Precipitation-hardened Aluminum Alloys Research Articles

Heat Treatment Influencing Porosity and Tensile Properties of Field Assisted Sintered AlSi7Mg0.6.

In this study, an attempt was made to improve the mechanical properties and in particular the strength of a precipitation-hardenable aluminum alloy while still maintaining high ductility. For this purpose, AlSi7Mg0.6 (A357) powder with an average particle diameter of d50 = 40 µm was consolidated using field assisted sintering technique (FAST), and two material conditions were compared: an as-sintered and an underaging heat treated condition (T61). Mechanical properties were determined using tensile tests and hardness measurements. In addition, the microstructure was investigated by optical microscopy. Further, porosity and density were analyzed after the different heat treatments. By the underaging heat treatment, the surface hardness was increased by 100% and the yield strength was increased by 80% compared to the as-sintered material. However, the elongation to failure dropped to one third of that of the as-sintered material. Presumably, this effect was a result of an increased porosity due to the heat treatment. It is assumed that the observed pores were generated by artefacts from the FAST process used to manufacture the samples. The internal gas pressure and equilibrium diffusion supported by heat treatment temperature, and the reduction in surface energy caused by coalescent micropores, led to the enlargement of previously undetectable inhomogeneities in the as-sintered material that resulted in pores in the heat-treated sintered alloy.

Scale-Dependent Structure–Property Correlations of Precipitation-Hardened Aluminum Alloys: A Review

Scale-Dependent Structure–Property Correlations of Precipitation-Hardened Aluminum Alloys: A Review

The concept of improvement high-strength aluminum alloys FSW joint properties via post-weld explosive treatment

The study describes the theoretical background and technological aspects of the post-weld explosive treatment of high-strength aluminum alloy FSW joints. Although FSW allows to effective join high-strength aluminum alloys, the heat generated during the process causes undesirable changes in the strengthening phase, giving a joint efficiency of about 80%. The load-carrying capabilities of these joints can be increased via post-weld treatment (e.g. shot peening, laser shock peening). The new, potential post-weld treatment that is presented in this paper is based on the affection of the welded joint by a shock wave generated during the detonation of explosive material. Such post-weld explosive treatment would result in the hardening of the low-hardness zone, which often determines the mechanical properties of precipitation-hardened aluminum alloy FSW joints. Studies show that explosive welding of annealed aluminum alloys increases their microhardness by about 25% as the result of a high-velocity collision. If a similar effect can be achieved in explosive hardening, the microhardness of the low-hardness zone will increase entailing an improvement of entire joint mechanical properties. The variety of explosives materials used in metalworking (covering the values of detonation velocity from about 2000 m/s to 8000 m/s) and different systems for shock-wave affection gives many technological possibilities. In this work are discussed two different explosive hardening systems: with direct placement of explosive material on a treated welded plate and with an additional driven plate, which provides a higher pressure impulse. Considering that affecting of high amplitude shock wave introduces defects into the structure and decreases residual stresses in the welded joints, the application of an appropriate technological system creates a potential for improving the load-carrying capacities of discussed joints, especially in a condition of cyclic loading.

Defect formation, microstructure evolution, and mechanical properties of bobbin tool friction–stir welded 2219-T8 alloy

Bobbin tool friction stir welding (BT–FSW) is a novel and effective welding method for aluminum alloys, but microvoid defects are usually formed in the nugget zones (NZ). Therefore, it is important to understand the formation mechanism of microvoid defects and their effect on the fracture behavior of joints. In the present study, 6 mm thick 2219–T8 aluminum alloy plates were subjected to BT–FSW under a constant rotation rate of 300 rpm and welding speeds of 150–500 mm min–1 with the objective of analyzing the evolution of microvoid defects and their effect. The results showed that, lower welding speeds resulted in microvoid defects in the band pattern structure on the retreating side (RS) of the NZ, and the size of the defects decreased with increasing the welding speed, while sound joints were obtained under higher welding speeds. In contrast to conventional FSW precipitation–hardened aluminum alloys with the lowest hardness zone (LHZ) at the heat affected zone (HAZ), the LHZs of the BT–FSW 2219–T8 joints were located at the thermal–mechanically affected zone (TMAZ). Digital image correlation (DIC) of large tensile samples showed that the microvoid defects exhibited an obvious effect on the tensile deformation and fracture behavior of the joints due to the decrease in the maximum strain and lack of necking. Mini-samples in different layers further confirmed that cracks initiated from the microvoid defects in the NZ. However, the sound BT–FSW 2219–T8 joints fractured along the LHZs on the RS, with a maximum joint strength coefficient of 78.1% being achieved.

