Article history:
a b s t r a c t To improve hardness and corrosion resistance of aluminum substrate, using laser cladding
Received 29 June 2007
technique, metal composite with Ni-based alloy, refining and dispersion strengthening Y2 O3 ,
Received in revised form
particle hardening W, Cr was obtained. The microstructure and morphology, phase identi-
16 December 2007
fication, element diffusion and composition analysis of the Ni-based alloy + Y2 O3 /substrate
Accepted 16 January 2008
laser clad and laser clad/aluminum substrate interface were examined using scanning electron microscopy (SEM), Electron Probe Microanalyzer (EPMA), energy-dispersive spectrometer (EDS) analysis. The corrosion resistance was investigated also. The results showed,
Keywords:
with the addition of Y2 O3 , more fine microstructures consisted of isometric crystal, white
Laser clad
acicular crystal and fringe crystal; primary phases were mainly Ni3 Al, NiAl, NiAl3 , W, ␣-Al
Yttrium oxide
and Crx C. The corrosion rate of aluminum substrate was nearly two times more than that
Ni-based alloy + Y2 O3
of laser clad metals with addition of Y2 O3 . © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
Strengthening phase structures were often introduced into alloys for surface modification purposes by different surface modification technique. An obvious example is the extremely fine second-phase particles used for precipitation and dispersion hardening (Qi et al., 2006; Manna et al., 2006). While using spraying technique, thicker coating could be obtained, but the bonding between coating and substrate belong to mechanical bonding with weak and such drawbacks as gas porosity. Therefore, it is of interest to develop a surface modification technique, e.g., by using laser cladding with a new material deposited metal (DM) system design that provides good combination of desirable mechanical properties, in particular, both high strength and good corrosion resistance properties. Laser cladding technique is steadily becoming a tool for synthesis of materials. Recently, a large number of new clad materials and composites have been developed (Navas et
∗
Corresponding author. Tel.: +86 21 67791202. E-mail address: [email protected] (P. Xu). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.01.026
al., 2006; Duraiselvam et al., 2005; Shangping Li et al., 2007; Xu et al., 2006a, 2006b). Clad material included ceramic phases, strengthening wearing phases or special crystalline materials, e.g., Al2 O3 , NiTi, in-situ synthesis TiC coating (Liu et al., 2006; Yue et al., 2006), WC/Ni coating (Ibrahim et al., 1991; Lloyd, 1994), quasicrystal and amorphous mixtures (Lavernia et al., 1995; Taya and Lucay, 1991; Lebrat et al., 1992; Munir and Umberto, 1989; Lalas et al., 2007; Salehi and Brandt, 2006), in which the hardening phases itself has a coarse scale structure. While such materials offer high strength, they usually have to suffer from porosity and contamination problems in laser cladding process. To achieve high strength clad with high corrosion resistance and high wear-resisting property, we desire a Ni-based alloys + Y2 O3 deposited metal that can be obtained on the surface of aluminum alloys by using laser cladding technique, properly choosing the deposited metals’ composition and controlling the laser cladding parameters.Such the
550
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 549–555
solidification products will produce the desired microstructure as discussed in the preceding paragraph. The multiple elements added also render the crystal growth more sluggish, favoring the refinement of coarse scales to fine scales.
HNO3 , 95% H2 O and Reagent (20% KOH + 20% K3 [Fe(CN)6 ] + 60% H2 O: vol%) at room temperature.
3. 2.
6061 aluminum alloy (Al–Mg–Si) was selected as base materials, the details were: dimension: 60 mm × 50 mm × 30 mm; surface roughness: 0.2 m, and its composite was shown in Table 1. The deposited alloys were mixture of Ni-based alloys and yttrium oxide after milling. The purity was 99.99%. Molecular formula: Y2 O3 ; molecular weight: 225.82.
2.1. Preparation method of Ni-based alloy/Y2 O3 deposited metal The grain size of mixture powers was 30 m. After milling, four pressed compacts were obtained with 1 mm thickness and dried in 120 ◦ C temperature. The pressed compacts were preset onto the surface of aluminum, and Ni-based alloy-Y2 O3 /aluminum alloy laser clad could be obtained by laser cladding using continuous wave 3 kW Nd:YAG laser. Parameters for laser clad and serial number of specimens were illustrated in Table 2. The overlap ratio was 30%, the welding speed was 10 mm s−1 , and the gas-flow rate is 1.4 L min−1 .
