Journal of Wuhan University of Technology-Mater. Sci. Ed. Apr.2012 389

Preparation of Ni60-Cr3C2 Coating by Plasma Spraying, Plasma Re-melting and Plasma

Spray Welding on W6Mo5Cr4V2

ZENG Zhiqiang1, WANG Huachang2, WANG Hongfu1, WANG Junyuan1

(1.School of Mechanical Engineering and Automation, North University of China, Taiyuan 030051, China; 2.School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China)

Abstract: In order to prepare heat-resistant inner layer of hot-forging die, plasma spraying, plasma re-melting and plasma spray welding were adopted. Cr3C2 coatings of Ni-Based were prepared respectively with 10%, 20% and 30% Cr3C2 powder and W6Mo5Cr4V2 substrate. The coating microstructure analysis, the micro-hardness test, and the measurement of thermal parameters of coating were conducted. The experimental results show that the coating has the better thermo-physical property by using plasma spray welding method with the powder ratio of 90% Ni60 and 10% Cr3C2, and by this way the micro-hardness of coating can achieve 1100 HV.

Key words: W6Mo5Cr4V2; plasma spraying; plasma re-melting; plasma spray welding; thermo-physical property; Ni60-Cr3C2

©Wuhan University of Technology and SpringerVerlag Berlin Heidelberg 2012(Received: Sep. 13, 2011; Accepted: Dec. 21, 2011)ZENG Zhiqiang(曾志强): E-mail: zzhqzhy@sohu.com

DOI 10.1007/s11595-012-0471-x

1 Introduction

Hot forging die is working at very poor conditions, especially in direct contact with the work piece surface. In order to adapt to harsh working environment and improve service life, the hot-forging die is demanded to be with high toughness and plasticity, small thermal expansion coefficient, good thermal conductivity and many other excellent mechanism properties as well as excellent thermo-physical properties[1,3].

The cavity damage of hot-forging die on surface layer was mainly caused by thermal stress, so the decrease of thermal stress on the mold cavity surface layer is the key to improve service life. And gradient materials can ease off the thermal stress, which provides a new way to solve this problem. In this paper preparing heat-resistant cermet layer on the inner surface of the hot-forging die by plasma spraying, plasma re-melting and plasma spray welding is an exploration of the problem that how to strengthen the inner surface of hot-forging die[4,12].

2 Experimental

2.1 Materials

The substrate is W6Mo5Cr4V2, and sample size was Φ35 mm×15 mm. The shot peening was used to strengthen the surface before the fi st sample handing, then high pressure airflow was used to clean dust on surface.

The main alloys used for plasma spray and plasma spray welding were Nickel-based self-fluxing alloy and cobalt-based self-fl uxing alloys. Nickel-based self-fluxing alloys not only have excellent oxidation resistance, wear resistance and corrosion resistance performance, but also have a certain toughness and excellent performance of the plasma cladding. And price is moderate, with a series of commodities powder supply. Therefore, Ni60 was chose as bonding material. The thermal expansion coefficient of Cr3C2 is small; and its thermal conductivity was large enough; so it was a kind of ideal thermal shock resistant ceramic material. The particle size of Ni60 powder in the test was 250 to +400 meshes, and the particle size of Cr3C2 powder was 150 to 300 meshes.2.2 Methods

The coating on substrate (W6Mo5Cr4V2) was prepared with 10%, 20% and 30% Cr3C2 powder. Parts of the samples were plasma sprayed on their surface to prepare coating layer, and then obtain remelted layer by plasma remelting; another parts of the samples were used to prepare spray welding layer by plasma spray welding.

390 Vol.27 No.2 ZENG Zhiqiang et al: Preparation of Ni60-Cr3C2 Coating by Plasma Spr...

2.3 Characterization The microstructure of coating was observed by

a 25800 type scanning electron microscope; micro-hardness of coating was tested by a HV-1000 type sclerometer; an U S production 0.01HZ-51HZ dynamic mechanical thermal analyzer was used in the detection of thermal expansion coeffi cient of the coating; a japan TC-7000H laser thermal constant tester was used to measure thermal conductivity and specifi c heat capacity of the coating .

3 Results and discussion

3.1 The micro-structure of coatingWith ratio of 90%Ni60, 10%Cr3C2 powder as an

example, the comparison of microstructure of coating prepared by plasma spraying, plasma re-melting and plasma spray welding respectively was obtained as below.

Micro-structure of cross-section of 10%Cr3C2

spray coating is shown in Fig.1, in which the coating is a typical layered structure and a rough black line evidently existed between coating and substrate. With uneven surface of the coating, the organization was not uniform and dense, and there were a certain number of porosities. The coating organization was characterized by different shapes of black specks scattered on the continuous substrate metallographic. The unmelted zone and melted zone could be observed in surface layer and buffer layer. The causes of these phenomena: condensation contraction of droplet; stress generated due to the different coefficients of thermal expansion between coating and substrate materials; the structure of coating was sandwich, which formed by repeatedly spraying layer; and spraying powder was uneven

heated by the plasma fl ame stream.Cross-section micro-structure of plasma re-

melting layer is shown in Fig.2. Re-melted layer was uniform white organization without black hard phase. There was closely interconnected transition layer between the substrate and remelting layer without stratifi cation.