Tensile Behaviour of Friction Stir Welded Joints of Different Aluminium Alloys

This paper investigates the outcome of base metals and their temper condition on tensile and fracture behavior of friction stir welds of various work and precipitation hardening aluminium alloys in different tempers (AA1100-O, 5052-H, AA5086-O & H25, AA2024-O & T6, and AA6061-T651). The type and temper of aluminium alloys affected the tensile, and fracture behavior of weld joints. The extent of improvement in tensile properties of welded joints increased with a decrease in hardness (i.e., from T6 or H to O) of base metals. Softening was not observed for welded joints when base metal was in an annealed temper and an opposite trend was observed for hardened tempers ‘H’ and ‘T6’. Fracture location moved towards the weld centre with the change in temper from ‘O’ to ‘H’ and ‘T6’, hardness minima were closer to base metal hardness in the first case than later. The mode of fracture was ductile for all the weld joints except AA2024-T6 and AA 6061-T651. Except for AA2024-T6 and AA 6061-T651, all weld joints had ductile fracture mode.

Tribological behaviour of metal matrix composites

Aluminum 6061 is a precipitation hardening aluminum alloy, has good mechanical properties, and exhibits good weldability. It is used in Aircraft and Aerospace components, Bicycle frames, Drive shafts and Brake components. The addition WC with Ali6061 helps to improve the wear rate of metal matrix composite. In this present investigation, Aluminum (Al 6061) is used as base matrix metal and Tungsten Carbide (WC) particulate as reinforcement. Fabrication of MMCs was done by stir-casting process. Microstructure examination was carried by using Scanning Electron Microscope (SEM) to obtain the distribution of tungsten carbide. Three body abrasion study was carried out to do the analysis. Dry sand abrader followed with ASTM G-65 test was also done.

Short Heat Treatments for the F357 Aluminum Alloy Processed by Laser Powder Bed Fusion.

Conventionally processed precipitation hardening aluminum alloys are generally treated with T6 heat treatments which are time-consuming and generally optimized for conventionally processed microstructures. Alternatively, parts produced by laser powder bed fusion (L-PBF) are characterized by unique microstructures made of very fine and metastable phases. These peculiar features require specifically optimized heat treatments. This work evaluates the effects of a short T6 heat treatment on L-PBF AlSi7Mg samples. The samples underwent a solution step of 15 min at 540 °C followed by water quenching and subsequently by an artificial aging at 170 °C for 2–8 h. The heat treated samples were characterized from a microstructural and mechanical point of view and compared with both as-built and direct aging (DA) treated samples. The results show that a 15 min solution treatment at 540 °C allows the dissolution of the very fine phases obtained during the L-PBF process; the subsequent heat treatment at 170 °C for 6 h makes it possible to obtain slightly lower tensile properties compared to those of the standard T6. With respect to the DA samples, higher elongation was achieved. These results show that this heat treatment can be of great benefit for the industry.

Effect of Post-Weld Heat Treatment on the Solid-State Diffusion Bonding of 6061 Aluminum Alloy

