Selective Laser Melted AlSi10Mg Alloy Research Articles

In situ X-ray imaging of fatigue crack growth from multiple defects in additively manufactured AlSi10Mg alloy

Defects introduced during additive manufacturing currently control fatigue resistance and lead to a large scatter in lifetime, with pancake shaped lack of fusion (LOF) defects being particularly potent. In this study the fatigue crack propagation life of selective laser-melted (SLM) AlSi10Mg alloy is considered in cases where single cracks and multiple cracks can initiate from LOF defects under high cycle fatigue (HCF). Firstly, the aspect ratios of initially long fatigue cracks were determined for critical LOF defects obtained from X-ray CT renderings using the critical defect regularization method, and the response surface method used to obtain the stress intensity factor of the crack front quickly and continuously. Then a single crack propagation model considering the evolution of the crack aspect ratio established to predict the crack propagation life which is in good agreement within in situ X-ray CT imaging of the crack front when a single crack is dominant. The crack propagation phase was predicted to represent 35–60% of the total fatigue life representing a larger fraction at high stress amplitudes. Multiple cracks were found to initiate cracks at the larger stress amplitudes. In cases where multiple cracks arise this is non-conservative and so a synergistic multiple fatigue crack growth (smFCG) model was developed based on multiple defects measured a priori by X-ray CT to depict the competitive cracking effect. Compared with the single crack model, the smFCG model predicts a shorter propagation life (by 5–10%) when multiple defects are involved since it considers all the initial defects within the crack initiation region. Given the propensity of large numbers of defects in AM material this approach may be more appropriate in many cases.

Investigation on pulsed electrolytically polished AlSi10Mg alloy processed via selective laser melting technique

Currently, the surface integrity-related issues of additively manufactured parts are limiting the potential high-end applications. The present work investigates the effectiveness of the pulsed-electropolishing technique to improve the surface integrity of aluminium alloys fabricated using the selective laser melting (SLM) technique. Due to its low density, high corrosion resistance, the aluminium alloys considerably enhance the performance of lightweight critical parts for different industrial applications. In this study, selective laser melted AlSi10Mg samples were subjected to microstructural examinations using optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction technique. The microhardness and tensile properties were determined using a microhardness tester and a universal testing machine, respectively. The pulsed-electropolishing process was employed for the surface finishing of the SLM-processed AlSi10Mg samples. The effect of current density on the electropolishing of selective laser melted AlSi10Mg alloy were also investigated. The microstructures of printed samples revealed a density of greater than 99.9% and weld beads along the build direction (longitudinal) and laser scan paths perpendicular to the build direction (transverse). The microhardness, yield and tensile strength properties were similar in both as-printed conditions. The pulsed electropolishing results showed a beneficial effect of higher current density values, resulting in decreased surface roughness of the SLM-processed AlSi10Mg samples. Compared to as-printed conditions, a significant decrease in surface roughness of about ∼73.35% under optimized electropolishing conditions was noted. The material ratio curve shows that the surface topography becomes more uniform with increased current density and has lesser surface undulations for as-printed samples. The 2D line profilogram and 3D surface topography of electropolished SLM-processed samples reveal the surface finish quality characteristics. The material ratio curve aids as an effective method to assess and qualify the surface topography of electropolished samples.

Effect of supporting structure design on residual stresses in selective laser melting of AlSi10Mg

Residual stress is a key indicator to measure the forming quality of selective laser melting (SLM) components, and its control method has received extensive attention. As an auxiliary structure for forming SLM components, the structural characteristics of the supporting structure will affect the residual stress distribution of the formed parts. Therefore, it is extremely meaningful to explore the influence of the supporting structure design on the residual stress of SLM AlSi10Mg alloy. In this study, an approach is proposed to select and design the supporting structure for forming SLM components with different structural characteristics to achieve the purpose of reducing the residual stress in the overhanging structure of the components. As the result shows, when the contact area of single supporting tooth structure and component overhanging structure is 0.25mm2 and the X/Y interval of main supporting structure is 2.5mm, the forming effect is relatively good. Furthermore, the block supporting structure is more suitable for the overhanging structure which has small areas and less height, and the contour support is more suitable for the overhanging structure with larger area.

