Aging Behaviour of a 12.2Cr-10Ni-1Mo-1Ti-0.6Al Precipitation-Hardening Stainless Steel Manufactured via Laser Powder Bed Fusion

The combination of precipitation-hardening stainless steels (PH-SS) and laser powder bed fusion (LPBF) enables the manufacturing of tools for plastic injection moulding with optimised geometry and conformal cooling channels, with potential benefits in terms of productivity, part quality, and tool duration. Moreover, the suitability of LPBF-manufactured PH-SS in the as-built (AB) condition to be age-hardened through a direct aging (DA) treatment enables a great heat treatment simplification with respect to the traditional solution annealing and aging treatment (SA). However, plastic injection moulding tools experience severe thermal cycles during their service, which can lead to over-aging of PH-SS and thus shorten tool life. Therefore, proper thermal stability is required to ensure adequate tool life and reliability. The aim of the present work is to investigate the aging and over-aging behaviour of a commercially available PH-SS (AMPO M789) manufactured by LPBF in the AB condition and after a solution-annealing treatment in order to evaluate the effect of the heat treatment condition on the microstructure and the aging and over-aging response, aiming at assessing its feasibility for plastic injection moulding applications. The AB microstructure features melt pool borders, oriented martensite grains, and a cellular solidification sub-structure, and was retained during aging and over-aging. On the other hand, a homogeneous and isotropic martensite structure was present after solution annealing and quenching, with no melt pool borders, cellular structure, or oriented grains. The results indicate no significant difference between AB and solution-annealed and quenched specimens in terms of aging and over-aging behaviour and peak hardness (in the range 580–600 HV), despite the considerably different microstructures. Over-aging was attributed to both the coarsening of strengthening precipitates and martensite-to-austenite reversion (up to ~11 vol.%) upon prolonged exposure to high temperature. Based on the results, guidelines to aid the selection of the most suitable heat treatment procedure are proposed

EVALUATION OF HIGH-TEMPERATURE TENSILE PROPERTIES OF HEAT-TREATED ALSI10MG ALLOY PRODUCED BY LASER-BASED POWDER BED FUSION

The AlSi10Mg alloy is widely used to produce complex-shaped components by Laser-based Powder Bed Fusion (L-PBF); these parts, characterized by light structures and high specific strength, are currently employed in high-performance room temperature applications in the automotive and aerospace industries. However, it is important to increase the data concerning the high-temperature mechanical properties of the L-PBF AlSi10Mg alloy to spread its use. This study aims to fulfill the lack of knowledge by investigating the mechanical behavior at 200 °C, a representative condition of the average temperature of engine heads, of the L-PBF AlSi10Mg alloy subjected to a T5 heat treatment (artificial aging at 160 °C for 4 h) and an innovative T6 heat treatment (solubilization at 510 °C for 10 min and artificial aging at 160 °C for 6 h). The influence of high temperatures on the mechanical behavior of the L-PBF AlSi10Mg alloy was assessed by tensile tests, while microstructural and fractographic analyses were carried out to correlate the mechanical behavior of the alloy to its microstructure, and consequently explain the failure mechanisms. The ultrafine cellular microstructure, characterizing the T5 alloy, led to higher tensile strength than the homogeneous composite-like microstructure of the T6 alloy, which makes it very interesting for future application in the automotive and aerospace industries

Tensile behaviour of a hot work tool steel manufactured via Laser Powder Bed Fusion: effect of an innovative high pressure heat treatment

The combination of outstanding mechanical strength of tool steels and design freedom ensured by Additive Manufacturing (AM) processes is of a great interest for automotive applications. However, AM techniques produce peculiar defects, which affect the resulting mechanical behavior. For this reason, safety critical AM components are often subjected to hot isostatic pressing (HIP) to heal process defects. At the same time, tool steels require a proper quenching and tempering (QT) heat treatment to achieve high hardness and strength. In the present work, the effect of an innovative high pressure heat treatment (HPHT) on the tensile properties of a hot work tool steel produced via Laser Powder Bed Fusion (LPBF) was investigated. LPBF samples were subjected to two post-process heat treatments: a conventional quenching and tempering heat treatment performed in vacuum (CHT), and an innovative HPHT combining HIP e QT in a single step, to heal LPBF defects and obtain high mechanical properties. HPHT featured the same quenching and tempering cycle of CHT but it was performed under high pressure in a HIP furnace and with longer austenitizing time to promote defect closure. Tensile tests indicated no significant effect on proof and tensile strength for HPHT compared to CHT, but a significant reduction of elongation after fracture. Fractographic analyses and fracture mechanics calculations indicated that both CHT and HPHT specimens failed via crack propagation from large LPBF defects when the stress intensity factor reached the material fracture toughness. Fractographic analyses indicated an incomplete defect closure during HPHT due to the presence of an oxide film on the inner surface of defects, thus justifying the same failure mechanism and strength of CHT samples. Instead, it was proposed that the reduced elongation could arise from coarsened grains due to the longer austenitizing

