Analysis of processing parameters and characteristics of selective laser melted high strength Al-Cu-Mg alloys: From single tracks to cubic samples

Abstract

The ever-increasing demands for lightweight alloys parts with complex shapes in the aerospace industries lead the development of selective laser melting (SLM) technology. The effect of changing scanning speed on the formation of single track, as well as the overlapping rate to form multi tracks during SLM processed (SLMed) Al-Cu-Mg alloy was investigated. A processing map for SLMed single track of Al-Cu-Mg alloys was developed. Under different processing parameters, the single track can be characterized as unstable, stable with cracks and stable without cracks. For geometrical characteristic of melt pool of single track, the width and remelted depth are positively correlated with energy input, whereas the height and the contact angle are to the contrary. As for multi tracks, four typical morphologies have been identified, and a processing map was established to select the optimum processing parameters for the SLMed parts without defects. Finally, high density cubic samples (99.91%) without imperfections and microcracks were obtained using optimum processing parameters. In addition, the relationship between microstructure and hardness was discussed. The results provide an important experimental guidance for obtaining high-strength Al-Cu-Mg alloys dense parts with good performance.

Introduction

Recently, considerable interest has been generated in the additive manufacturing, as stated by Olakanmi et al. (2015) and Zhang and Attar, (2016). Selective laser melting (SLM), as one of the layer-based additive manufacturing (AM) technologies, is a processing manner that forms the three-dimensional parts from powder by using a high power density laser beam to melt metallic powder. Unlike conventional manufacturing processes, SLM enables the production of complex-geometry components with enhanced design flexibility, low material wastage and little post processing. Nowadays, more and more literatures have been focused on this emerging area. Materials such as stainless steel, titanium-based and nickel-based alloys have been successfully fabricated using SLM and applied in aerospace, biomedicine and many manufacturing industries.

The SLM involves building a 3D part layer-by-layer through by using a laser beam to scan a powder bed track-by-track. Therefore, the single track formation is the basic element in complex SLM processes. The single track formation of stainless steel, titanium alloys and Co-Cr-Mo alloys with SLM has been intensively investigated. For instance, Yadroitsev et al. (2010) investigated the formation of single tracks of 304L and 904L powder by SLM. The results show that there exist stability and instability zones characterized by the formation of a stable melt pool and continuous tracks. Instabilities appear at low scanning speed in the form of irregularities, and high scanning speed gives rise to the balling effect. Song et al. (2012) proposed a way to optimize the processing parameters for the SLM of Ti6Al4V alloy, and three different types of single tracks were identified: melting with cracks, continuous melting and partial melting. Wei et al. (2017) studied the processing parameters for 3D Ti-5Al-2.5Sn components through single track and single layer formation processes. Ciurana et al. (2013) demonstrated the effect of energy density on the formation of single tracks from CoCrMo powder. The minimum value of energy density for forming continuous tracks is 151 J/mm³.

However, it is challenging to use SLM technology to produce the aluminum alloys, due to their high reflectivity and conductivity (mentioned by Louvis et al. (2011)). At present, most of the researches focus on Al-Si casting alloys (Kempen et al., 2014). Microstructure, static mechanical properties, different heat treatments and dynamic mechanical properties of Al-Si casting alloys fabricated by SLM were concerned more (Li et al., 2015). In contrast, there are only a few literatures focused on the processing parameters optimization. Kempen et al. (2014) investigated the single tracks of AlSi10Mg, and a processing parameter window was defined, in which the formed melt pool is stable and meets the set requirements. Later, Aboulkhair et al. (2016) discussed the effect of parameters on the formation of single tracks from AlSi10Mg powder, as well as their overlap to form a single layer. It is found that the dimensions of tracks decrease linearly as the scanning speed increasing.

In contrast, fewer works focused on SLM of 2XXX series high strength aluminum alloys. Al-Cu-Mg alloys have been generally considered as a valuable structural material with extensive application in aerospace and automobile industry due to its low density, high strength and fatigue strength. SLM implies complete melting, high cooling rates, high temperature gradient and multiple heat cycles. The solidification rate induced by high energy laser melting can reach values as high as 10⁶–10⁸ K/s (Das et al., 2010). Inevitably, processing high strength aluminum alloys by SLM is confronted with great difficulties because of its poor flowability, high reflectivity, high thermal conductivity, large solidification range and hot cracking susceptibility. Ahuja et al. (2014) studied SLM of AW-2618 and AW-2219, good quality single tracks and fully dense 3D structures were observed. SLM of high-strength aluminum-copper alloys AW-2022 and AW-2024 were also performed. The scanning speed between 44.9 mm/s and 250 mm/s seems to be better suited for single tracks formation. The processing windows of single tracks and thin walls were obtained. But the microstructure and mechanical properties were not mentioned by Karg et al. (2014). Zhang et al. (2016) successfully fabricated nearly fully dense Al-4.24Cu-1.97Mg-0.56Mn cubic samples with good mechanical properties. Whereas, the formation of single track and multi tracks was not investigated in detail.

To achieve the stable formation with excellent performance, a comprehensive understanding of single tracks of Al-Cu-Mg alloys is required. However, few works have been done to investigate the evolution from single tracks to cubic specimens for SLMed Al-Cu-Mg alloys in detail. In this study, the effect of processing parameters on the formation of single track, multi tracks and cubic samples for SLMed Al-Cu-Mg alloys was investigated firstly. Then the optimal processing windows were obtained. Finally, the mechanical properties were analyzed in correlation to the changes in microstructure.