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Effect of heat treatments on the microstructure and mechanical properties of Al-Mg-Sc-Zr alloy fabricated by selective laser melting

https://doi.org/10.1016/j.optlastec.2021.107312Get rights and content

Highlights

  • A rapid ageing at 1 h 330 °C for an Al-Mg-Sc-Zr alloy was explored with a good mechanical performance.

  • Homogenization at >2 h 500 °C causes a reduced tensile strength & ductility compared with as built.

  • Strengthening mechanism in an SLMed Al-Mg-Sc-Zr alloy is fine grains and precipitation of Orawan.

Abstract

High-strength aluminum alloy with Sc-Zr is specially optimized for Selective Laser Melting (SLM). The alloy has a great potential in structurally optimized lightweight parts in the fields of aerospace, nuclear power and automobile due to its good formability, fine microstructure, and excellent mechanical properties. An appropriate heat treatment for this alloy is critical to improve its ultimate tensile strength. However, the effect of heat treatments on the microstructure and mechanical properties of SLMed Al-Mg-Sc-Zr alloy is not fully understood and heat treatment needs to be optimized. In this investigation, the effect of 500 °C homogenization and 330 °C ageing on the microstructure and properties of the SLMed parts were investigated. The results show that a homogenization treatment at 500 °C does not increase the solubility of the alloy elements, but accelerates the precipitation of Sc/Zr elements and recrystallization. Moreover, homogenization with holding time longer than 2 h results in a decrease of mechanical properties including both tensile properties and ductility compared with those of the as-built state. Ageing treatment at 330 °C for 1 h drives the second phase of Al3(ScxZr1-x) to fully precipitate, and long-term ageing for 8 h neither leads to recrystallize nor deteriorates the properties of the alloy. A rapid ageing heat treatment at 330 °C for 1 h was explored, with a tensile strength of 479 MPa, a yield strength of 441 MPa, an elongation after break of 14.5%, and a shrinkage of 45%. Specimens of SLMed Al-Mg-Sc-Zr alloy are strengthened by fine grains whose formation is driven by extremely high temperature gradients and solidification rates, and precipitation of the second phase Al3(ScxZr1-x) caused by ageing. The Orawan bypass mechanism plays an important role in the strengthening of the aged SLMed Al-Mg-Sc-Zr alloy.

Introduction

Selective Laser Melting (SLM) is an eruptive metal additive manufacturing (or 3D printing) method which can be used to build three-dimensional metal parts from a digital model by adding thin layers of metallic or pre-alloyed powders progressively. This embedded principle provides SLM distinctive advantages in the direct manufacture of customized metal function parts, without the constrains of complicated structures and the need of eliminating the expensively time-consuming tooling compared to other conventional manufacturing processes. Therefore, SLM is becoming a significant method in various industries fields including aerospace, medical, energy, automotive, marine and other industries [1].

Aluminum alloys are widely used in aerospace, automotive [2], and aircraft industries for their recyclability, excellent strength-to-weight ratio, thermal and electrical conductivity, corrosion resistance, formability and other advantages [3]. The most widely used aluminum alloys in the field of SLM are Al-Si series alloys such as AlSi10Mg, AlSi12, AlSi7Mg, etc., owing to their good fluidity and building performance. However, the yield strength (YS) of the Al-Si alloys without post heat treatment is lower than 290 MPa and ductility only about 5-10% [4], [5], [6], [7], respectively. Such mechanical properties cannot meet the requirements of some medium- and high-strength structural parts in aerospace fields [8]. Therefore, many researchers have carried out investigations on SLMed high-strength aluminum alloys.