High‐Temperature Behavior of the Heat‐Treated and Overaged AlSi10Mg Alloy Produced by Laser‐Based Powder Bed Fusion and Comparison with Conventional Al–Si–Mg‐Casting Alloys

In recent years, the AlSi10Mg alloy produced by laser-based powder bed fusion (L-PBF) has gained more attention for increasing the strength-to-weight ratio in structural parts subjected to severe operating conditions. Herein, the effects of thermal exposure (0–48 h at 200, 210, and 245 °C) on the metastable microstructure of the L-PBF AlSi10Mg alloy and the high-temperature (200 °C) tensile properties post-overaging (41 h at 210 °C) of the heat-treated alloy is investigated. In particular, two specific heat treatment conditions, currently neglected in the literature, are analyzed: i) T5 heat treatment (direct artificial aging: 4 h at 160 °C), and ii) the innovative T6R heat treatment (rapid solution: 10 min at 510 °C, and artificial aging: 6 h at 160 °C). The T5 shows a lower decrease in mechanical properties after thermal exposure and during the high-temperature tensile test than the T6R. This behavior is related to the higher efficiency of the submicrometric cellular structure in hindering the dislocation motion. In addition, the T5 has good tensile properties compared to high-temperature Al–Si–Mg- and Al–Si–Cu–Mg-casting alloys, representing an attractive option in future industrial applications characterized by operating temperatures up to 200 °C

Room- and High-Temperature Fatigue Strength of the T5 and Rapid T6 Heat-Treated AlSi10Mg Alloy Produced by Laser-Based Powder Bed Fusion

The AlSi10Mg alloy produced by laser-based powder bed fusion (L-PBF) is widely used to produce high-value-added structural parts subjected to cyclic mechanical loads at high temperatures. The paper aims to widen the knowledge of the room- and high-temperature (200 ◦C) fatigue behavior of the L-PBF AlSi10Mg alloy by analyzing the fully reversed rotating bending test results on mechanically polished specimens. Two heat-treated conditions are analyzed: T5 (direct artificial aging: 4 h at 160 ◦C) and novel T6R (rapid solution: 10 min at 510 ◦C, artificial aging: 6 h at 160 ◦C). The study highlights that (i) the T6R alloy is characterized by higher fatigue strength at room (108 MPa) and high temperatures (92 MPa) than the T5 alloy (92 and 78 MPa, respectively); (ii) thermal exposure at 200 ◦C up to 17 h does not introduce macroscopical microstructural variation; (iii) fracture surfaces of the room- and high-temperature-tested specimens show comparable crack initiation, mostly from sub-superficial gas and keyhole pores, and failure propagation mechanisms. In conclusion, the L-PBF AlSi10Mg alloy offers good cyclic mechanical performances under various operating conditions, especially for the T6R alloy, and could be considered for structural components operating at temperatures up to 200 °

Heat treatment response and influence of overaging on mechanical properties of C355 cast aluminum alloy

The research activity was focused on the optimization of heat treatment parameters for C355 (Al-Si-Cu-Mg)cast aluminum alloy and on its microstructural and mechanical characterization in T6 condition, also evaluatingthe effect of subsequent high temperature exposure. Differential thermal analyses were carried out to identifythe solution heat treatment optimal temperature. After solution heat treatment and quenching, samples weresubjected to artificial aging, at different times and temperatures, as to obtain the corresponding hardnesscurves. Samples for successive hardness and tensile tests were subjected to hot isostatic pressing (HIP) and T6heat treatment, according to the parameters optimized in the foregoing research phase. Some of the T6 heattreated samples were also characterized after overaging, induced by holding at 210 °C for 41 h. Aiming to carryout a comparative study, tensile properties of C355 alloy, both in T6 and overaged conditions, were comparedto those of A356 alloy (results from a previous study), which is currently more widely employed than C355.Experimental results showed how C355-T6 alloy is characterized by superior mechanical properties as comparedto A356-T6, especially in the overaged condition, due to the higher thermal stability induced by Cu-basedstrengthening precipitates

