Development of High-Strength Aluminum Alloys for Additive Manufacturing

Marvin Lennard Schuster
EPFL, 2022
EPFL thesis

Abstract

Metal additive manufacturing (AM) offers the possibility to rapidly produce complex geometries that are not achievable with conventional manufacturing methods. The two most common technologies, Laser Powder Bed Fusion (LPBF) and Direct Metal Deposition (DMD), are characterized by high process temperatures, fast heating, and cooling rates, and an associated far-from-equilibrium solidification. This is problematic when processing materials that were developed and optimized many years ago for conventional manufacturing processes. Common difficulties are the occurrence of gas pores in the component, which are caused by the evaporation of volatile alloying elements during the process, the formation of hot cracks due to large solidification intervals, and the formation of metastable phases. For the processing of high-strength, precipitation-hardening Al-Cu aluminum alloys, this prevents the use of non-modified conventional alloys. However, the development of novel alloys tailored to these processes not only allows these difficulties to be overcome but also opens up the possibility of improving material properties through the targeted exploitation of process-inherent properties. This includes the processing of oxide particle-reinforced alloys, which can not be produced by conventional casting processes. The standard heat treatment usually performed for precipitation hardening, which has also been optimized for conventional materials, must be adapted accordingly to achieve optimum material properties. Within the scope of this work, the microstructure, precipitation, and defect formation after LPBF as well as the heat treatment behavior are thoroughly investigated based on an Al-Cu-Mg-Zr alloy for the first time. The obtained findings are applied to design a novel 2618 Al-Cu alloy for LPBF and DMD, with suitable heat treatment being developed for LPBF. The alloy is characterized with respect to microstructure, precipitation formation, and mechanical properties before, during, and after heat treatment. Furthermore, by modifying elemental Al-Zr powder blends with nm-sized Al2O3, an oxide dispersion strengthened alloy is produced by LPBF and investigated in detail.

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https://infoscience.epfl.ch/record/297170?ln=en

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Marvin Lennard Schuster
EPFL, 2022
EPFL thesis

Abstract

Official source

https://infoscience.epfl.ch/record/297170?ln=en

About this result

Related concepts (12)

An aluminium alloy (or aluminum alloy; see spelling differences) is an alloy in which aluminium (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon

Precipitation hardening, also called age hardening or particle hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of

An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting materia

Related concepts

Related publications (11)

Related publications

Metal Additive Manufacturing (AM) technologies have enabled the manufacturing of parts with complex geometries that were previously not feasible with conventional manufacturing. Unfortunately, many commercial engineering alloys (with the exception of alloys for welding) were designed with conventional manufacturing in mind and can behave poorly in the rapid solidification and thermal conditions that go along with AM. Research is needed to optimize the higher performance engineering alloys, such as pre-cipitation hardened alloys, for AM. One important class of precipitates in both the nick-el-base and aluminum-base superalloys is the intermetallic L12 âstructured (Strukturbericht notation) intermetallics. Many scientific challenges must be overcome prior to widespread industrial usage of these alloys. AM of L12-strengthened Ni-base superalloys results in extensive micro-cracking and AM of L12-strengthened Al-base alloys has been successful but requires further research to understand the material be-havior. This thesis focuses on overcoming the scientific challenges for the AM of L12-strengthened alloys. A new Al-Mg-Zr alloy strengthened via L12-structured Al3Zr precipitates, AddalloyÂ®,was successfully fabricated for the first time with Laser-Powder Bed Fusion (L-PPF). The as-fabricated Al-Mg-Zr alloy displayed a unique bimodal grain structure: (i) the bottom of the melt pools consisted of fine equiaxed grains (~1 Âµm) and (ii) the top of the melt pool consisted of columnar grains (up to 40 Âµm long). The bimodal microstructure was at-tributed to the changing solidification conditions during the lifetime of the melt. A new microstructure modification strategy, laser rescanning, was shown to refine the micro-structure. The grain refinement was attributed to the reduced melt pool depths of the additional scans, thus breaking up the previously solidified columnar grains. Scanning transmission electron microscopy (STEM) investigations of peak-aged (400ËC, 8 h) samples reveal both continuous (~2 nm in diameter) and discontinuous (~5 nm wide and hundreds of nanometers long) coherent, secondary L12-Al3Zr precipitates. The Al-Mg-Zr in the peak aged (400ËC, 8 h) condition had a 385 MPa yield strength and a 19% elongation at fracture. The excellent combination of strength and ductility was attribut-ed to the bimodal microstructure. For short-term elevated temperature yield tests, as-fabricated samples displayed higher yield strengths than peak-aged samples at temper-atures above 150ËC (e.g., 87 vs 24 MPa at 260ËC). For longer term creep tests at 260 ËC, both as-fabricated and peak-aged samples displayed near-identical creep behavior. The reduced elevated temperature performance of the peak aged samples was attributed to coarsening of grain-boundary precipitates during aging, decreasing their ability to in-hibit grain-boundary sliding (GBS) of the fine equiaxed grains. The micro-cracking of CM24LC, a Ni-base superalloy, was attributed to both solidifica-tion and liquation cracking. Based on experimental evidence, reduction in the solidifica-tion interval of CM247LC was investigated as a candidate for micro-crack mitigation and a new alloy was developed. As Hf is found to have a significant influence on the freezing range of the alloy, a new CM247LC without Hf was produced and tested. Sam-ples fabricated with the Hf-free CM247LC, CM247LC NHf, in combination with opti-mized processing conditions exhibit a reduction in crack density of 98%.

