Tuning the Surface Characteristic of Al-Si Alloys and Its Impacts on the Formation of Micro Arc Oxidation Layers

Kang Li,Wen Zhu,Zhongmiao Liao,Weimin Li,Ken Chen,Aihua Yi,Wenfang Li

doi:10.3390/coatings11040453

Abstract

Aluminum Silicon (Al-Si) alloys are the most important among cast alloys and have widespread application. The β-Si phase is detrimental to the growth of micro arc oxidation (MAO) layers on Al-Si alloys. In this paper, the β-Si in the skin layer of different Al-Si alloys was removed by an acid etching pretreatment. Then, the impacts of different etching condition on their MAO process were investigated. Results show that, as etching time was prolonged, the Si content on the surface of Al-12 Si was decreased greatly (less than 0.5 wt.%), and the average size of pores left on the sample surface was increased gradually. This then led to the occurrence of some bigger and more drastic discharge sparks at the earlier oxidation stage. The corresponding layers had a higher ratio of Al2O3 and better corrosion resistance after 30 min of oxidation. Moreover, a 60 s etching pretreatment benefitted and boosted the growth rate and energy efficiency of layers on the Al-9 Si, Al-12 Si and Al-15 Si alloys. Such pretreatment was believed to be applicable to restrain the negative effects of Si on the MAO of different Al-Si alloys.

Highlights

IntroductionThe presence of the Si phase makes it difficult to form a uniform anodic film on the surface of Aluminum Silicon (Al-Si) alloys
The etching acid was composed of HF (40 wt.%) and HNO3 (65 wt.%) with a volume ratio of 1: 4. Some samples was immersed entirely into the acid to remove the β-Si phase in the skin layer
The structure and composition at the skin layer of Aluminum Silicon (Al-Si) alloys was modified by an acid etching pretreatment, and the effect on the microarc oxidation (MAO) characteristic of Al-Si alloys was investigated

Summary

Introduction

The presence of the Si phase makes it difficult to form a uniform anodic film on the surface of Al-Si alloys This is because it is difficult to oxidize β-Si under a low temperature, which eventually exists in the alumina film as an inclusion and degrades the film property [8,9]. As a simple and eco-friendly technique, the MAO can fabricate coatings with high performance, e.g., high hardness, good anticorrosion and wear resistance [10,11]. In this regard, the MAO technique is more suitable for the surface strengthening of Al-Si alloys [12,13,14].

Methods

Results

Conclusion