In situ synthesis and characterization of Fep/Cu composite coating on SAE 1045 carbon steel by laser cladding

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Abstract

Fe particulate composite coatings have been successfully in situ synthesized with a preplaced mixture of Cu+5 wt.% Al on SAE 1045 carbon steel substrate using a 2 kW CW CO2 laser. Microstructural analysis reveals that Fe-rich spherical particles are embedded in the Cu-matrix. At a low scanning speed of 5 mm s−1, the Fe-rich spherical particles are homogeneously dispersed in the matrix other than near the bonding interface where there are dendrites. Upon increasing the scanning speed (9 or 13 mm s−1), the dendritic microstructures disappear, the distribution of the Fe-rich spherical particles becomes heterogeneous and the Fe-rich belt along the convection currents can be observed in the clad. Area percentage of these Fe-rich particles and electrical resistivity decreased with an increase of the scanning speed. Finally, the in situ formation mechanism of the Fe-rich spherical particles is discussed in detail.

Introduction

Synthesis of metal matrix composites (MMCs) by surface incorporation of reinforcement particles on various substrates for providing a better performance has attracted extensive attention in recent years. Among the various synthesizing techniques, laser surface alloying (LSA) or laser cladding is a promising technique because it can produce the desired shape and distribution of the reinforcement phase [1], thus providing remarkable enhancement on corrosion resistance, wear resistance and thermal conductivity without impairing the bulk properties.

Usually, the additional reinforcement particles such as WC, TiC, SiC, ZrO2 or Al2O3 ceramics are directly injected into the high temperature molten pool created in the substrate by the laser beam (one-step method) or preplaced on the surface of the substrate with a chemical binder to form a coating and then to be irradiated by laser (two-step method) [2], [3], [4]. However, the shape and chemical composition of additives hardly remain unchanged due to the dissolution into the metal liquid or metallurgical reactions with the environment resulting in the deterioration of the toughness and crack resistance of MMCs [5], [6]. In order to overcome these limits, in situ synthesis of composite coatings by laser cladding or LSA as a promising technique has been investigated recently [7], [8], [9], [10], [11]. Park and his colleagues [7] have successfully synthesized TiC particulate composite coatings on cast iron surfaces by laser alloying CP Ti and reacting in situ with carbon contained in cast iron. Also, copper particulate composite coatings on iron substrate have been produced by LSA of copper for enhancing the thermal conductivity of steel surfaces [10]. However, there is limited research on the in situ synthesis of the composite coatings.

During laser cladding, only a few substrate materials are melted and brought into the molten pool resulting in a good metallurgical bond. When the components of an immiscible system such as Cu–Fe alloys are selected as the substrate and clad materials, respectively, the reinforced particles are in situ synthesized via laser cladding or LSA [9], [10]. The distribution and percent content of the particles can be controlled by the laser processing parameters such as the power density or scanning speed. In the present study, an attempt was made to in situ synthesize an Fe particulate composite coating by laser cladding of copper on SAE 1045 carbon steel using a CO2 laser with the particular emphasis on the formation mechanism of the particle phase and its relation to the scanning speed. This particulate composite coating as an electric contact material may have a potential application in the electrical engineering industry.

Section snippets

Experimental procedures

SAE 1045 carbon steel with an area of 40×30 mm2 and thickness of 8 mm was used as substrate material in the present study. Its chemical composition in wt.% is 0.46% C, 0.24% Si, 0.64% Mn, 0.012% P, 0.028% S and Fe in balance. The alloying powders were pure copper powder (99.9%, about 20 μm in size) mixed with 5 wt.% aluminum (99.9%, about 20 μm in size) as additives which can decrease the specific energy and improve the clad quality [9]. Laser cladding was carried out with the two-step method.

Results

A transverse cross-sectional view of a single track clad sample at a scanning speed of 5 mm s−1 as shown in Fig. 1(a) exhibits three regions: clad zone, heat affected zone (HAZ), and substrate. Smooth and crack-free clad coatings were formed and metallurgically bonded with the substrate. The height of clad is approximately 1 mm. There are a few gas pores in the alloyed zone (marked by an arrow), which can be eliminated by adjusting the laser parameters. At 5 mm s−1, the microstructure in the

Temperature profile in the clad and cooling rate at the solid/liquid interface

To conveniently discuss about the formation mechanism of the Fe-rich spherical particles and effect of the scanning speed on the microstructural evaluation, an analytical heat flux model for laser cladding established by Piscasso [20] was used to calculate the temperature profile and the cooling rate at the solid/liquid interface under three scanning speeds V (5, 9, 13 mm s−1). According to the model with a constant moving Gaussian heating resource, the temperature T within the material is

Conclusions

The synthesis and characteristics of an Fep/Cu composite coating on an SAE 1045 carbon steel by single track laser cladding using a CO2 laser, especially the in situ formation mechanism of the Fe-rich spherical particles were investigated, yielding the following conclusions: (1) Fep/Cu composite coatings were successfully synthesized in situ by laser cladding of Cu+5% Al on SAE 1045 carbon steel. The phase-separated microstructure clearly indicates the existence of a large undercooling of the

Acknowledgements

Financial support by Natural Science Foundation of Hubei, China under research grant (Number 99J046) and Open Foundation of the State Key Laboratory of Plastic Forming Simulation and Mould Technology is gratefully acknowledged.

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