Influence of heat treatment on the high-cycle fatigue properties and fatigue damage mechanism of selective laser melted AlSi10Mg alloy

The high-cycle fatigue properties and fatigue damage mechanism of AlSi10Mg alloy manufactured by selective laser melting (SLM) were investigated. The influence of the post-heat treatment on the microstructure and high-cycle fatigue properties was also examined. The SLM-manufactured AlSi10Mg alloy was heat-treated under two different conditions, namely T6 and direct aging (DA). The results of the high-cycle fatigue tests, which were carried out at room temperature, imply that, under all three conditions (as-built, T6, DA), the fatigue strength decreases as the number of cycles increases along with the estimated endurance limit. Predominantly, the fatigue properties were influenced by the morphology and size of the Si particles, both controlled by adjusting the heat treatment. The alloy that was processed using DA heat treatment had superior fatigue properties due to its cellular structural morphology characterized by a significant amount of fine Si precipitates. Compared with the as-built and T6 specimens, the superior fatigue limit of the DA alloy is attributable to its outstanding strength characteristics. The reliability of the fatigue life of the DA alloy is relatively high. Fatigue fracture samples revealed that the DA alloy had evenly distributed dislocations along the cellular boundaries. The fine Si particles were found to play an important role in preventing fatigue crack propagation, thereby enhancing fatigue properties. The effect of the heat treatment on the microstructure and mechanical properties of the SLM AlSi10Mg specimens was investigated with a specific focus on fatigue strength. The results highlighted that an appropriate DA process could significantly enhance the fatigue performance of the AlSi10Mg alloy under investigation. The resulting characteristics are highly advantageous and are superior to those obtained with other conventional heat treatment processes.

Effect of Heat Treatment and Electric Discharge Alloying on the Tribological Performance of Selective Laser Melted AlSi10Mg

Abstract Selective laser melting (SLM) is an emerging additive manufacturing (AM) technology for fabrication of complex lightweight components along with improved mechanical properties. However, the properties are highly influenced by the continual heating and cooling during deposition, variation in local temperature, size, and shape of melt pool, and solidification growth rate. Therefore, postprocessing is very often required to control various properties of additive manufactured components. The present work investigates the influence of various postprocessing methods such as heat treatment and electric discharge alloying (EDA) on ambient and elevated temperature wear behavior of selective laser melted AlSi10Mg alloy and compared with its tribological behavior with cast AlSi10Mg. The dry wear tests were conducted using a pin on disk (POD) tribometer with EN-31 as counter body. The EDA treated SLM AlSi10Mg showed the least wear-rate and coefficient of friction (COF) at both ambient and elevated temperatures (1.05 × 10−4 mm3/Nm and 0.434 and 3.12 × 10−5 mm3/Nm and 0.531, respectively) due to its higher hardness (189.8 HV) as compared with other samples. The wear-rate and COF of cast specimen are found to be highest among all specimens at both ambient and elevated temperatures (1.34 × 10−4 mm3/Nm and 0.528 and 4.49 × 10−5 mm3/Nm and 0.724, respectively). Lower wear-rate and higher COF are observed at elevated temperature due to the excessive formation of wear-resistant oxides (Al2O3, SiO2, and MgO) and glaze layers for all samples compared with ambient temperature wear behaviors of its counterparts. Abrasive wear, adhesive wear, oxidation wear, and surface delamination are the prominent wear mechanisms observed for ambient and elevated temperatures for all the specimens.

