Aluminizing of Metal Surfaces by Electric-Spark Alloying

Abstract

The analysis of the influence of the parameters of electrospark alloying with an aluminium electrode on the quality (roughness, microstructure of the coating, its continuity, phase composition, and microhardness) of the aluminized layer is presented. The effect of finishing methods after aluminizing is evaluated. The heat resistance of the obtained coatings is studied. Metallographic analysis shows that the coating consists of three sections: a ‘white’ layer, a diffusion zone, and the base metal. With an increase in the discharge energy, such quality parameters of the surface layer as thickness, microhardness of both a ‘white’ layer and a transition zone, and roughness are increased. The continuity of a ‘white’ layer at the discharge energy Wp = 0.52 J is low (of 50–60%); with a subsequent increase in the discharge energy, it increases and, at Wp = 6.8 J, it is of 100%. An increase in the discharge energy during electric-spark alloying (ESA) leads to a change in the chemical and phase compositions of the layer: at low discharge energies, a layer is formed, consisting mainly of α-Fe and aluminium oxides. As Wp increases, the layer consists of iron and aluminium intermetallic compounds, as well as free aluminium, that is confirmed by the data of local x-ray microanalysis. For practical application, it is possible to recommend the process of aluminizing by the ESA method, using the modes (discharge energy in the range of 4.6–6.8 J and productivity of 2.0–3.0 cm2/min). Such process provides the formation of a ‘white’ layer with a thickness of 70–130 µm, microhardness of 5000–7500 MPa, roughness (Ra) of 6–9 µm, and continuity of 95–100%. In order to increase the thickness of the aluminized layer, it is recommended to preliminarily apply grease containing aluminium powder to the steel surface and, without waiting for it to dry, carry out ESA with an aluminium electrode. In this case, the coating continuity is of 100%, the layer thickness is of up to 200 µm, and the microhardness is of 4500 MPa. The paper presents the results of a study of the quality parameters of multicomponent aluminium-containing coatings of Al–S, Al–C–S, and Al–C–B systems. Replacing the aluminium electrode with graphite one leads to a decrease in the thickness and continuity of a ‘white’ layer, respectively, to 50 µm and 30%. In turn, the microhardness on the surface increases to 9000 MPa. The addition of 0.7 boron to the consistency substance leads to an increase in the thickness and continuity of a ‘white’ layer, respectively, up to 60 µm and 70%. The microhardness on the surface rises to 12000 MPa. In order to reduce the roughness of the surface layer and to obtain continuous coatings, it is recommended to carry out ESA with an aluminium electrode, but at lower modes.

Keywords

• electrospark alloying
• coating
• aluminizing
• microhardness
• continuity
• roughness
• structure
• x-ray diffraction analysis
• x-ray spectral analysis