Evaluation of corrosion resistance of AlSi10Mg and AlSi7Mg alloys manufactured by LPBF

Introduction: Laser powders bed fusion (LPBF) is a technique based on fusion of metal powder - layer by layer - by using a laser. This method has advantages in terms of cost reduction and manufacturing time and it is able to eliminate the metal scrap by traditional subtractive machining. However, it leaves rough surfaces and can greatly influence the corrosion behavior of the alloy compared to traditional casting. Materials and Methods: tests were performed on two aluminium casting alloys having different silicon content - AlSi10Mg and AlSi7Mg alloys. The specimens were obtained along two building directions, with the test surface parallel or perpendicular to the deposition plane – i.e. the scanner laser plane – on as-processed or polished surfaces. After manufacturing, the specimens were stress relived at 300°C for 2 h. Pitting potential were estimated by potentiodynamic tests at room temperature in Harrison solution at 10 mV/min scan rate. The susceptibility to intergranular corrosion was evaluated according to ISO 11846 standard. Results and Discussion: The results show modifications of passive film and metal microstructure induced by LPBF, relevant for the corrosion resistance of aluminum-silicon alloys. The results show a marked influence of surface conditions on pitting potential. The lowest pitting potentials measured on both as-processed alloys denotes a worse resistance to localized attack due to the formation of less protective film during manufacturing and heat treatment. The pitting potentials are in any cases similar for both the alloys. During the LPBF process, the film forms into inert atmospheres with very low oxygen partial pressures and afterwards it evolves during heat treatment at high temperatures – i.e. under conditions quite different from atmospheric exposure. The mechanical polishing removes such poor film and restore the intrinsic resistance of metal matrix. However, the unique microstructure produced by the LPBF process also affects the corrosion behavior of aluminum silicon alloy. The propagation of localized attack occurred on both the alloys by selective dissolution of aluminum along the border of melt pools, which distinguishes the microstructure of alloys produced by LPBF. Selective corrosion strictly depends upon the presence, distribution and dimensions of silicon particles