Evolution of Heterogeneous Microstructure and its Effects on Tensile Properties of Selective Laser Melted AlSi10Mg Alloy

To figure out the evolution of the heterogeneous microstructure across the molten pool boundaries (MPBs) and its effects on tensile properties of selective laser melted (SLM) AlSi10Mg alloy, two of the most commonly used scanning strategies, i.e., bidirectional scanning within a layer and with a 90° and 67° rotation for the successive layers (zigzag-90 and zigzag-67) were adopted. Grain morphology, solidification microstructure, texture and room temperature tensile properties of the SLMed AlSi10Mg alloy under the two scanning strategies were studied. The coarse columnar and fine equiaxed grains were observed in a single solidified track under the two scanning strategies. The columnar grains dominated the bulk of the molten pool (MP) while the equiaxed grains mainly distributed along the MPBs. The equiaxed grains along MPBs break the continuous growth of the columnar grains and lead to a weak texture. A new mechanism based on heterogeneous nucleation and constitutional supercooling was proposed to explain the formation of heterogeneous microstructure across the MPBs. The tensile test was performed in the horizontal (perpendicular to the build direction) and the vertical (parallel to the build direction) directions fabricated under the two scanning strategies. The mechanical properties showed obvious fluctuation, and the yield strength of the horizontal direction is generally higher than that of the vertical direction. The distribution of pores and MPBs is the main factors to influence the tensile properties.

Aging Response in Selective Laser Melted AlSi10Mg Alloy as Function of Distance from the Substrate Plate

Selective Laser Melting (SLM) builds a metallic part in a layer-by-layer mode with growth occurring along the vertical axis. Metallic powder layers are melted by a laser beam by programmed scan sequences inducing specific mechanical properties in the as-built samples according to process parameters. Post heat treatments are usually performed to optimise the mechanical behaviour. In this work, the effects induced by heat treatments at 175°, 200° and 225°C on SLMed bars of Al10SiMg were investigated as function of distance from the substrate plate. The bars were 300 mm height and in the as-built condition, Vickers microhardness and tensile strength decreased along the built direction, while the elongation increased from the bottom to the top of the billet. After heat treatments, Vickers microhardness resulted lower of 10HV at the top of the bar compared to its bottom in contact with the hot substrate; microhardness decreased with time at constant temperature compared to the as-built. Tensile properties showed variations of 50 MPa and 1% elongation between the top and the bottom of the billet when aging was performed at 175°C for 4h; the strength and ductility gradients were reduced to 20 MPa and 0,5% respectively by increasing the aging time to 6h. Microstructure investigations performed by scanning electron microscopy confirmed the different evolution of Silicon particles and precipitated particles at different height of the bars.

Uncovering the roles of LaB6-nanoparticle inoculant in the AlSi10Mg alloy fabricated via selective laser melting

The effects of inoculation treatment with LaB6 nanoparticles (0–2 wt% additions) on the microstructural evolution and mechanical performance in a selective laser melted AlSi10Mg alloy were comprehensively investigated. The addition of 0.2–0.5 wt% LaB6 nanoparticles was identified to be optimal to achieve substantial grain refinement, microstructural homogeneity and thus remediation in the mechanical property anisotropy in the AlSi10Mg alloy. The substantial grain refinement was attributed to the coherent Al/LaB6 interfaces, which facilitated the heterogeneous nucleation of Al on the LaB6 nanoparticles during solidification. Increasing the LaB6 addition up to 2 wt% only marginally further refined the equiaxed grains, which can be understood in terms of the concept of nucleation free zone formed in the liquid at front of the growing solid-liquid interfaces. The LaB6 nanoparticles within the nucleation free zone could not be activated to be nucleants for α-Al. As a result, random orientation relationships between LaB6 nanoparticles within the nucleation free zone and the Al matrix were determined. Those excessive LaB6 nanoparticles weakened the melt pool boundaries, and therefore deteriorated the longitudinal ductility of the SLMed AlSi10Mg alloy.

Dimensional Change of Selectively Laser-Melted and Gravity-Cast AlSi10Mg Alloy During Heat Treatment

AlSi10Mg alloy, processed by selective laser melting (SLM) in as-built state and gravity casting in solution-treated state, was heat-treated at several temperatures for different durations, and the resulting dimensional changes were investigated. For the cast AlSi10Mg alloy, the linear dimensional change heat-treated at 200 ˚C was approximately 0.10%, which agreed with that calculated by a theoretical model. The linear dimensional change of the SLM AlSi10Mg alloy heat-treated at 150 ˚C was 0.17%. This indicated that, in as-built state, the concentration of supersaturated solid solution Si in α-Al phase was 1.7 wt% on average, and the lattice constant of α-Al phase was 0.4047 nm. SLM AlSi10Mg alloy contained more than 3 cm3/100 g-Al hydrogen and showed a linear dimensional change of more than 0.6% after the heat treatment at 530 ˚C because of porosity expansion. The relative densities of the SLM AlSi10Mg alloy heat-treated at 150 ˚C for 1800 ks and 530 ˚C for 18 ks were 99.89% and 98.12%, respectively.

