Research article
Thermal fatigue testing of laser powder bed fusion (L-PBF) processed AlSi7Mg alloy in presence of a quasi-static tensile load

https://doi.org/10.1016/j.msea.2020.139617Get rights and content

Abstract

The AlSi7Mg (A357) alloy is a suitable material to be used in different industrial applications. Even though the majority of parts is produced by casting, a large interest is raising on laser additive manufacturing of these alloys owing to their good processability. Service conditions of several applications, including cylinder heads and exhaust manifolds, cause the alloy to undergo fluctuating thermal and mechanical loads. The thermal fatigue of A357 alloy processed by Laser Powder Bed Fusion is here investigated using a Gleeble® 3800 equipment. The material is thermo-mechanically tested in the artificially aged condition. Microstructural and mechanical behavior of the alloy was investigated and it was found that by keeping fixed the maximum and minimum temperatures of thermal cycles between 100 and 280 °C, the fatigue life of the alloy deteriorates significantly by increasing the mechanical load from 90 to 120 MPa. Fractographic analyses showed the occurrence of ductile fracture nucleated from large process-induced pores and numerous fine dimples created due to plastic deformation. Secondary cracks were observed on the fracture surface of samples, which nucleated and propagated from large cavities and micro-dimples. Results based on the analysis of the elasto-plastic behavior of the material at high temperature showed that the inelastic strain of the broken samples was about 5 times higher than that of the run-out samples. Hardness drop occurred in all specimens after thermal fatigue experiments due to the coarsening of strengthening phases and modification of the Si-rich particles.

Introduction

Laser powder bed fusion (L-PBF) is one of the most studied additive manufacturing techniques that is used for the production of industrial parts. In L-PBF process, a laser beam is used for selectively melt a layer of powder, based on a 3D digital model [1,2]. During the melting process of a powder bed, also the underlying layers down to a certain depth are re-melted, resulting in complete consolidation into a solid volume. The quality of L-PBF-processed parts depends on various process parameters, including laser power, scanning strategy, hatch distance, scanning rate, process temperature, quality of feedstock material and build chamber atmosphere. The parts manufactured by this method feature a near-net shape, which is a great advantage with respect to the traditional manufacturing methods in which extensive machining is often required. As a result, L-PBF gives the possibility to produce parts with complex geometry and low material usage and wastes. All these factors drive the attention on L-PBF for the design of high performance parts for the aerospace, automotive and biomedical sectors [3]. Integrity of parts has a great impact on mechanical properties, hence on safety and reliability. Defects in parts produced by L-PBF can be divided in two categories: surface features such as balling, partially melted powder particles and spatters [[4], [5], [6]] and volume defects, including porosity and cracks [7,8]. Porosity in turn can be process-induced or powder-induced. Process induced pores can be of different types, such as those due to lack of fusion, keyhole formation or shrinkage [9,10]. Powder induced pores with spherical shapes, can form due to the entrapment of the inert gas during gas atomization, which can translate directly to the printed component [1].

Among aluminum-based alloys, already existing cast grades are the preferred materials to be processed by L-PBF due to their low thermal expansion coefficient, good fluidity and narrow solidification range inherited from cast alloy requirements. Several studies have been done on mechanical properties of hypoeutectic AlSi10Mg and AlSi7Mg alloys [11,12]. The results show that the mechanical properties of the L-PBF components, including ductility and ultimate tensile strength, are comparable or higher than those of similar alloys produced by casting [[13], [14], [15], [16]]. Debroy et al. [10] presented the values of the yield strength, ultimate tensile strength, ductility and hardness of both as-built and heat treated AlSi10Mg and AlSi12 alloys processed by L-PBF and compared the results with the traditionally produced counterparts by casting methods, by gathering the data from several research studies. Based on their database, except for few cases, the mentioned properties were similar or higher in L-PBF manufactured alloys. This has been attributed to the fine cellular microstructure, reduced size of the eutectic Si phase and limited segregation in parts made by L-PBF.

Fatigue failures caused by the combination of mechanical and thermal loads are referred to thermo-mechanical fatigue (TMF) effects. Under such conditions the total strain induced in the component is a combination of the thermal and mechanical contributions [17,18]. Damage mechanisms due to TMF can be divided in different categories. The main mechanisms are caused by mechanical fatigue, oxidation and creep [[19], [20], [21]]. High thermal conductivity, low coefficient of thermal expansion, low content of porosity and intermetallic phases and a stable microstructure are some of the features that improve the TMF life of the cast Al alloys [[22], [23]].

