Microstructural Refinement and Improvement of Microhardness of a Hypoeutectic Al–Fe Alloy Treated by Laser Surface Remelting

Ricardo Oliveira,Amauri Garcia,Karina C B Cangerana,Rafael Kakitani,Noé Cheung

doi:10.3390/ciwc2020-06813

Abstract

In laser surface remelting (LSR) treatment, only a small region is affected by heat, surpassing the melting temperature, followed by rapid cooling at 103–108 K/s, thus producing an extremely refined microstructure. The treated region shows a more homogeneous microstructure and better mechanical properties as compared to the substrate. Iron is a common impurity found in Al-based alloys but in the 2618 commercial alloy, around 1 wt.% of Fe is intentionally added to improve the high temperature strength and the corrosion resistance. In this work, LSR experiments were performed, by using a CO2 laser operating in a continuous-wave mode, to investigate the influence of process parameters on the treated surface of an as-cast Al-1 wt.% Fe alloy. These parameters encompass work distance (z), laser beam speed (v) and laser average power (P), setting a total of 18 combinations. The configuration of z = 6 mm, v = 500 mm/s and P = 800 W resulted in a molten pool with 710 µm of width for 242 µm of length without major porosities, therefore being the largest stable pool amongst all parameter combinations. The resulting cellular microstructure is shown to have an average interphase spacing of 0.93 ± 0.17 µm, a decrease of about 14 times in relation to that of the substrate. The effects of LSR on microhardness were remarkable, with the remelted track presenting Vickers microhardness of 50.1 ± 2 HV, which corresponds to increase of about 43% as compared to that of the original substrate.

Highlights

In commercial Al-based alloys, the usual alloying elements are Si, Mg, Cu and Zn, but Fe is generally present as an impurity, even though in some alloys the presence of Fe is shown to favor the mechanical properties at high temperatures [1]
This work aims to investigate the influence of process parameters of a laser surface remelting (LSR) treatment, on the remelted tracks at the surface of an as-cast Al-1wt.%Fe alloy, in order to evaluate their influence on the dimensions of the molten pool and on the resulting microstructure, as well as on the microhardness of the laser treated region
The refinement result of the treatment can already be seen, as the cellular microstructure of the substrate can be clearly observed with a horizontal growth direction, the remelted pool microstructure needs a much higher magnification to be analyzed

Summary

Introduction

In commercial Al-based alloys, the usual alloying elements are Si, Mg, Cu and Zn, but Fe is generally present as an impurity, even though in some alloys the presence of Fe is shown to favor the mechanical properties at high temperatures [1]. The main drawbacks of Fe are the low solubility in Al (maximum of 0.052 wt.% [2]), leading to the precipitation of brittle intermetallics (IMCs) with low cohesion with the aluminum matrix, e.g. β-AlFeSi and Al3Fe [3]. Another disadvantage concerns the disturbance on heat treatment efficiency, due to the formation of phases with low diffusivity, such as Al7Cu2Fe and π-Al8Mg3FeSi6 [4]. Wang et al [10]

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