High-cycle fatigue and fracture behaviours of SLM AlSi10Mg alloy

Abstract The high-cycle fatigue and fracture behaviours of the selective laser melting (SLM) AlSi10Mg alloy were investigated. Flat specimens were designed directly in the shape required for the fatigue tests under pulsating loading in tension (R=0, R is the dynamic factor). The fatigue–life (S–N) curves were modelled with a conditional Weibull's probability density function, where the real-valued genetic algorithm (GA) and the differential ant-stigmergy algorithm (DASA) were applied to estimating the needed Weibull's parameters. The fractography of the fatigue specimens showed that the fatigue cracks initiated around the surface defects produced by SLM and then propagated in an unstable manner. However, the presence of large SLM defects mainly influenced the crack initiation period and did not have a strong influence on the crack propagation. The obtained experimental results present a basis for further investigation of the fatigue behaviour of advanced materials and structures (e.g. cellular metamaterials) fabricated by additive manufacturing (AM). Especially, in the case of two-dimensional cellular structures, the cross-section of cellular struts is usually rectangular which corresponds to the specimen shape considered in this work.

Effect of melting modes on microstructure and tribological properties of selective laser melted AlSi10Mg alloy

ABSTRACT This paper focuses on the effect of melting modes on microstructural evolution and tribological properties of AlSi10Mg alloy fabricated by selective laser melting (SLM). The results showed that the microstructures of SLM AlSi10Mg consisted of primary α-Al surrounded by cellular Si networks (∼500 nm) when fabricated in conduction mode, but has a finer cellular-like Si phase (∼200 nm) when fabricated in keyhole mode. The strong convection caused by the melt reflow and Marangoni convection under keyhole mode also resulted in deposition of nano-scale Si particles at the bottom of the molten pool. The SLM AlSi10Mg fabricated in keyhole mode exhibited better wear resistance than that fabricated in conduction mode. Compared to traditional as-cast specimens, both SLM specimens showed better wear resistance due to the unique cellular-like networks. The SLM technique offers a new approach for material processing that can be used to refine microstructures for improved tribological properties.

Heterogeneous microstructure and voids dependence of tensile deformation in a selective laser melted AlSi10Mg alloy

Microstructure and voids evolution of a selective laser melted (SLM) AlSi10Mg alloy during tension were systematically investigated. The SLM AlSi10Mg sample is featured with a multi-level heterogeneous microstructure that is composed of melt pools (MPs), columnar Al grains and sub-cells. According to the Si morphologies and thermal history, the MP could be divided into three regions, including the fine structure zone (FSZ), remelted zone (RMZ) and heat-affected zone (HAZ), respectively. The Si segregation phenomenon on sub-cells boundaries is observed and may attributed to the constitutional supercooling. The tensile deformation behaviors of the vertical sample (V-sample) vary from the horizontal sample (H-sample) for that heterogeneous microstructure arises strain localization in V-sample. In addition, the voids have no obvious effect on the uniform plastic deformation, while they may affect the non-uniform deformation during tension. The combined effects of strain localization and void contribute to the earlier break in V-samples than in H-samples.