The precipitation-hardenable aluminum alloy 6061 (AA 6061) is favored for aerospace components and automotive parts. However, the tenacious oxide layer on the surface greatly limits the quality and applicability of joining AA 6061. In this study, the joining method of solid-state diffusion bonding was implemented for AA 6061 plates, and the effects of post-weld heat treatment (PWHT) on the joint interface were investigated. The bonding temperatures were within the range of 500–530 °C, and the time periods varied from 30 to 240 min under a static pressure of 5 MPa in a vacuum. The diffusion bonded specimens were subjected to T4- and T6-PWHT to improve the bonding quality. The interfacial microstructure of the joints was analyzed by scanning electron microscopy, and the mechanical properties were evaluated with shear tests. The experimental results showed that the shear strength of the diffusion bonded joint could reach around 71.2 MPa, which was highly dependent on bonding temperature and holding time, and T6-PWHT further enhanced it to over 100 MPa. The effects of PWHT on the diffusion bonded AA 6061 joint were investigated, and the fractography on the sheared surfaces indicated that PWHT-T6 played an important role in enhancing joint strength, which was consistent with the measured shear strength. The sequential PWHT for AA 6061 after diffusion bonding was proven to be feasible for bonding of AA 6061 parts, and the joint strength was sufficient for industrial needs.

Repair of aluminum 6061 plate by additive friction stir deposition

The deposition of new alloy to replace a worn or damaged surface layer is a common strategy for repairing or remanufacturing structural components. For high-performance aluminum alloys common in the automotive, aerospace, and defense industries, however, traditional fusion-based deposition methods can lead to solidification cracking, void formation, and loss of strength in the heat-affected zone. Solid-state methods, such as additive friction stir deposition (AFSD), mitigate these challenges by depositing material at temperatures below the melting point. In this work, a solid-state volumetric repair was performed using AFSD of aluminum alloy 6061-T6 to fill grooves machined into the surface of a plate of 6061-T651. The groove-filling process is relevant to replacement of cracked or corroded material after removal by localized grinding. Three groove geometries were evaluated by means of metallographic inspection, tensile testing, and fatigue testing. For the process conditions and groove geometries used in this study, effective mixing of the substrate and deposited alloy were achieved to a depth of 3.1–3.5 mm. Below that depth, the interface between the substrate and AFSD alloy exhibited poor bonding associated with insufficient shear deformation. This suggests a practical limitation of approximately 3 mm for the depth of repair by groove filling using the current combination of process parameters, materials, and tool design. The mechanical properties of the filler alloy, the depth of the heat-affected zone, and areas for further optimization are discussed within the context of precipitation hardened aluminum alloys.

Comparative assessment on weldability of Al 6061 to polycarbonate for dissimilar sheets placement in friction stir lap welding using sensor signals

The thermoplastic polymers and precipitation hardened aluminium alloys are highly popular in the aerospace and automobile sectors as a replacement of metallic materials to improve the strength to weight ratio. Thus, the unlike aluminium alloy to polycarbonate assembled structures are often necessary for which mechanical fastening and adhesive bonding are the primary methods for joining as fusion welding processes are inadequate. However, the dissimilar joint efficiency is found to be less. Thus, the ultrasonic and friction welding processes are developed. The friction stir welding is one such advanced material stirring technique without any melting of base materials. The present work addresses metallic aluminium (Al6061) to polycarbonate sheet materials joining using friction stir welding in overlap configuration using tapered H13 tool steel. The thrust force with associated tool stirring torque has been acquired in real time during plunging followed by welding phase. The weld bead profile with respective force-torque signals was analysed for the process monitoring. The tensile test has been carried out on the lap welds. The weld interface of the unlike sheets have also been scrutinised. Initially, the aluminium sheet was partially overlapped on polycarbonate for the parametric study. The highest joint efficiency was found to be 40.2% at 1400 rpm tool rotational speed and 75 mm/min traverse speed due to improper material mixing at the weld interface. Therefore, the feasibility of the process have been tested by placing thermoplastic polycarbonate over aluminium alloy through which the joint efficiency was further improved (48.57%) at comparatively low tool rotational speed (1100 rpm) with lower welding speed (55 mm/min) as the minute metallic particles uniformly mixed with melted and solidified polycarbonate due to more uniform torque in the welding phase. The tool stirring torque and axial thrust was found to be higher in this overlap position.

