Influence of Cr Addition on Molten Aluminium Corrosion Resistance of Flame Sprayed Aisi H13
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Influence Of Cr Addition On Molten Aluminium Corrosion Resistance Of Flame Sprayed AISI H13
Kalla Swaroopa and Kakumani Manish Chowdaryb & K. Gokul Kumarc and N. Arivazhagand School of mechanical and building sciences, VIT University, Vellore, India E-mail: akallaswaroop@gmail.com, kmanishchowdary@yahoo.com, ckgokulkumar@vit.ac.in, dnarivazhagan@vit.ac.in
Abstract - In the present investigation, molten aluminium corrosion tests have been performed on flame spray coated AISI H13 samples, with bare AISI H13 as reference. The commercially used EWAC 1001 EB Nickel based powder was used to coat the samples by using the flame spray technique. The commercially used EWAC 1001 EB powder was modified by adding 20% Cr to study the influence of chromium in molten aluminium corrosion resistance. The molten aluminium corrosion tests were performed on AISI H13, Flame sprayed samples of EWAC 1001 EB powder and Flame sprayed samples of modified EWAC 1001 EB powder. Apparently the corrosion resistance in molten aluminium becomes better in the flame sprayed sample with modified EWAC 1001 EB powder. Keywords- molten aluminium corrosion; flame spray; EWAC 1001 EB;AISI H13.
I. INTRODUCTION Die casting is a very versatile method for aluminium and magnesium castings but, life of casting dies has been a big problem since a long time [1]. Today, high-quality aluminium castings are usually produced using permanent metallic moulds in gravity or pressure die-casting. This is because they offer high volume production of more uniform castings with closer dimensional tolerances. When compared to sand casting they also offer superior surface finish and improved mechanical properties, for example for automotive components [2]. Two different mechanisms are claimed to limit die lifetime, the washout phenomenon i.e. corrosion, erosion and soldering as a result of affinity of tool steel towards molten aluminium and thermal fatigue, caused by thermal cycling of the dies [3]. Erosive wear is due to the high velocity of the incoming molten aluminium [4]. The corrosion is by two mechanisms, corrosion by molten aluminium and high temperature cyclic air oxidation. Molten aluminium has been a serious problem in aluminium processing industry and aluminium casting industry due to high reactivity and affinity of molten aluminium, molten aluminium reacts with almost all metals and metal oxides [5, 6]. Due to this molten aluminium corrosion, soldering results and a rough surface results on the die wall. Hence surface finish and tolerances of casting dies are greatly affected leading to much higher production cost. A number of researches have been done to access the molten aluminium corrosion and to improve the corrosion resistance. Ceramic has a good corrosion resistance but suffers rapid strength degradation. PVD and CVD coatings also provide a good corrosion resistance but suffer rapid corrosion once the coating wears off [5, 6]. Flame spray coating is mostly used for repair work of dies to improve edge retention after the original uncoated H13 die is no longer usable due to corrosion and wear. Flame spray is used in a different way from PVD and CVD, PVD and CVD are used to protect dies from the initial usage. But flame spray is used for repair work. Initially castings are performed in the H13 dies, after 25000 cycles when the dies gets damaged the inner walls of the dies are flame sprayed with EWAC 1001 EB/EWAC 1003 EBZ Ni based powder and machined according to dimensions so that, the die can be used for 8000 cycles again. PVD and CVD coatings are used to coat coating thickness of 5-6 µm, but using flame spray coating technology coating thickness up to 6mm can be coated. Ni and Cr are the frequently used alloying elements for improving the performance of various cast irons such as the wear-resistance, corrosion-resistance and thermal-resistance. Ni can dramatically improve the chemical stability of the iron alloys so as to increase their corrosion resistance. Cr is one of the strong carbide formation elements, which significantly enhances the strength. More Ni content in the powder mixture improves the formability and surface smoothness while
International Conference on Advanced Research in Mechanical Engineering (ICARME-2012), 13th May, 2012, TRIVENDUM, ISBN : 978-93-81693-59-9
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Influence Of Cr Addition On Molten Aluminium Corrosion Resistance Of Flame Sprayed AISI H13
more Cr content results in higher cracking tendency. Therefore, the commercially used EWAC 1001 EB nickel based powder was modified by adding 20% Cr to improve the corrosion resistant properties [7]. II. MATERIALS AND EXPERIMENTAL PROCEDURE A. Materials The materials used in the present investigation are H13 and flame sprayed H13, the coating method used in the present work is powder flame spray. Two powders are used, the first powder is EWAC 1001 EB and second powder is EWAC 1001 EB modified with 20% Cr here after referred as EWAC 1001 EB + 20% Cr. EWAC 1001 EB is a commercially used nickel based powder for flame spraying the damaged H13 dies to improve red hardness, heat resistance, corrosion resistance and edge retention. The typical composition of H13 is shown in Table 1. The chemical composition of EWAC 1001 EB and EWAC 1001 EB + 20% Cr are shown in Table 2. The average size of EWAC 1001 EB powder particle was 95 µm and the average size of the chromium powder particle was 150 µm, the EWAC 1001 EB + 20% Cr powder was prepared by mechanically alloying with chromium powder by a typical ball milling process for six hours.