Computational Fatigue Analysis of Auxetic Cellular Structures Made of SLM AlSi10Mg Alloy

In this study, a computational fatigue analysis of topology optimised auxetic cellular structures made of Selective Laser Melting (SLM) AlSi10Mg alloy is presented. Structures were selected from the Pareto front obtained by the multi-objective optimisation. Five structures with different negative Poisson’s ratios were considered for the parametric numerical analysis, where the fillet radius of cellular struts has been chosen as a parameter. The fatigue life of the analysed structures was determined by the strain–life approach using the Universal Slope method, where the needed material parameters were determined according to the experimental results obtained by quasi-static unidirectional tensile tests. The obtained computational results have shown that generally less auxetic structures tend to have a better fatigue life expectancy. Furthermore, the fillet radius has a significant impact on fatigue life. In general, the fatigue life decreases for smaller fillet radiuses (less than 0.3 mm) as a consequence of the high-stress concentrations, and also for larger fillet radiuses (more than 0.6 mm) due to the moving of the plastic zone away from the edge of the cell connections. The obtained computational results serve as a basis for further investigation, which should be focused on the experimental testing of the fabricated auxetic cellular structures made of SLM AlSi10Mg alloy under cyclic loading conditions.

Research on the limitations of laser energy density and microstructure characteristics of selective laser melted AlSi10Mg alloy

In this study, deficiencies in using the laser energy density (LED) to combine different process parameters to represent the heat input in selective laser melting (SLM) process are proposed. The change in melt pool shape could not always be clarified by the LED input, since the vary of LED input caused by altering hatch spacing cannot change the melt pool shape. In addition, the microstructure of SLM fabricated AlSi10Mg material was observed, and this paper gives a further elucidation on the formation of the hierarchical microstructures that discriminated by the morphology of Si phase. Microhardness is enhanced by the supersaturated α-Al matrix, and the hardness value of coarse zones and fine zones show different performance.

Effect of internal pores on fatigue properties in selective laser melted AlSi10Mg alloy

Effect of internal pores on fatigue properties in selective laser melted AlSi10Mg alloy

Effect of Trace Addition of Ceramic on Microstructure Development and Mechanical Properties of Selective Laser Melted AlSi10Mg Alloy

Selective laser melting (SLM) is an emerging additive manufacturing technology for fabricating aluminum alloys and aluminum matrix composites. Nevertheless, it remains unclear how to improve the properties of laser manufactured aluminum alloy by adding ceramic reinforcing particles. Here the effect of trace addition of TiB2 ceramic (1% weight fraction) on microstructural and mechanical properties of SLM-produced AlSi10Mg composite parts was investigated. The densification level increased with increasing laser power and decreasing scan speed. A near fully dense composite part (99.37%) with smooth surface morphology and elevated inter-layer bonding was successfully obtained. A decrease of lattice plane distance was identified by X-ray diffraction with the laser scan speed decreased, which implied that the crystal lattices were distorted due to the dissolution of Si and TiB2 particles. A homogeneous composite microstructure with the distribution of surface-smoothened TiB2 particles was present, and a small amount of Si particles precipitated at the interface between reinforcing particles and matrix. In contrast to the AlSi10Mg alloy, the composites showed a stabilized microhardness distribution. A higher ultimate tensile strength of 380.0 MPa, yield strength of 250.4 MPa and elongation of 3.43% were obtained even with a trace amount of ceramic addition. The improvement of tensile properties can be attributed to multiple mechanisms including solid solution strengthening, load-bearing strengthening and dispersion strengthening. This research provides a theoretical basis for ceramic reinforced aluminum matrix composites by additive manufacturing.

Post heat treatment of additive manufactured AlSi10Mg: On silicon morphology, texture and small-scale properties

Post-fabrication heat treatment, including solution treatment (at 520 °C for 1 h) followed by water quenching (WQ), air cooling (AC) and furnace cooling (FC), was performed on a selective laser melted AlSi10Mg alloy. The objective is to assess the effect of various cooling rates on the microstructure (specifically eutectic-Si morphology) and small-scale mechanical properties, measured by employing a depth-sensing nanoindentation platform, of the selective laser melted AlSi10Mg alloy. Results show extensive evolutions in the microstructure and the mechanical properties of the heat-treated materials relative to the as-fabricated sample. Upon solutionizing treatment, the eutectic Si is first fragmented, then spheroidized, and finally coarsened when cooled with slow rates. The microstructural evolution directly affects the mechanical properties, where the as-fabricated and the FC are the hardest and the softest structures, respectively. This is directly attributed to the size and morphology of the eutectic-Si within the microstructure. The findings of this study could help to adjust the optimized heat-treatment process to fabricate SLM AlSi10Mg parts with desirable microstructure and mechanical properties.

