Effects Of The Content Of Li Salt On Formation And Characterization Of Micro-arc Oxidation Coatings Formed On ZAlSi12Cu2Mg1 In Electrolyte

ZAlSi12Cu2Mg1 aluminum alloy is an multivariate alloy of Al, Si, Mg and Mn, it is common used of piston manufacturer. ZAlSi12Cu2Mg1 alloy has some excellence properties, such as good castability and fluidity, small linear shrinkage and the higher air tightness, etc. However, its low hardness,wear resistance and corrosion resistance has limited its application.Microarc oxidation is a surface treatment technique, which can in-situ form ceramic coatings on the surface of some non-ferrous metals, it is expected to improve the properties of ZAlSi12Cu2Mg1 by microarc oxidation treatment.Li and Na play an important role in sintering process of ceramic and can improve its properties. In industry, Li2O and Na₂O are often added in sintering process of ceramic as cosolvent, the aim is to reduce the viscosity of melt, improve the ceramic density and thermal conductivity and promote the process of low-temperature sintering of the ceramics. In this work, Li2SO₄, Li2CO₃ and Na₂CO₃ with different content were added into Na₂SiO₃ system electrolyte separately.Their thickness and cross-section morphology of oxidation coatings were measured and observed. The surface morphology,phase compositions and the content of element of the oxidation coatings were also investigated by SEM,XRD respectively.The influences of content of Li and Na ions in basic electrolyte for characteristics of the micro-arc oxidation coatings were studied.The results show that the micro-arc oxidation coatings on ZAlSi12Cu2Mg1 alloy were prepared using the microarc oxidation technique in Na₂SiO₃ 8.0 g/L+NaOH 2.0 g/L+EDTA 2.0 g/L electrolyte（electrolyte E1）, its thickness is 89μm; It is found by cross-section morphology that the coating is composed of the two-layer structure with compact layer and loose layer,the substrate and coating penetrate each other,presenting metallurgy combination.The thickness of oxide film increases from 133μm to 164μm with the increasing of content of Li₂SO₄ in electrolyte E1 from 0.5 to 2.5g/L;But the thickness of compact layer relatively lower and there are a lot of microspores exist in it,its compactability in coatings decreases and the roughness of the surface of coatings increases obviously by adding Li₂SO₄.According to SEM it is found that there are a large number of pits on the surface of coatings while the content of Li2SO₄ was in excess of 1.0g/L. When the content of Li₂SO₄ rises to 2.5g/L, spot flaking on the parts of the coatings occurs. The thickness of oxide film varying from 45μm to 165μm with the increasing of content of Li₂CO₃ in electrolyte E1 from 0.5 to 2.5g/L,the thickness of coating is about 45μm while the content of Li₂CO₃ reaches to 1.5g/L; the compact coating in microarc oxidation is c.a.75% of entire film, while the content of Li₂CO₃ reaches to 1.5g/L,and there is few microspores existed in it. Furthermore, the viscosity of the oxide on coatings surface reduce evidently and the oxidation coatings uniformly distributed on the sample surface with the increasing of content of Li₂CO₃.The content of Na₂CO₃ was increased from 0.25 to 1.0g/L in electrolyte E1,the thickness of oxide film ranges from 126μm to 135μm.The thickness ratio of compact layer increases,the number of microspores become less and less and the compactability of coatings is also enhanced with addition Na₂CO₃ into the electrolyte E1. But continue to change the content of Na₂CO₃, the characteristics of the oxidition coatings have less change.The thickness of oxide film increases from 36μm to 112μm with the increasing of content of Li₂CO₃ from 3.0 to 6.0 g/L in the Na₂SiO₃ 8.0g/L+NaOH 1.0 g/L+EDTA 2.0 g/L electrolyte （electrolyte E2）, and the thickness ratio of compact layer and the compactability of coatings increase gradually.It is found by SEM that the viscosity of the oxide on coatings surface is very small while the content of Li₂CO₃ reaches to 3.5g/L the roughness of the surface of coatings and the number of oxidation granules on coatings surface increase with the increasing of content of Li₂CO₃. But the bottom of film is still relatively flat.The XRD results show that the microarc oxidation films were mainly composed ofα-Al2O₃,γ-Al2O₃, mullite and amorphousness;The results of EDS analysis indicate that the distribution of chemical elements of films surface are mainly O,Al and Si,as well as a trace amount of Na and Mg.