Combining SLM with heat treatment to get 25% elongation in AlSi12 alloys

Where the coarse acidular silicon acts as crack initiation sites in conventional cast alloys, refined eutectic microstructure of Al–Si alloys machined by SLM tends to dramatically improve their mechanical properties - specifically strength and ductility [1]. Here we report the latest findings on AlSi12 where SLM combined with solution heat treatment generates 25% tensile ductility. [2]

The mechanical properties of Al–Si alloys are largely dictated by the morphology of the eutectic silicon [3,4], where finer microstructure commonly generates better mechanical properties.

Conventionally, the refinement of Si phases is achieved by controlling nucleation and growth of eutectic grains via a) chemical modification and elemental additions [5-8] or b) rapid solidification [9-12]. The latter is limited to small or thin parts [9-12]. It’s not a suitable option for large commercial components that require microstructural uniformity and where the entire component must solidify at high cooling rate.

However, high heating and cooling rates (10^3 -10^8 K/s) [13] occur naturally during SLM and results in formation of metastable phases [14-15]. These superheating and supercooling mechanisms enhance nucleation rate and suppress grain growth in Al-Si alloys.

Thermal history (ie remelted areas from overlapping laser tracks or subsequent layers) affects the size and morphology of  as-built dendrites to create a bimodal distribution of  coarse and fine Si particles as well as chemical inhomogeneity in the resolidified melt pool [2]. This post reports the influence of conventional solution heat treatment (T=500C for up to 4h+Water Quench) on the microstructure and mechanical properties of SLMed Al-Si12 samples built on a Realizer SLM machine [2].

Influence of solution heat treatment on eutectic microstructure of SLMed AlSi12 samples

In the as-built condition, the SLMed aluminium matrix is supersaturated with 7% Si nanosized spherical particles [2]. As a comparison, the solubility equilibrium amount in AlSi12 is 1.6%.

As solution heat treatment (SHT) takes place, a few phenomena happen:
-          In early stages of SHT:
o    SHT brings quickly down the amount of Si to a plateau equal to its maximum solubility equilibrium amount (=1.6%) in Al while the excess of Si precipitates out
o   the number of Si particles decreases while their size increases. This is due to particles coalescence and Ostwald ripening.
-          Si particles continue to grow with increased SHT times and remain mostly spherical 

Influence of SHT on tensile strength
SHT reduces tensile strength to its plateau equilibrium value (95Mpa) after 2h at 500C. This is due to:
o   A reduction in solution strengthening as the Si content trapped in the Al matrix decreases rapidly and precipitates out to feed the Si particles localised at the Al grains boundaries;
o   A coarsening of the Si particles during the early stages of SHT as the particles increase in size  and decrease in numbers;

Influence of SHT on enhancement of ductility
The decrease of Si particles numbers associated to their increase in size induces a reduction of localised stress or strain. In addition, SHT reduces residual stresses created during SLM.

High elongation E25%
It is believed [2] that  the distribution of small Si particles (<100nm) at the boundaries of Al grains (due to high cooling rates) and the inhomogeneous chemical distribution due to temperature history combined with SHT is responsible for high elongation value. Yet, the respective influence of these mechanisms is still unknown.

The variation of mechanical properties of the Al–12Si alloy upon solution heat treatment at different times. delta, sigma0.2 and sigmaUTS represent the ductility, yield strength and ultimate tensile strength, respectively [1].

Takeaway

In SLMed ALSi12 components, rapid cooling forms ultrafine eutectic microstructure characterised by spherical  nano-sized Si particles embedded in the Al matrix. Thermal history (ie remelted areas from overlapping laser tracks or subsequent layers) affects the size and morphology of  as-built dendrites to create coarse or fine features. Melt pool temperature can affect the homogeneity of the chemical distribution. Ultrafine microstructure gives rise to better tensile properties than conventional cast Al-12Si parts. Eutectic microstructure of as-built SLMed components can be tailored by SHT times to obtain superior mechanical properties without need for refinement elements.

Experimental parameters: Realizer SLM-100 + AlSi12 (d50=38um) powder – high purity argon – P=200W, s=500mm/s, hatch-150um, layer=50um – substrate=200C 

References
[1] X.J. Wang, L.C. Zhang, M.H. Fang, T.B. Sercombe, Mater. Sci. Eng., A 597 (2014) 370–375.
[2] X.P. Li, X.J. Wang, M. Saunders, A. Suvorova, L.C. Zhang, Y.J. Liu, M.H. Fang, Z.H. Huang, T.B. Sercombe, A selective laser melting and solution heat treatment refined Al–12Si alloy with a controllable ultrafine eutectic microstructure and 25% tensile ductility, Acta Materialia 95 (2015) 74–82
[3] S.D. McDonald, K. Nogita, A.K. Dahle, Acta Mater. 52 (2004) 4273–4280.
[4] Y.-C. Tsai, C.-Y. Chou, S.-L. Lee, C.-K. Lin, J.-C. Lin, S.W. Lim, J. Alloys Compd. 487 (2009) 157–162.
[5] H.Y. Geng, Y.X. Li, C. Xiang, W. Xue, Scripta Mater. 53 (2005) 69–73.
[6] S. Farahany, A. Ourdjini, T.A. Abu Bakar, M.H. Idris, Metall. Mater. Trans. A 45 (2014) 1085–1088.
[7] F. Wang, Z. Liu, D. Qiu, J.A. Taylor, M.A. Easton, M.-X. Zhang, Acta Mater. 61 (2013) 360–370.
[8] K. Narayan Prabhu, B.N. Ravishankar, Mater. Sci. Eng., A 360 (2003) 293–298.
[9] L. Pedersen, L. Arnberg, Metall. Mater. Trans. A 32 (2001) 525–532.
[10] Y. Birol, J. Alloys Compd. 439 (2007) 81–86.
[11] A.M. Bastawros, M.Z. Said, J. Mater. Sci. 28 (1993) 1143–1146.
[12] Y. Birol, J. Mater. Sci. 31 (1996) 2139–2143.
[13] T. Vilaro, V. Kottman-Rexerodt, M. Thomas, C. Colin, P. Bertrand, L. Thivillon, et al., Adv. Mater. Res. 586 (2010) 89.
[14] X.P. Li, C.W. Kang, H. Huang, L.C. Zhang, T.B. Sercombe, Mater. Sci. Eng., A 606 (2014) 370–379.
[15] X.P. Li, C.W. Kang, H. Huang, T.B. Sercombe, Mater. Des. 63 (2014) 407–411.