TABLE I. C 0.38 S 0.03 Si 1.0
B. Flame spraying The Flame spray coating is a part of family of thermal spray coating. Thermal spray is a generic term for a group of coating processes used to apply metallic or non metallic coatings. In the powder flame spray process used in the present research work, powdered feed stock is aspirated in the oxy fuel flame, melted and carried by the flame and air jets to the work piece. The apparatus used for coating is EWAC SuperJet Eutalloy system which can give a coating thickness between 0.075mm to 6mm. The hardness of the deposit varies from 90 Rb to 64 Rc depending on the alloy used. The size of the AISI H13 samples used for flame spraying is 8×12×17 mm3; the samples were grounded with 220 grit sheets to impart surface roughness. All the samples were ultrasonically cleaned before the coating. The parameters of the flame spray process are mentioned in Table 3. The images of the uncoated and coated samples are shown in Fig. 1. The images of the coated surface magnified at 50x are shown in the Fig. 2.
CHEMICAL COMPOSITION OF AISI H13
Mn 0.4 Fe Bal Mo 1.3 Mg 0.02 Cr 5.3 V 0.9 Ni 0.0 5
(a)
(b)
TABLE-II : COMPOSITION OF EWAC 1001 EB AND EWAC 1001 EB + 20% CHROMIUM (IN WEIGHT PERCENT)
Parameters The pressure of acetylene The pressure of oxygen Torch angle with respect to plate Distance from sample to torch tip Torch speed Pre heat temperature Flame Temperature Particle Velocity
Quantity 1.2 kpa 3 kpa 60 15 to 20mm 0.1 m/min 300 C 2800 to 3000°C 80 to 100 m/s
International Conference on Advanced Research in Mechanical Engineering (ICARME-2012), 13th May, 2012, TRIVENDUM, ISBN : 978-93-81693-59-9
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Influence Of Cr Addition On Molten Aluminium Corrosion Resistance Of Flame Sprayed AISI H13
C. Molten aluminium corrosion testing Prior to molten aluminium corrosion testing, flame sprayed and uncoated samples were cleaned with acetone and dried. All the samples were initially weighed, to determine the weight losses after the test. Flame sprayed and uncoated samples were immersed in molten aluminium (type 6063) at a temperature of 680 to 690 °C, afterwards the coupons were withdrawn from molten aluminium and put into 20% NaOH to remove the residual aluminium attached with the samples. The weight losses were determined after the test, the weight losses (gm/cm2).h-1 was converted into volume losses (mm3/cm2).h-1
III. RESULTS AND DISCUSSION A. Powder XRD The XRD for the EWAC 1001 EB, Chromium and EWAC 1001 EB + 20% Cr powders used for flame spraying are shown in Fig. 3. The presence of Cr was not detected in EWAC 1001 EB powder due to lower content, but in EWAC 1001 EB + 20% Cr due to the increased percentage of Cr it was detected. The XRD for Cr powder is also shown in the figure. The powders had a spherical morphology as indicated by K Gokul Kumar Et al [8]. B. Optical microscopy of coatings 1. EWAC 1001 EB Coatings:
EWAC 1001 EB
α‐Ni
Optical microscopy of coating cross section is shown in the Fig. 4(a). The hardness of the coating was in the range of 840.9 - 874.1 Hv, while the hardness of the substrate was 611.7 to 653.9. 2. EWAC 1001 EB + 20% Cr coatings:
(a)
Optical microscopy of coating cross section is shown in the Fig. 4(b). The hardness of the coating varied between 700 and 765.9, while the hardness of the substrate varied between 503.8 and 735.5.