Feasibility study of FSW, LBW and TIG joining process to fabricate light combat aircraft structure

In this research, friction stir welding (FSW), laser beam welding (LBW) and tungsten inert gas (TIG) welding are used to join precipitation hardening aluminum alloy 2014-T6. This alloy is a potential candidate in heavy structural fabrication industries due to strength ratio. However, the joining of this alloy is a major problem in assembly line. Hence, to find an ideal method for welding this alloy, three processes were compared in terms of mechanical and metallurgical properties of welded joints. Results showed that the FSW joints disclosed better strength than other LBW and TIG joints. In comparison to LBW and TIG joints, FSW conceived narrow heat-affected zone and recorded maximum hardness of 135 VHN. Similarly, the maximum hardness in the LBW and TIG welded joints were 110 VHN and 92 VHN respectively. The reason for maximum strength in FSW joints is the finer size and uniform distribution of strengthening precipitate. (Particle size diameter of 39 nm, thickness of 29 nm).

Enhanced aging kinetics in Al-Mg-Si alloys by up-quenching

Precipitation-hardened aluminium alloys typically obtain their strength by forming second-phase particles, which, however, often have a negative effect on formability. To enable both lightweight construction and forming of complex parts such as body panels, high strength and formability are required simultaneously. Cluster hardening is a promising approach to achieve this. Here, we show that short thermal spikes, denoted as up-quenching, increase aging kinetics, which we attribute to the repeated process of vacancies being formed at high temperatures and retained when cooled to lower temperatures. Combined with further heat treatment, the up-quenching process promotes rapid and extensive cluster formation in Al-Mg-Si alloys, which in turn generates significant strengthening at industrially relevant heat treatment time scales. The high elongation values also observed are attributed to reduced solute depleted zones along grain boundaries.

Influence of Hot Deformation on the Precipitation Hardening of High-Strength Aluminum AA7075 during Thermo-Mechanical Processing

The aim of this work was to investigate the effect of hot deformation on the aging behavior of precipitation-hardenable aluminum alloy AA7075 within a novel thermo-mechanical forming process, in order to gain insight into its precipitation kinetics. For this purpose, the material was formed at 420 °C after undergoing solution treatment to different strain levels ranging from 2% to 10% to obtain different dislocation densities. After undergoing hot deformation, aging at 120 °C with different parameters was carried out to improve the material hardness. The resulting material properties and microstructure evolution were characterized afterward using hardness measurements and a transmission electron microscope (TEM). TEM investigations revealed the formation of very fine particles for the material formed at 2%, as well as at 10%, of formed material, which act as effective barriers to dislocation motion. It was found that the response of artificial aging on the deformation degree in hot forming was less than expected due to the thermally activated mechanisms, leading to a decrease in dislocation density. Therefore, a dramatic increase in material hardness with the increase in hot deformation was not observed.

Experimental and modelling assessment of ductility in a precipitation hardening AlMgScZr alloy

Precipitation hardening is the most effective strategy to enhance the mechanical properties of metals. Dislocation mechanisms to control strengthening during precipitation have been demonstrated extensively. However, owing to the complexity of different precipitates in alloys, variations in ductility caused by precipitation are complex and have not been clarified so far. In this study, the effects of precipitation on ductility in precipitation hardening aluminium alloys are investigated based on a modified dislocation-based approach and experimental characterisation. The AlMgScZr alloy with spherical Al3(Sc, Zr) precipitates is used as a model alloy system to understand the effects of precipitation on ductility. Via heat treatment, shearable and nonshearable Al3(Sc, Zr) precipitates are introduced in the AlMg matrix. The results show that the ductility of AlMgScZr alloy decreases when shearable precipitates occur, while it increases with shearable precipitates being replaced by nonshearable precipitates. The variation in ductility of AlMgScZr alloy is mainly controlled by the dynamic recovery rate of the dislocations. Finally, by analysing the different precipitate–dislocation interactions and evaluating the dislocation density evolution during deformation, the dislocation mechanisms of ductility during precipitation for AlMgScZr alloy are demonstrated. This study reveals the dislocation mechanism for controlling ductility during precipitation for AlMgScZr alloy which can provide a theoretical foundation for the design of high-performance structural materials.