EPFL2020

Measurements and modelling of residual stresses during quenching of thick heat treatable aluminium components in relation to their precipitation state

Nicolas Paul Henry Chobaut

In the fabrication of heat treatable aluminium parts, solutionising and quenching are key steps in order to obtain the required mechanical characteristics. Fast quenching is necessary to avoid coarse precipitation as this one reduces the mechanical properties after heat treatment. However fast quenching gives birth to high residual stresses, which cause unacceptable distortions during machining and can reduce service life drastically. For this reason, plates and forgings such as those considered in this thesis are subjected to a stress relieving operation, removing most of the residual stresses after quenching. For thick products, fast quenching throughout the thickness is no longer possible owing to the high but still limited thermal diffusivity of aluminium alloys, but residual stress occurrence remains problematic. The objective of this work is to develop a comprehensive model of quenching including precipitation effects for understanding the development of residual stresses in thick heat treatable aluminium components. Two industrial contexts are studied in this work: - the cold-water quenching of hot-rolled AA7040 and AA7449 thick plates for aerospace applications, - the boiling-water quenching of large AA2618 compressor wheel forgings used in turbochargers. Besides an accurate knowledge of the thermal field in the components during quenching, this work has involved an extensive thermo-mechanical characterisation of the three alloys in different precipitation states by interrupted quench-tests achieved in a Gleeble machine. Plastic strain recovery at high temperature is considered in a simple way in the description of the material behaviour and the Bauschinger effect is neglected as suggested by dedicated experiments. Three numerical models are developed to predict as-quenched residual stresses: - a thermo-mechanical model ignoring precipitation, - a physically-based thermo-metallurgical-mechanical model where a yield strength model is coupled with a precipitation model, - a thermo-mechanical model accounting for precipitation in a simple but realistic way. Instead of modelling precipitation that occurs during quenching, the model parameters are identified using a limited number of tensile tests achieved after representative interrupted cooling paths in a Gleeble machine. The results are compared to as-quenched residual stress measurements achieved by neutron diffraction and layer removal in thick AA7449 and AA7040 plates and in large AA2618 forgings. The thermo-mechanical model ignoring precipitation is found to be sufficient for relatively thin products. For thicker products, precipitation hardening by small precipitates is found to increase as-quenched residual stresses. Since the thermo-mechanical model ignoring precipitation underestimates these residual stresses, precipitation must thus be accounted for. The second model gives an excellent agreement between measurements and simulations provided that the precipitation model is well calibrated. This physically-based model is necessary for modelling complex parts with different thicknesses. Nevertheless, a lot of effort has to be dedicated to the characterisation and modelling of the precipitation of metastable non-stoichiometric phases and GP zones during quenching. This tedious work is not needed for the third model which requires only a few interrupted quench-tests while giving an excellent agreement between measurements and simulations for all the investigated components. In particular, it is verified that the two models taking into account precipitation give identical residual stress profiles in thick AA7449 plates.

EPFL2015

This study concerns the evolution of the thermomechanical properties of new high-performance materials. In particular, we focus on the relationship between the microstructure and the mechanical properties of a superalloy based on Co-Ni-Cr. Superalloys are metallic alloys with superior characteristics to conventional alloys, especially in extreme conditions, and find application in all that areas requiring materials with high elastic modulus, yield strength and resistance. The investigated alloy is used with satisfaction since several years, but the origin of its outstanding mechanical properties is still poorly understood, as well as the role of small alloying elements additions on mechanical properties. To understand the influence, on the mechanical properties, of the elements added to the base composition, the study is carried out simultaneously on two variants of the material, differing for the presence of a small amount of beryllium in one them. The variant containing beryllium exhibits superior mechanical properties with respect to the base variant. The microstructural evolution and mechanical properties was monitored by using several complementary techniques. Series of heat treatments were performed on samples from the two variants, and both TEP and hardness were systematically measured. This has allowed us to establish the different stages of microstructural changes with the temperature. The TEP is a parameter very sensitive to the change in matrix's composition. It indicates that exposure of this material to a temperature between 500°C and 600°C produces a precipitation stage (A), involving titanium. Such a precipitation is responsible for a remarkable hardening, as well as an increase in the elastic limit. Mechanical spectroscopy allows us to obtain information about the mobility of structural defects which dissipate energy when undergone to a periodic stress. By developing a phenomenological model based on the ADIF curves, we can affirm that the hardening comes from the pinning of dislocations by A precipitates. A second precipitation stage (B) occurs only on the variant containing beryllium, undergone a complex sequence of heat treatments up to 700°C. The analysis of samples by TEM and APT provided essential support to the interpretation of results, and to characterization of the precipitation B. Precipitates B are nanostructured intermetallic compounds NiBe, analogous to GP zones. Their growth is favored by the segregation of titanium at the precipitate-matrix interface to relieve the coherency strains. The hardening provided by B precipitates, together with the structural hardening due to the cold working and the precipitation hardening due to the heating at 550°C, allows to obtain the maximum hardness values. The peaks on the internal friction spectra, also provide informations about the influence of beryllium on the mobility of grain boundaries. This parameter determines the recrystallization and the relaxation mechanism at high temperature, which can be observed by mechanical spectroscopy. Finally, the work here presented yields an important contribution to the understanding of the performance's origin of a Co-Ni-Cr superalloy. It also points the way for other studies aiming the optimization of production processes, in terms of temperature and time of annealing, to obtain the desired properties. This thesis allowed to discover a new hardening phase (B) previously unknown, which could lead to even higher hardness values.

EPFL2016

Measurements and modelling of residual stresses during quenching of thick heat treatable aluminium components in relation to their precipitation state

Nicolas Paul Henry Chobaut

EPFL2015

EPFL2020

EPFL2016