Many researchers have tried to strengthen Al-Si series alloys by adding Cu and other elements and improved their mechanical performance. Lin et al [9] investigated the effects of conform continuous extrusion and subsequent heat treatment on the mechanical properties and wear-resistance of the high-alloying Al–13Si–7.5Cu–1Mg alloy. They found that for this alloy, heat treatment greatly improved its properties, especially the ultimate tensile strength (UTS) from 112.6 MPa to 486.8 MPa with a solid-solution treatment at 494 °C for 1.5 h and ageing at 180 °C for 4 h. A.V. Pozdniakov et al [10] found that fine eutectic microstructure of AlSi11CuMg alloy was formed by SLM. After solution treatment at 515 °C and ageing at 180 °C for 8 h, the YS, UTS and percentage elongation (El) of the alloy reach 288 MPa, 354 MPa and 5.4%, respectively. Dario Rafael Manca et al [11] selective laser melted a novel Al-12Si-1.4Fe-1.4Ni alloy with minor additions of Cu. The alloy is strengthened by fine structure formed by Si, Al5Fe(Ni, Cu) and Al3(Ni,Cu) phases and has a high compression strength of 355 MPa at 200 °C. J. Fiocchi et al [12] revealed that the mechanical behavior of the SLMed AlSi9Cu3 alloy outperformed the conventional cast alloy, which according to the authors, is caused by the highly oversaturated Al matrix and the refinement of the eutectic structure by the high cooling rates of the SLM process. A precipitation hardening performance (UTS = 318.7 MPa, YS = 206 MPa, El = 7.8%) of the SLMed specimens was obtained by ageing treatment (T6). M. Roudnická et al [13] investigated the strengthening mechanism of AlSi9Cu3Fe alloy fabricated by SLM. The alloy has a UTS of 380 ± 13 MPa, YS of 326 ± 23 MPa and El of 2.6 ± 0.2% after T6 treatment. It has been revealed that the specific response of the material includes simultaneous precipitation of semi-coherent θ’ precipitates and Si platelets. N.V. Dynin et al [14] presented the microstructure and corresponding mechanical properties of an advanced aluminum alloy Al-10Si-0.9Cu-0.7 Mg-0.3Zr-0.3Ce. The alloy has a UTS > 390 MPa, YS > 330 MPa and El > 6% after T6 heat treatment.

Martin et.al [15] introduced nanoparticles of nucleates to 7075 and 6061 high-strength aluminum alloys to control solidification during SLM. Montero Sistiaga found that 7075 alloy can be tailored by adding 4% silicon which increases the density of SLMed specimens to 99%. Moreover, microcracks were reported to be significantly reduced owing to the grain refining effect caused by the addition of silicon [16]. Li et.al from the University of Leuven in Belgium developed an in-situ nano-TiB2 decorated AlSi10Mg composite powder fabricated by gas-atomization for SLM [17]. The corresponding SLMed parts have UTS of ~530 ± 16 MPa, El of 15.5 ± 1.2% and the microhardness of 191 HV0.3. The preparation process of the powder, however, is so complicated that large-scale engineering application are facing many difficulties.

Jia et al [18] introduced a high strength Al alloy Al-Mn-Sc specifically for SLM by employing Mn and Sc as major strengthening elements. This new high strength Al alloy has a YS of up to 560 MPa and a ductility of about 18% after a post heat treatment at 300 °C for 5 h. Such multiple strengthening mechanisms include grain boundary strengthening, solid solution strengthening and precipitation strengthening. This Al-Mn-Sc alloy also has a fine equiaxed-columnar bimodal grain structure.

D. R. Manca et al [19] developed a novel heat resistant SLMed Al–3Ce–7Cu alloy. The as-printed alloy has a YS of 274 MPa, a UTS of 456 MPa and an El of 4.4% and shows high stability under high temperature. A. Plotkowski et al. [20] developed an Al-10Ce-8Mn (wt.%) alloy for SLM, whose strength is 2–4 times higher than that of the general aluminum alloys at elevated temperatures ranging from 300 °C to 400 °C.

As to Al-Mg-Sc-Zr alloys for SLM technology, Airbus Group Innovations developed a new Sc- and Zr-modified 5xxx Al-Mg alloy, Scalmalloy® [21], [22]. During the traditional forming of aluminum alloy like casting, Sc and Zr are often added to improve the mechanical properties because of precipitation [23]. With appropriate ageing treatments, Sc inside Al matrix completely precipitates out, which increases the UTS of the aluminum alloy [24]. Although SLMed alloys have unique microstructure including columnar crystals and equiaxed crystals unlike casting alloys [25], Sc and Zr can still be utilized to improve the mechanical performance of aluminum alloys [26].

Scalmalloy® serves as an ageing hardenable alloy and displays exceptional properties such as UTS of 500 MPa, YS of 450 MPa and El of 8.6 ± 1.9%, while the associated anisotropy in ultimate values (UTS, El) remains less than 5%. It has very fine-grained bi-modal microstructure with grain sizes varying from ≈ 200 nm to less than 15 μm due to significant grain boundary pinning by different particles including Al3(Sc/Zr) [27]. In that paper, an optimized post-process heat treatment was proposed at between 325 °C and 350 °C, with an ageing duration between ≈ 4 h and ≈ 10 h to achieve the optimum material strength as mentioned above.