Several studies investigated the effects of process parameters and microstructural properties on TMF life of cast Al alloys. Grieb et al. [24] reported that the decrease in strength due to the exposure to high temperature in combination with plastic deformation during TMF loading is responsible for early crack initiation. They showed that during TMF loading of AlSi7Mg-T6 and AlSi5Cu3-T7 alloys, incoherent Mg2Si and AlCu2 precipitates formed, resulting in over-aging effects and in the decrease in strength of the alloys. Javidani et al. [23] showed that some transition elements that precipitate as fine, stable and coherent particles significantly improve the TMF life of hypoeutectic Al alloys. Huter et al. [25] demonstrated that under TMF conditions, Cu additions cause stabilization of the matrix by precipitation hardening in hypoeutectic Al–Si alloys, hence provide higher TMF strength. Takahashi et al. [22] investigated the effect of aging time and strain range on TMF behavior of A356-T6 alloy subjected to additional aging at 250 °C for 1, 10 or 100 h. They concluded that the TMF life could increase as the aging time was increased and strain range reduced.

Although several studies focused on Thermal Fatigue (ThF) and TMF behavior of cast Al alloys are available in the literature, to the authors’ knowledge, the ThF and TMF behavior of L-PBF processed Al alloys have not been reported in the literature yet. In this study, experimental methods and results of quasi-static ThF behavior of L-PBF processed A357 alloy aged to peak hardness are described. An approach used for the evaluation of the inelastic strain induced during ThF is first presented. Then, the results on modification of hardness and microstructure induced by ThF tests are discussed.

Section snippets

Material and experiments

A gas atomized commercial powder of AlSi7Mg (A357) alloy supplied by LPW South Europe Srl with particle size distribution in the range of 20–63 μm was used for the experiments. The chemical composition is reported in Table 1.

Samples for metallography and cylindrical dogbone-shaped specimens for ThF were produced by L-PBF technology using a Renishaw AM250 system that employs a single mode pulsed fibre laser with a maximum laser power of 200 W and an estimated focused spot size of 75 μm. Meander

ThF resistance measured by the Gleeble simulator

The number of cycles to failure for each sample during thermal cycling from 100 to 280 °C is shown in Fig. 5 as a function of the applied stress. After the first tests, it was observed that run-out (set at 1000 cycles) was achieved under the average stress values of 90, 95, 100, 105 and 107.5 MPa. Under the stress value of 110 MPa, one sample out of the two tested broke after 913 cycles. At the stress levels of 115 MPa and higher, the samples systematically failed after few hundreds of cycles.

Thermal analysis

Conclusion

In this study the thermal fatigue behavior of L-PBF processed AlSi7Mg alloy was investigated. The results revealed that:

  • 1)

    The relative density of the L-PBF samples, measured by Archimedean method, was in the range of 99.6–99.8%. No direct relation between the relative density and the fatigue life under ThF tests was found.

  • 2)

    The ThF tests showed that ThF failure with fluctuation from 100 to 280 °C took place at applied constant loads of 110, 115 and 120 MPa, while run-outs were measured below

Statement of originality

We hereby declare that this submission is our own work and to the best of ourknowledge it contains no materials previously published or written by another person.

CRediT authorship contribution statement

Zahra Sajedi: Conceptualization, Methodology, Validation, Writing - original draft, Visualization, Formal analysis, Investigation, Data curation. Riccardo Casati: Conceptualization, Investigation, Writing - review & editing, Resources. Maria Cecilia Poletti: Validation, Writing - review & editing, Resources. Mateusz Skalon: Investigation, Writing - review & editing. Maurizio Vedani: Conceptualization, Methodology, Validation, Writing - review & editing, Funding acquisition, Resources,

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

The authors would like to acknowledge L. Rovatti for her technical support at Politecnico di Milano, R. Wang and K. Pradeep for their support with the Gleeble experiments and R. H. Buzolin for his technical support at TU Graz. The present research was also supported by the Italian Ministry for Education, University and Research (MIUR) through the project “Department of Excellence LIS4.0” (Integrated Laboratory for Lightweight and Smart Structures).

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