C. XRD Analysis of ‘as coated’ samples ‘As coated’ stands for the coating without any tests being carried out on it. The XRD analysis of the ‘as coated’ samples revealed that the Cr present in the coatings was completely oxidized into Cr2O3 [9]. Hence the Cr2O3 is expected to protect the coating by forming a protective layer over the coating which further protects it from corrosion. D. Hardness profiles A Microhardness tester (SMV 1000 Series) was used to measure the microhardness of the samples; a load of 100 grams with a dwell time of 10 sec was used for studying the microhardness. The microhardness profiles are shown in Fig. 5. The highest hardness
International Conference on Advanced Research in Mechanical Engineering (ICARME-2012), 13th May, 2012, TRIVENDUM, ISBN : 978-93-81693-59-9
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Influence Of Cr Addition On Molten Aluminium Corrosion Resistance Of Flame Sprayed AISI H13
achieved in the EWAC 1001 EB coating was 874.1, while for the modified EWAC 1001 EB + 20% Cr coating had a maximum hardness of 765.9. The Hardness of the substrate varied between 732.1 and 503.8. E. Corrosion resistance The results from corrosion testing are expressed in terms of volume losses (mm3/cm2).h-1. The results are shown in Fig. 6. The results show that molten aluminium corrosion resistance was improved in the EWAC 1001 EB + 20% Cr coating compared to the EWAC 1001 EB coating.
coating. Addition of Cr has increased the corrosion resistance by a factor of 1.61.Further examination was carried out by Optical microscopy.
F. Characterization of corroded sample
1000.0 EB 1001+20% Cr 800.0
Microhardness (HV)
1.
EB 1001
Examination of samples by optical microscopy:
600.0 400.0 200.0 0.0
In the Fig. 7(b), it can be seen that large number of pits are formed in the EB 1001 coating compared to the uncoated AISI H13 steel and EB 1001 + 20% Cr coating. The corrosion mechanism is not electro chemical it may be due to the chemical reactions and diffusion processes [10]. The pits in the EB 1001 coating is much deeper when compared to AISI H13 steel and EB 1001 + 20% Cr coating attributed by the fact that AISI H13 steel and EB 1001 + 20% Cr coating has a more stable corrosion resistant condition. IV. CONCLUSION In the present investigation, the influence of chromium in EWAC 1001 EB powder on molten aluminium corrosion resistance has been studied on AISI H13, Flame sprayed EWAC 1001 EB coating and Flame sprayed EWAC 1001 EB + 20% Cr. The conclusions are as follows: • Flame spraying was carried out on AISI H13 steel using the EWAC SuperJet Eutalloy system to prepare coating of the thickness of 100 to 120 µm. Microhardness measurements through the coating revealed that both coatings have higher hardness than substrate material. A ranking between the materials towards molten aluminium corrosion resistance is in the following order EWAC 1001 EB + 20% Cr > AISI H13 > EWAC 1001 EB. The corrosion resistance of EWAC 1001 EB + 20% Cr coating measured in volume loss was increased by a factor of 1.61. The corrosion resistance of the EWAC 1001 EB + 20% Cr was due to the formation of chemically stable Cr2O3.
-0.15
-0.05
0.05
0.15
0.25
0.35
Distance from interface (µm)
Figure 5.
Vickers microhardness profiles of EWAC 1001 EB and EWAC 1001 EB + 20% Cr coatings along the cross section. 2
Volume Loss (mm3/cm2).h-1
1.5 1 0.5 0
A B C Material
A ‐ AISI H13 B ‐ EWAC 1001 C ‐ EWAC 1001+20% Cr
•
•
•
Figure 6.
Volume loss in molten aluminium corroded sample •
The increased corrosion resistance in the EWAC 1001 EB + 20% Cr powder was due to the formation of corrosion resistant Cr2O3 layer on the surface, not only the weight losses even formation of pits was considerably reduced in the EWAC 1001 EB + 20% Cr
International Conference on Advanced Research in Mechanical Engineering (ICARME-2012), 13th May, 2012, TRIVENDUM, ISBN : 978-93-81693-59-9
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Influence Of Cr Addition On Molten Aluminium Corrosion Resistance Of Flame Sprayed AISI H13
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Minlin Zhong, Wenjin Liu, Hongjun Zhang, “Corrosion and wear resistance characteristics of NiCr coating,” Wear, Vol. 260, pp.1349-1355, October 2005. K. Gokul Kumar, N. Arivazhagan, “Studies on influence of WC on scratch and hot corrosion resistance of AISI H13 steel by flame sprayed coatings of EWAC 1001,” ISRN Material Science, Vol. 2011, 10 pages, September 2011. W.J. Tomlinson, A.S. Bransden, Cavitation erosion of laser surface alloyed coatings on Al12%Si, Wear 185 (1995) 59–65. S. Shankar, D. Apelian, Metall. Mater. Trans. 33B (2002) 465.
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International Conference on Advanced Research in Mechanical Engineering (ICARME-2012), 13th May, 2012, TRIVENDUM, ISBN : 978-93-81693-59-9