Influence of a local short-term heat treatment on the formability of orbital formed functional components

Conventional processes like shearing dispose a lack in efficiency and formability to manufacture functional components with a high geometric variety. A possibility to allow the efficient and sustainable manufacturing is the application of innovative forming operations like orbital forming. Thus, a strain-hardened component with a load-adapted thickness profile can be manufactured in a single-stage process. Thereby, the control of the material flow could be identified as major challenge due to the three-dimensional stress and strain state. In order to reduce the parts weight and simultaneously guarantee the same level of performance, conventional steel is substituted by lightweight materials such as precipitation hardenable aluminum alloys. However, new challenges arise due to the reduced formability of aluminum compared to conventional steel. In recent research, the potential of a short-term heat treatment to enlarge the formability could be shown. The presented process locally reduces the materials strength, thus allowing a control of the material flow by the interaction between softened areas and areas, which offer the initial conditions. In this contribution, the influence on the formability of orbital formed components manufactured out of the precipitation hardenable aluminum alloy EN AW 6016 by a local short-term heat treatment is investigated. Different geometry-based heat treatment layouts are applied in order to maximize the thickening on different radial positions. The potential to enhance the formability is evaluated by comparing geometrical and mechanical properties of the components manufactured in the conventional process route and with a previous heat treatment. The analysis of radial cross-sections and the hardness distribution reveals the positive influence of the heat treatment but also points out the geometrical dependency of the effect due to the characteristic material flow in orbital forming.

The opportunity to join through laser braze-welding of aluminium alloys with galvanized low alloy steels

The welding process of aluminium alloys with steels presents a series of difficulties due to the metallurgical incompatibility between this two materials.This paper deals to abilitate the laser braze-welding process of a precipitation hardened aluminium alloy with a galvanized C-Mn steel, using as filler material an aluminium alloy wire, AlSi12. Avoiding braze-welding defects which may constitute breaking triggers, ensuring a good fusion between the filler material and the base material of the aluminum alloy, obtaining a sufficient thickness of braze-welded seam and formation of intermetallic compounds FexAly layer, represents the main aspects of the analyzed issue.

Eddy Current at High Temperatures for in-Situ Control of Heat Treatment Precipitation in Hardening Aluminum Alloys

Aluminum alloys are commonly used in commercial aircraft due to their low density and excellent mechanical properties. However, they need to become increasingly competitive in order that aluminum plate manufacturers maintaining market share. One way to ensure this is to improve or control parameters used in heat treatments. It is well known that electrical conductivity of metal alloys varies with microstructural change. Therefore, it is believed that In-Situ control of microstructural evolution can lead to optimization of treatment time, reduced energy consumption, increased production and final product quality, as well as reduction of costs, nonconformities and rework, having low implementation cost. In this way, eddy current sensors can be coupled to the furnace controller to optimize time and quality of heat treatment through the analysis of electrical conductivity with low implementation cost. Therefore, the objective of this work is to design, model and validate an eddy current sensor to monitor heat treatment of precipitation hardenable aluminum alloys in real time. Samples of 2024 alloy were prepared by solution treatment at 495 °C for 3 h and monitorization of aging treatment was performed at 190 °C for 9 h. Hardness, metallography, electrical conductivity (outside furnace) and relative electrical conductivity (inside furnace) were analyzed. The results were compared with the literature. There is a lot of discrepancy in the results of other authors. The eddy current sensor showed that behavior is similar to electrical conductivity through four-terminal method outside furnace.

First Principles Prediction of Corrosion Kinetics As a Function of Microstructural Features in Precipitation Hardened Aluminum Alloys