2.2.
Analyzing position and testing methods
Specimens for aluminum alloy matrix composite were obtained by laser cladding of the deposited metals (Ni-based alloys + Y2 O3 ) on 6061 aluminum alloy in argon atmosphere. The specimens were then investigated along the position of transverse cross section (clad–interface–Al matrix) direction and longitudinal direction (1st pass– 2nd pass–3rd pass), respectively. The specimen was dried, weighed and immerged in solution of 10% H2 SO4 until 4, 8, 12, 16 and 20 h for static corrosion, respectively. After corrosion, specimen was cleaned, dried and weighed in TG328B analytical balance with 0.1 mg precision. The corrosion was carried out at 50 ◦ C in HH-2 type digitalconstant-temperature water bath. The corrosion rate could be calculated as following equation: vloss =
Results and discussion
Experimental procedure
(m0 − m1 ) St
(1)
where, vloss is corrosion rate; m0 is mass before corrosion; m1 is mass after corrosion; S is surface area of specimen; t is corrosion time. The microstructures of the specimens were characterized by means of optical observation, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM). A Hitachi S-2700 SEM and Electron Probe Microanalyzer EPMA-8705 equipped with an energy-dispersive spectrometer (EDS) were used for microstructure observations and composition analyses. The samples for optical and SEM observations were etched in a solution of mixed acid consisted of 1% HF, 1.5% HCl, 2.5%
The whole morphology distribution of laser clad specimen along transverse cross section direction and laser scanning longitudinal direction was shown in Fig. 1, the macrostructure analysis results showed weld bead of deposited metal/Al matrix with well-metallurgical bonding was obtained with acicular structure on the surface, free from faults such as porosity (air hole), fine hair crack and slags etc.; SEM image showed that interface reaction had occurred near the fusion line with two significant transition zones.
3.1.
Microstructures
Microstructure of specimen 2# using Ni-based alloy and 2% Y2 O3 as clad metal on 6061 aluminum matrix was illustrated in Fig. 2(a). From the image, along the transverse cross section, upper microstructure of deposited metal was consisted of gray isometric crystal, white acicular crystal and fringe crystal at interface. The average diameter of isometric crystal was 1.6 m. The microstructure was mainly fine crystal under the deposited near deposited metals/6061 aluminum matrix interface, a significant transition belt with maximum size of 30 m was formed. Typical microstructure of deposited metals/6061 aluminum matrix interface near 6061 aluminum side was isometric crystal and arborescent crystal. Some agglomerated particle with finer crystals was formed. The arborescent crystals near interface were against to deposited metals/6061 aluminum matrix interface and priority grown diffusion along this direction. The transition belt was 150 m at the most. Contrast to different laser scanning passes, analysis along longitudinal direction, in the 1st pass, penetration, width and heat affected zone (HAZ) were smallest. In the 2nd pass, width and heat affected zone were smaller, in the 3rd pass; width and HAZ were small with coarse microstructure. Microstructure of specimen 4# using Ni-based alloy and 8% Y2 O3 as clad metal on 6061 aluminum matrix was illustrated in Fig. 2(b). From the image, we could see, along the transverse cross section, upper microstructure of deposited metal was consisted of more fine gray isometric crystal, white acicular crystal and fringe crystal at interface than specimen 2# . The average diameter of isometric crystal was less than 1 m. The microstructure was mainly fine crystal under the deposited near deposited metals/6061 aluminum matrix interface, a significant transition belt with maximum size of 20 m was formed. Typical microstructure of deposited metals/6061 aluminum matrix interface near 6061 almuinum side was isometric crystal and arborescent crystal. Some agglomerated particle with finer crystals was formed. The arborescent crystals near interface were against to deposited metals/6061 aluminum matrix interface and priority grown diffusion along this direction. The transition belt was 30 m at the most. The morphology of composite for specimen 4# was illustrated in Fig. 3, from it we could see, besides above microstructure, acicular crystal, bulk and ellipse reinforced crystals were formed.