H. Fang et al [28] reported that the nanometer-size second phase Al (Sc, Zr) precipitates from Al-Mg-Sc-Zr alloy could hinder grain boundary movement and strongly inhibit alloy recrystallization. After annealing at 500 °C for 1 h, there was no obvious recrystallization and only a few sub crystals appeared. They also found that an increase of annealing temperature resulted in a decrease in alloy strength and an increase in plasticity. When annealed at 300 °C, the YS, UTS and El of the alloy were 278 MPa, 398 MPa and 19%, respectively. This alloy also has the best corrosion resistance.

R. Li et al [29] investigated the effect of ageing treatment on the microstructure and mechanical properties of Al-3.02 Mg-0.2Sc-0.1Zr alloy fabricated by SLM. The UTS of 400 MPa and YS of 327 MPa can be obtained with the optimal ageing parameters.

A.Y. Churyumov et al [30] proposed a novel Al-4.5 Mg-0.32Sc-0.66Zr alloy for selective laser melting with minimum porosity of 0.5% at optimized parameters. One stage annealing at 360 °C/6h and two stage annealing at 300 °C/3h + 360 °C/4h were used, and the resulting plasticity is 6% higher after two-stage annealing than after one-stage annealing owing to more homogeneously distributed Al3(Sc, Zr) precipitates. An optimum strength-plasticity combination was obtained after two-stage annealing of the specimens fabricated in the XY direction: YS = 435 MPa, UTS = 478 MPa and El. = 16%.

D. Gu et al. [31], [32] revealed the effect of “island” laser scanning strategy of SLMed Al-4.2 Mg-0.4Sc-0.2Zr alloy on its performance. The improved mechanical properties with UTS > 500 MPa, El between 10% ~ 12% and high thermal stability are caused by the formation of high density nano-sized Al3(Sc, Zr) precipitate phase.

Z. Wang et al. [33] reported that the YS and ductility of Al-4.66 Mg-0.72Sc-0.48Mn-0.33Zr-0.12Fe-0.028Si alloy prepared by SLM reached 335 ± 4 MPa and 23.6 ± 1.9%, respectively. The corresponding microstructure has a heterogeneous grain structure with ultrafine equiaxed grains bands and columnar grains domains.

Heat treatment plays an important role on the structure and properties of high-strength aluminum alloys. The effect of heat treatments on the structure and properties of high-strength aluminum alloys is still not fully understood, especially for high-strength aluminum alloys with different chemical compositions. Furthermore, holding time of heat treatments for this Al-X-Sc/Zr high strength aluminum alloy is a bit too long.

In this paper, a series of SLM experiments with Al-Mg-Sc-Zr alloy were carried out. A variety of homogenization and ageing heat treatments with different hold time were performed with the SLMed specimens. The microstructure and performance of these specimens were investigated, and the strengthening mechanism was discussed.

Section snippets

Materials

The chemical composition of the Al-Mg-Sc-Zr alloy powder used in this investigate is shown in Table 1. The major elements are Al, Mg, Sc, Zr, and the metallurgical impurity elements of Cu and Cr remain relatively low. At room temperature, the solubility of Mg in Al is only 1.9%. The eutectic Mg5Al8 should precipitates out at slow cooling rates. However, only α-Al phase was detected by X-ray diffraction as shown in Fig. 1. On one hand, most of the Mg is still supersaturated in the α-Al phase

Microstructure and mechanical properties of selective laser melted parts of Al-Mg-Sc-Zr as built

The optical microscopic metallography of the Al-Mg-Sc-Zr alloy fabricated by SLM is shown in Fig. 4. A photomicrograph of a section perpendicular to the building direction (Z axis) or XOY plane is presented in Fig. 4(a). From this figure, the structural feature is composed of the melted beads which is caused by the scanning strategy of the 67° rotation angle between the upper and lower layers. No obvious defects can be found in the parallel melted bead of each layer which can be seen only on

Conclusions

In this study, the influence of homogenization and ageing heat treatments on the microstructure and mechanical properties of the Al-Mg-Sc-Zr high strength aluminum was investigated. The following conclusions can be drawn:

  • 1)

    A homogenization treatment at 500 °C does not increase the solubility of the alloy elements but accelerates the precipitation of Sc\Zr elements and recrystallization. This homogenization with holding time less than 10 min causes an increase of tensile strength and a decrease of

CRediT authorship contribution statement

X.F. Shen: Conceptualization, Writing - original draft. Z.Y. Cheng: Methodology. C.G. Wang: . H.F. Wu: . Q. Yang: . G.W. Wang: Formal analysis, Supervision. S.K. Huang: Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by supported by NSAF (grant number U1930207) and CAEP Foundation (grant number CX20210005).

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