In this poster, first principles density functional theory (DFT) implemented in the Vienna Ab Initio Simulation Package (VASP)1 is coupled with micro-kinetic model2 to simulate the electrochemical corrosion kinetics of some commonly identified intermetallic particles3 (IMPs) associated with precipitation hardened aluminum alloys such as AA7075. First principles DFT calculation is used to characterize the adsorption energies for the cathodic reaction intermediates on intermetallic surfaces. The species include atomic oxygen (O) and hydroxide (OH) as intermediates for the oxygen reduction reaction. We will show that when coupling the thermodynamic calculations with an appropriate micro-kinetic model, we can predict the corrosion kinetics of IMPs. One of the most desirable properties obtained from the simulated polarization curve is the corrosion potential of the IMPs. The corrosion potential can indicate the relative nobilities of IMPs with respect to the alloy matrix. Experimentally the corrosion potential of IMPs on the range of microns are measured via a micro-capillary cell technique. With the state of the art TEM, localized corrosion even down to the nm scale can be characterized4. On the simulation side, however, a method that can be directly compared to experimental data has not yet been fully realized. Attempts to estimate corrosion potentials from the surface work functions obtained by DFT calculations have been made in recent literature but the direct correlation of work function to the corrosion potential has a significant band of uncertainty associated with it. Rather than finding direct correlation, we incorporate work function into the kinetics equation as one of the input parameters. Our simulated for Al7FeCu2, one of the most commonly identified IMP in high strength aluminum alloy, is -337 mVNHE, which is in good agreement with experimental data (-307 mVNHE). We contend here that a first-principles model founded in electrode kinetics is a promising path forward for bridging first-principles simulation and experiment in corrosion science.Reference G. Kresse and J. Furthmüller, Comput. Mater. Sci., 6, 15–50 (1996).H. A. Hansen, V. Viswanathan, and J. K. Nørskov, J. Phys. Chem. C, 118, 6706–6718 (2014) http://pubs.acs.org/doi/abs/10.1021/jp4100608.N. Birbilis and R. G. Buchheit, J. Electrochem. Soc., 152, B140 (2005) http://jes.ecsdl.org/cgi/doi/10.1149/1.1869984.Y. Zhu et al., Acta Mater., 189 (2020).

Improving the mechanical properties and reducing the residual stresses of AA7075 by combination of cyclic close die forging and precipitation hardening

As one of the major strengthening mechanisms, the precipitation hardening is used to enhance the mechanical properties of aluminum alloys. Based on the results of the residual stress measurement, after fast quenching, the core of the material is under tensile stress while the surfaces are under compressive residual stress. Distortion commonly happens during the machining of precipitation-hardened aluminum alloys due to the residual stresses created during the quenching step in the heat treatment process. In this study, the combination of cyclic close die forging and precipitation hardening was used to improve the mechanical properties and control the residual stresses of AA 7075, simultaneously. According to the results, a considerable level of residual stresses was developed in the sample after the quenching step. Performing the cyclic close die forging process immediately after the quenching step changed the pattern of the residual stresses and reduced them significantly. The reduction of the residual stresses after the first pass of cyclic close die forging was about 50%, while after two passes, the sample was almost fully stress relieved. Besides, the results of the microhardness and uniaxial tensile tests demonstrated the improvement of the mechanical properties of the processed samples when compared to the T6 condition. Also, in comparison to AA 7075-T6, the yield stress, ultimate tensile stress, and microhardness were increased by about 24%, 22%, and 48%, respectively.

Application of aberration-corrected scanning transmission electron microscopy in conjunction with valence electron energy loss spectroscopy for the nanoscale mapping of the elastic properties of Al-Li-Cu alloys.

The stress and strain play an important role in strengthening of the precipitation-hardened Aluminum (Al) alloys. Despite the determination of relationship between the mechanical properties and the precipitation existing in the microstructure of these alloys, a quantitative analysis of the local stress and the strain fields at the hardening-precipitates level has been seldom reported. In this paper, the microstructure of a T8 temper AA2195 Al alloy is investigated using aberration corrected scanning transmission electron microscopy (AC-STEM). The strain fields in Al matrix in the vicinity of observed precipitates, namely T1 and β' , are determined using geometric phase analysis (GPA). Young's modulus (Ym ) mapping of the corresponding areas is determined from the valence electron energy loss spectroscopy (VEELS) measured bulk Plasmon energy (Ep ) of the alloys. The GPA-determined strains were then combined with VEELS-determined Ym under the linear theory of elasticity to give rise the local stresses in the alloy. The obtained results show that the local stresses in Al matrix having no precipitates were in the range of 138 ± 2 MPa. Whereas, in the vicinity of thin and thick T1 platelet shape precipitates, the stresses were found to be about 202 ± 3 MPa and 195 ±3 MPa, respectively. The stresses measured in the vicinity of β' spherical shape precipitates found out to be 140 ± 3 MPa which was near to the local stress value in Al matrix. Our findings suggest that the precipitate hardening in T8 temper AA2195 Al alloy predominantly stems from thin T1 precipitates.