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Table 1 – Chemical composition of 6061 Al alloy and Ni-based alloy powder (wt%) Materials 6061 Al Ni-based alloy
Si
Fe
Ni
Cu
0.4–0.8 –
0.7 6
– 48
0.15–0.4 –
Co
Mn
Mg
Cr
Mo
C
Zn
W
Al
– 8
0.15 –
0.8–1.2 –
0.04–0.35 14
0.15 5
– 1
0.25 –
– 12
Bal. 6
Table 2 – The numbering and processing parameters of laser ladding layer Specimen 2# 4#
Compositions of clad layer Ni-Based alloy + 2% Y2 O3 Ni-Based alloy + 8% Y2 O3
Output power (kW) 1.688 1.688
Scanning speed (mm s−1 ) 10 10
Laser-beam Shape Narrow band (diameter: 1 mm) Narrow band (diameter: 1 mm)
Gas-flow rate (L min−1 ) 1.4 1.4
Fig. 1 – SEM observation for morphology of three passes laser clad.
Therefore, for specimen 2# or 4#, microstructure distributions abided by aluminum matrix → dendritic crystal + fringe crystal → fine dendritic, disperse reinforced particles and acicular crystal → coarse dendritic crystal. Contrast to different laser scanning passes, analysis along longitudinal direction, in the 1st pass, penetration, width and HAZ were smallest. In the 2nd pass, width and HAZ were smaller, in the 3rd pass; width and HAZ were large with coarse microstructure.
SEM observation of typical dendritic crystals near deposited metals/6061 aluminum matrix interface was shown in Fig. 3. Dendritic crystals grew from fusion line and bulk cell coalescence (see Fig. 3(a)), prior grew along perpendicular to interface. White in black microstructure could be formed while cell growth. Form the high resolution SEM image (see Fig. 3(b)), we could see that there was black bulk microstructure and abscission zone besides dendritic crystals and matrix.
Fig. 2 – Microstructure of deposited metal and interface with Ni-based alloy and 2-wt% Y2 O3 or 8-wt%Y2 O3 . (a) Microstructure in deposited metal with Ni-based alloy + 2%Y2 O3 ; (b) microstructure in deposited metal with Ni-based alloy + 8%Y2 O3 ; (c) microstructure near interface with Ni-based alloy + 2%Y2 O3 ; (d) microstructure near interface Ni-based alloy + 8%Y2 O3 .
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Fig. 3 – SEM observation of the Ni-based alloy + 2% Y2 O3 /Al matrix interface, taken the upper of the interface of the transverse cross section, showing the isometric dendrite crystal and matrix morphology. (a) Several isometric dendrite crystals distribution; (b) black–white morphology of isometric dendrite crystal.
3.2.
Composition and element diffusion
Composition analysis and element diffusion analysis were investigated using EPMA-8705 analyzer. The distribution of elements Ni, Cr, Al, Y and W near deposited metals/6061 aluminum matrix interface was studied in 500 m distances between 6061 aluminum matrix and deposited metals. Taking specimen 2# for example, the analysis spectrum 1 was illustrated in Fig. 4. The results showed: average composition in white bulk crystal was 57.65% Al, 35.01% Ni, 2.14% Cr and 1.38% Fe (at.%); average composition in acicular crystal was 64.63%
Al, 21.94% Ni, 1.10% Cr, 8.11% Si and 3.74% Fe (at.%). According to theory calculation, the microstructure might be Nix Al, primary phase near deposited metals/6061 aluminum matrix interface was Nix Al, Crx C was discovered also in this region. According to literature (Shangping Li et al., 2007) (see Table 3), the matrix is composed of NiAl, Ni3 Al, NiAl3 etc. From the results, white bulk crystal might be tungsten, and the particles were CrC, which led to dispersion strengthening. Dendritic crystal near deposited metals/6061 aluminum matrix interface was made up of NiAl3 , and the clad matrix was NiAl, which led to corrosion resistance. The distribution of elements Ni, Cr, Al, Y and W near deposited metals/6061 aluminum matrix interface in 255 and 500 m distance was investigated for specimen 2# and 4# , the results were illustrated in Fig. 5. From the profiles, gradient of Al, Cr, Ni is much larger than those of specimen 2# and specimen 4# . Significant Y and W peak could be found in specimen 2# or 4# .
4.
Mechanical properties
Micro-hardness distribution on near deposited metals/6061 aluminum matrix interface of 2# and 4# specimen and their corrosion resistance were carried out, respectively.
4.1.
Micro-hardness test
After polishing treatment, HXD-1000 type micro-hardness tester was used to investigate the distribution on near deposited metal/6061 Al matrix interface and the results were showed in Fig. 6. From the profile, micro-hardness of clad metal (780–1100 HV0.2 ) was much higher than that of aluminum matrix (62 HV0.2 ) significantly. In details, for specimen 2# , much larger width transition zones was achieved from aluminum matrix to deposited metal than those of specimen 4# . This agreed with former microstructure analysis. Fig. 4 – Composition analysis position of deposited metals/6061 aluminum matrix interface. (a) Position of spectrum 1 composition analysis; (b) composition analysis result.