The cross-section micro-structure of 10%Cr3C2

Plasma spraying welding layer is demonstrated in the Fig.3 and a thin flat ribbon crystal could be observed between coating and substrate. It appeared bright white when observed under metallurgical microscope after corrosion, which consisted of a flat ribbon crystal of

Journal of Wuhan University of Technology-Mater. Sci. Ed. Apr.2012 391

γ-Ni. The existence of thin fl at ribbon crystal provided evidence that there was good metallurgical bonding between the solder layer and the substrate. It was the reason why Cr3C2 spray welding layer appeared superior performance during the hardness testing. Near the boundary of the substrate and coating there were some black particles which might be unmelted Cr3C2

particles. With the more Cr3C2 content, the more un-melted Cr3C2 particles appeared.

3.2 Micro-hardness of coating As the micro-hardness distribution curves of

coating with three Cr3C2 powder ratios 10%, 20%, 30% are shown in Figs.4-6. These coatings were respectively prepared by plasma spraying, plasma remelting and plasma spray welding.

As shown in Fig.4, micro-hardness curves of three powder coating layer trended consistently. With the increase of distance from the surface, the hardness gradually increased, and then there was a hardness value plummeted, and then the curves gradually increased to reach the value of the substrate. Hardness value suddenly dropped at this point because the layered structure of plasma spraying coating. And the middle part of the binding area was mechanical combination, so micro-hardness was lower than the substrate and coating.

When Cr3C2 content increased, the micro-hardness of the coating layer increased as a whole. So increasing

the Cr3C2 content would improve the micro hardness of the coating layer. But the coating hardness was lower than the hardness of the substrate.

It could be seen in Fig.5, the curves’ trend of the micro-hardness of the re-melted layer were similar. The hardness curves were flat on the cocoting below 500 HV. On the junction area between the coating and substrate, the hardness values improved rapidly until they reached the hardness value of substrate. To improve the hardness of the forging die material was important way to improve the life of the forging die. The hardness of mold chamber should be higher than substrate hardness. It was fail to improve hardness of coating by plasma spray re-melting. which coused by three reason: the sample size was too small, which caused the substrate overheating; strong stirring of plasma fl ame; and the heating time was too long.

In Fig.6, the curves’ trends of the micro-hardness were the same. The micro-hardness reached 1100 HV on the surface and then gently down to the value of substrate. Surface hardness was higher than the hardness of substrate close to 300 HV, and the material had been strengthened very well. the percentage of Cr3C2 The coating hardness was inverse ratio to the percentage of Cr3C2.

The microstructure indicated that the number of black particles at the binding area of coating and substrate increased with the increase of Cr3C2 content.

392 Vol.27 No.2 ZENG Zhiqiang et al: Preparation of Ni60-Cr3C2 Coating by Plasma Spr...

The reason was that the extra Cr3C2 did not melt completely and joined in the metallurgical bonding with substrate; therefore plasma spray welding did not play the role of improving hardness. But in general, the coating of Cr3C2 could be strengthened by spray welding method.3.3 Thermal stress parameters

Theoretical coefficient of thermal expansion, thermal conductivity and specific heat capacity could be calculated according to the composite rule of functionally gradient material parameters. 3.3.1 Thermal expansion coeffi cient of the coating

The stress on forging die was most sensitive to changes of the thermal expansion coefficient, which produce a slighter elastic deformation in the course of their work. The linear expansion coeffi cient α should be a smaller value. The thermal expansion coefficient of the substrate material W6Mo5Cr4V2 is 11.2 × 106 K1; Cr3C2 is 10.9 × 106 K1; and Ni60 is 18 × 106 K1.

In order to obtain such coating with similar thermal expansion coefficient, in theory, should increase the Cr3C2 the content.

The diagram of thermal expansion coefficient listed in Fig.7, It demonstrated that the actual values of plasma re-melted layer were close to theoretical values.But it was quite different to thermal expansion coeffi cient of substrate. In plasma rem-elted layer, with the increase of the Cr3C2 content, thermal expansion coefficient decreased. This trend was consistent with the theoretical value predicted. With the former analysis results, it could deduce that plasma remelting process did not play the role of improving coating performance.

The actual value of thermal expansion coeffi cient of plasma spray welding layer was smaller than the theoretical value. Properties of three kind of coating prepared by plasma spray welding respectively with three kinds of ratio were relatively similar. The actual values of three kind of coating increased while the

Cr3C2 content increased, but change was not obvious.3.3.2 Specifi c heat capacity and thermal conductivity of

plasma spray coatingLarger specific heat capacity of hot-forging die

materials flatted the temperature gradient at the inner chamber of hot-forging die, and made heat pass by successfully in the forging die at the same time. The specifi c heat capacity of Cr3C2 was lower than Ni60 so that powder with less Cr3C2 had higher specific heat capacity. Specific heat capacity of remelted layer and spray welding layer was higher than theoretical value, close to substrate and inverse proportional to Cr3C2 content.

Hot-forging die withstood cycle impact of alternating cooling and heating in the course of working. The internal temperature gradient of forging die should be slight. The thermal conductivity value(k) of forging material should be as large as possible. Thermal conductivity of plasma spray welding coating and re-melting coating was much smaller than theoretical value, while the actual thermal conductivity value of spray welding was close to substrate’s. By the comparison of the three proportions of powder in Fig.9, thermal conductivity of coating with 20%Cr3C2 was the largest, and 10%Cr3C2 the second.

4 Conclusion

The organization of spraying welding layer

Journal of Wuhan University of Technology-Mater. Sci. Ed. Apr.2012 393

was uniform and compact. The hardness of the spray welding was the highest, spraying the second, re-melting the least. The specific heat capacity of the spray welding layer was close to re-melted layer. Both of them were inverse ratio to Cr3C2 content. The actual thermal conductivity of remelted layer and spraying welding layer were smaller than theoretical values. The actual value of spraying welding was close to the substrate.

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