Research articleThermal fatigue testing of laser powder bed fusion (L-PBF) processed AlSi7Mg alloy in presence of a quasi-static tensile load
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).
References (49)
- et al.
Very high cycle fatigue and fatigue crack propagation behavior of selective laser melted AlSi12 alloy
Int. J. Fatig.
(2017) - et al.
Design for additive manufacturing: trends, opportunities, considerations, and constraints
CIRP Ann.
(2016) - et al.
Laser powder-bed fusion additive manufacturing: physics of complex melt flow and formation mechanisms of pores, spatter, and denudation zones
Acta Mater.
(2016) - et al.
Investigation into spatter behavior during selective laser melting of AISI 316L stainless steel powder
Mater. Des.
(2015) - et al.
On morphological surface features of the parts printed by selective laser melting (SLM)
Addit. Manuf.
(2018) - et al.
Observation of keyhole-mode laser melting in laser powder-bed fusion additive manufacturing
J. Mater. Process. Technol.
(2014) - et al.
Spatter and oxide formation in laser powder bed fusion of Inconel 718
Addit. Manuf.
(2018) - et al.
Additive manufacturing of metallic components – process, structure and properties
Prog. Mater. Sci.
(2018) - et al.
Fatigue properties and micromechanism of fracture of an alsimg0.6 cast alloy used in diesel engine cylinder head
Proc. Eng.
(2010) - et al.
Selective laser melting of AlSi10Mg alloy: process optimisation and mechanical properties development
Mater. Des.
(2015)
Oxides, porosity and fatigue performance of AlSi10Mg parts produced by selective laser melting
Int. J. Fatig.
Mechanical properties of AlSi10Mg produced by selective laser melting
Phys. Procedia
Low cycle thermal fatigue of aluminum alloy cylinder head in consideration of changing metrology microstructure
Proc. Eng.
Thermomechanical fatigue of cast aluminium alloys for cylinder head applications–experimental characterization and life prediction
Proc. Eng.
A model for thermal fatigue in an aluminium casting alloy
Int. J. Fatig.
Thermo-mechanical fatigue life assessment of aluminium components using the damage rate model of Sehitoglu
Int. J. Fatig.
Simulation and evaluation of thermal fatigue cracking of hot work tool steels
Int. J. Fatig.
Thermal fatigue testing of chromium martensitic hot-work tool steel after different austenitizing treatments
J. Mater. Process. Technol.
DSC analyses of the precipitation behavior of two Al–Mg–Si alloys naturally aged for different times
Mater. Lett.
Effect of heat treatment on AlSi10Mg alloy fabricated by selective laser melting: microstructure evolution, mechanical properties and fracture mechanism
Mater. Sci. Eng., A
The metallurgy and processing science of metal additive manufacturing
Int. Mater. Rev.
Effects of defects on mechanical properties in metal additive manufacturing: a review focusing on X-ray tomography insights
Mater. Des.
Aluminum Alloy Castings: Properties, Processes and Applications
Effect of different heat treatment routes on microstructure and mechanical properties of AlSi7Mg, AlSi10Mg and Al-Mg-Zr-Sc alloys produced by selective laser melting
Cited by (8)
Comparative thermal fatigue behavior of AlSi7Mg alloy produced by L-PBF and sand casting
2021, International Journal of FatigueCitation Excerpt :It can be mentioned that, even though the load was defined as a constant during the tests, some oscillations in its signal could be observed during the transient periods, which could be attributed to the adjustment of the PID control loop. Further details about the experimental set-up can be found in ref. [35]. The fracture surfaces of broken samples were observed by means of a Zeiss EVO 50XVP Scanning Electron Microscope.
Enhancing as-built microstructural integrity and tensile properties in laser powder bed fusion of AlSi10Mg alloy using a comprehensive parameter optimization procedure
2021, Materials Science and Engineering: AInsight into the effect of different thermal treatment routes on the microstructure of AlSi7Mg produced by laser powder bed fusion
2021, Materials CharacterizationCitation Excerpt :In recent years, several scientific papers on microstructure and mechanical properties of Al-Si-Mg alloys produced by L-PBF have been published. Most of them focusses on the AlSi10Mg alloy, and only a small fraction investigated the AlSi7Mg alloy (corresponding to A357 grade) [5–14]. The research works published on L-PBF of AlSi7Mg focus on optimization of process parameters [6], properties of lattice structures [7], thermal and mechanical fatigue properties [8–10], fracture toughness [11] and corrosion behavior of the alloy [12].
High-Efficiency Dynamic Scanning Strategy for Powder Bed Fusion by Controlling Temperature Field of the Heat-Affected Zone
2024, Chinese Journal of Mechanical Engineering (English Edition)Experimental, computational, and data-driven study of the effects of selective laser melting (SLM) process parameters on single-layer surface characteristics
2022, International Journal of Advanced Manufacturing Technology