4.2.
Corrosion resisting property
Corrosion resistance experimental results were illustrated in Table 4. From it we could see that the corrosion rate of alu-
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Table 3 – Heats of formation, aluminum compositions, and melting points of nickel aluminides (Shangping Li et al., 2007) Intermetallics
Heat of formation Hf 298 (kcal/mol) −36.6 −28.3 −67.5 −36.6
Ni3 Al NiAl Ni2 Al3 NiAl3
± ± ± ±
Weight percent of aluminum
1.2 1.2 4.0 2.0
13.28 31.49 40.81 57.96
Melting point (◦ C) 1395 1639 1133 854
Fig. 5 – Line-scanning analysis for element Ni, Cr, Al, Y and W diffusion near interface. (a) EDS analysis of specimen 2# ; (b) Y in specimen; (c) W in specimen.
Table 4 – The numbering and processing parameters of laser ladding layer Specimen
Al alloy 2# 4#
Mass before corrosion, m0 (g) 0.76685 0.02184 0.00631
Mass after corrosion, m1 (g) 0.67213 0.02145 0.00621
Surface area S (m2 )
Corrosion time, t (h)
285.4926 × 10–6 2.4687 × 10–6 0.7831 × 10–6
minum matrix is nearly the double than that of deposited metal. After corrosion, the specimen was dried and analyzed using S-2700 SEM Analyzer. The results were shown in Fig. 7. From it we could see that the main erosion-resistance phase was Nix Al, Ni matrix and remainder Crx C reinforced particles. While the aluminum matrix is consist of porous ␣-Al.
5.
20 20 20
Corrosion rate, vloss (g m−2 h−1 ) 16.5889 7.8465 6.3669
Discussion
According to the above experimental results, the microstructure and mechanical properties changed with quantity of addition Y2 O3 . The details were as followed. The addition of Y2 O3 not only improved the weldability of deposited metal on aluminum matrix, but altered
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Fig. 6 – Micro-hardness distribution near interface of different composites. (a) Specimen 2# ; (b) specimen 4# .
Fig. 7 – Erosion-resistance test results of 6061 aluminum matrix and deposited metals in specimen 4 (a), (b) morphology of Al matrix after erosion; (c), (d) morphology of specimen 4# after erosion.
the microstructure and mechanical properties. From Fig. 2, contrast to 2% Y2 O3 addition, microstructure of deposited metals with 8% Y2 O3 manifested more fine crystals, this is because added Y2 O3 played a role of grain refinement in microstructure systems. However, there is Y element detected during composition analysis using EMPA 8705 Microanalyzer while too less Y2 O3 was added to deposited metals. This was might be the Y2 O3 burning loss. Contrast Fig. 2(a) with Fig. 2(b), we could see, with the increase of Y2 O3 , the crystals near deposited metals/6061 aluminum matrix interface became shorter and the transition
zone became smaller, this agreed with element diffusion analysis results in Fig. 5. From Table 4, with the increase of Y2 O3 , the better corrosion resistance could be obtained. This was might be the formation of Ni3 Al, NiAl and NiAl3 , the dispersion strengthening W and Crx C are the main strengthening model.
6.
Conclusions
(1) The addition of Y2 O3 improved the weldability of deposited metal on aluminum matrix, altering the
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 549–555
microstructure from coarse scales to fine scales and shortening the transition zone near deposited metals/6061aluminum matrix interface. (2) Well corrosion property of deposited metal was contributed to Nix Al matrix, and with the increase of addition of Y2 O3 , the corrosion rate became slower with Crx C and W dispersion strengthening.
Acknowledgments The authors acknowledge the support of the Scientific Program of Science & Technology department of Shanghai, People’s Republic of China (Grant No. 061111034), Specialized Research Fund for Excellent Young Teachers of Shanghai, People’s Republic of China (Grant No. 06xpyq17), Shanghai Leading Academic Discipline Project (No.J51402) and, many thanks would be given to Prof. Shun Yao, Dr. Xinhua Tang, Fenggui Lu, Hailiang Yu, J.J. Ding from Welding Engineering Institute, Shanghai Jiaotong University.
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