Study on the fatigue behaviour of selective laser melted AlSi10Mg alloy

Selective laser melted (SLM) AlSi10Mg alloy was subjected to high cycle fatigue test to elucidate the effect of microstructural characteristics and sub-microscopic defects in its dynamic behaviour. The microstructure of the processed materials revealed scan tracks on the scanning surface and solidified melt pool/melt pool tracks on the surface in the build direction. The processed material exhibits equi-axed morphology with 〈100〉 fibre texture on the scanned surface and elongated grains preferably 〈100〉 orientation in the build direction. The pore size and distribution in the selective laser melted materials were measured using computed micro-tomography method μ-(CT). The cellular structures constituting 500 nm sized grains were observed. The boundary of the cell was a eutectic mixture of Al and Si while the matrix comprised of primary α-Al as observed from TEM results. The influence of grain morphology, microtexture, pores, and cellular structure on the deformation behaviour of processed materials under cyclic loading were investigated. Load controlled axial fatigue test was conducted at a stress ratio of 0.1 and 25 Hz frequency. Fractographs revealed crack initiation occurred, typically, due to the pores and the Si particles in surface or sub-surfaces. The cellular structures in the microstructure played a significant role in influencing the fatigue crack path, depends on its location in the microstructure. Dislocations in the cellular matrix and dislocation fringes in the cellular boundaries observed through transmission electron microscopy affirms this influence. Anisotropy and shielding gases observed to exhibit no significant influences on the fatigue life of the processed material. The crack initiation and propagation are heavily influenced by surface/subsurface pores and cellular structure as evident from the detailed microscopic investigation made in the present work.

Research on metallurgical bonding of selective laser melted AlSi10Mg alloy

The densification behavior, surface morphology and attendant microstructural characteristics of the selective laser melting (SLM) processed AlSi10Mg alloy affected by the processing parameters were systematically investigated. Increasing the laser scanning speed or hatch spacing will deteriorate the metallurgical bonding between melt pools, resulting in the increase of irregular pores. Scanning speed and hatch spacing affect the liquid metallurgical bonding of melt pools in different ways. By manipulating scanning speed, the shape of the melt pool changes, resulting in different extents of metallurgical bonding. Whereas hatch spacing influences the resultant metallurgical bonding by controlling the overlapping rate between neighboring scan tracks simply. The formations of the hierarchical microstructures which discriminated by the Si phase are elucidated. Coarse zones are formed by the instantaneous existence of extremely high ratio of thermal gradient (G) and solidification rate (R) at the melt pool boundary, where solidification microstructure grows planar. Fine zones are formed by columnar-dendritic growth of microstructure. During the solidification process, the contraction forces that generated by the trapped gas in the pores and gravity are applied to the liquid around irregular pores and, forms the porous microstructures different from that in dense areas eventually. The tensile tests reveal that the tensile properties of SLM-processed samples are significantly affected by the formation of porosity.

Ad Hoc Heat Treatments for Selective Laser Melted Alsi10mg Alloy Aimed at Stress-Relieving and Enhancing Mechanical Performances

The response of an AlSi10Mg alloy produced by selective laser melting (SLM) to specifically designed heat treatments is explored. Thermal phenomena of the rapidly solidified material were studied by differential scanning calorimetry (DSC), and the results have been used to define specific post-processing thermal treatment temperatures. In detail, three heat treatments were studied: two innovative ones, able to selectively trigger (1) Mg2Si precipitation and (2) the rupture and spheroidization of the silicon network, and (3) a T5-like aging were applied for different times and their effects on mechanical properties were studied. The first and the second heat treatments were selected by combining DSC analysis and aging curves for the optimization of the temperature and duration. And these were more deeply investigated in terms of microstructural evolution and of residual stresses. The optimized thermal treatments for SLM-built AlSi10Mg are recommended as valid alternatives to the usually applied T6 or high-temperature stress release treatments.