Optimisation of heat treatment of Al–Cu–(Mg–Ag) cast alloys

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Influence of sludge particles on the fatigue behavior of Al-Si-Cu secondary aluminium casting alloys

Al-Si-Cu alloys are the most widely used materials for high-pressure die casting processes. In such alloys, Fe content is generally high to avoid die soldering issues, but it is considered an impurity since it generates acicular intermetallics (\u3b2-Fe) which are detrimental to the mechanical behavior of the alloys. Mn and Cr may act as modifiers, leading to the formation of other Fe-bearing particles which are characterized by less harmful morphologies, and which tend to settle on the bottom of furnaces and crucibles (usually referred to as sludge). This work is aimed at evaluating the influence of sludge intermetallics on the fatigue behavior of A380 Al-Si-Cu alloy. Four alloys were produced by adding different Fe, Mn and Cr contents to A380 alloy; samples were remelted by directional solidification equipment to obtain a fixed secondary dendrite arm spacing (SDAS) value (~10 \ub5m), then subjected to hot isostatic pressing (HIP). Rotating bending fatigue tests showed that, at room temperature, sludge particles play a detrimental role on fatigue behavior of T6 alloys, diminishing fatigue strength. At elevated temperatures (200\u25e6C) and after overaging, the influence of sludge is less relevant, probably due to a softening of the \u3b1-Al matrix and a reduction of stress concentration related to Fe-bearing intermetallics

A Novel T6 Rapid Heat Treatment for AlSi10Mg Alloy Produced by Laser-Based Powder Bed Fusion: Comparison with T5 and Conventional T6 Heat Treatments

AlSi10Mg is the most widely studied Al alloy used to produce components by laser-basedpowder bed fusion (LPBF), also known as selective laser melting. Several papers have alreadyinvestigated the effects of conventional heat treatment on the microstructure and mechanicalbehavior of the LPBF AlSi10Mg alloy, overlooking, however, the particular microstructureinduced by rapid solidification. This paper reports on the effects of a T5 heat treatment and anovel T6 heat treatment on microstructure and mechanical behavior of the LPBF AlSi10Mgalloy, consisting of rapid solution (10 minutes at 510°C) followed by artificial aging (6 hours at160°C). The short solution soaking time reduced the typical porosity growth occurring at thehigh temperature and led to a homogeneous distribution of fine globular Si particles in the Almatrix. In addition, it limited the diffusion processes, increasing the amount of Mg and Si insolid solution available for precipitation hardening and avoiding the microstructuralcoarsening. As a result, the strength-ductility balance was improved by increasing both yieldstrength and elongation to failure, respectively of about 14 and 7 pct compared with the bestsolution among those reported in the literature for conventional T6 heat treatment of LPBFAlSi10Mg alloy

Influence of Microstructure on Fracture Mechanisms of the Heat-Treated AlSi10Mg Alloy Produced by Laser-Based Powder Bed Fusion

Few systematic studies on the correlation between alloy microstructure and mechanical failure of the AlSi10Mg alloy produced by laser-based powder bed fusion (L-PBF) are available in the literature. This work investigates the fracture mechanisms of the L-PBF AlSi10Mg alloy in as-built (AB) condition and after three different heat treatments (T5 (4 h at 160 degrees C), standard T6 (T6B) (1 h at 540 degrees C followed by 4 h at 160 degrees C), and rapid T6 (T6R) (10 min at 510 degrees C followed by 6 h at 160 degrees C)). In-situ tensile tests were conducted with scanning electron microscopy combined with electron backscattering diffraction. In all samples the crack nucleation was at defects. In AB and T5, the interconnected Si network fostered damage at low strain due to the formation of voids and the fragmentation of the Si phase. T6 heat treatment (T6B and T6R) formed a discrete globular Si morphology with less stress concentration, which delayed the void nucleation and growth in the Al matrix. The analysis empirically confirmed the higher ductility of the T6 microstructure than that of the AB and T5, highlighting the positive effects on the mechanical performance of the more homogeneous distribution of finer Si particles in T6R