Precipitation-hardened Aluminum Alloys Research Articles

Thermal Modeling of Al-Al and Al-Steel Friction Stir Spot Welding

This paper presents a finite element thermal model for similar and dissimilar alloy friction stir spot welding (FSSW). The model is calibrated and validated using instrumented lap joints in Al-Al and Al-Fe automotive sheet alloys. The model successfully predicts the thermal histories for a range of process conditions. The resulting temperature histories are used to predict the growth of intermetallic phases at the interface in Al-Fe welds. Temperature predictions were used to study the evolution of hardness of a precipitation-hardened aluminum alloy during post-weld aging after FSSW.

Process Design of Aluminum Tailor Heat Treated Blanks.

In many industrials field, especially in the automotive sector, there is a trend toward lightweight constructions in order to reduce the weight and thereby the CO2 and NOx emissions of the products. An auspicious approach within this context is the substitution of conventional deep drawing steel by precipitation hardenable aluminum alloys. However, based on the low formability, the application for complex stamping parts is challenging. Therefore, at the Institute of Manufacturing Technology, an innovative technology to enhance the forming limit of these lightweight materials was invented. The key idea of the so-called Tailor Heat Treated Blanks (THTB) is optimization of the mechanical properties by local heat treatment before the forming operation. An accurate description of material properties is crucial to predict the forming behavior of tailor heat treated blanks by simulation. Therefore, within in this research project, a holistic approach for the design of the THTB process in dependency of the main influencing parameters is presented and discussed in detail. The capability of the approach for the process development of complex forming operations is demonstrated by a comparison of local blank thickness of a tailgate with the corresponding results from simulation.

Influence of Pre-Strain and Simulated Paint-Bake on Mechanical Properties of High Strength Aluminum Alloy AA7020

In modern car bodies lightweight structures are used to achieve a reduction of energy consumption and CO2-emissions. One of the most important lightweight materials is aluminum and its alloys. Current state of the art in automotive industry is utilizing precipitation hardenable aluminum alloys of the 6000 series whereas the crash-relevant strength is achieved by artificial ageing during the paint drying process. Due to newly developed coatings which provide faster curing at lower temperatures, post-forming ageing of the 6xxx alloys to satisfactory strength levels becomes more difficult. The aim of this study is to investigate the feasibility of employing high strength aluminum alloys of the 7000 series in order to reduce artificial ageing time and temperature while keeping required strength. Within this contribution, the influence of pre-straining and subsequent heat treatment on mechanical properties will be presented.

Integrated FEM-DBEM Simulation of Crack Propagation in AA2024-T3 FSW Butt Joints Considering Manufacturing Effects

This paper deals with a numerical and experimental investigation on the influence of residual stresses on fatigue crack growth in AA2024-T3 friction stir welded butt joints. An integrated FEM-DBEM procedure for the simulation of crack propagation is proposed and discussed. A numerical FEM model of the welding process of precipitation hardenable AA2024-T3 aluminum alloy is employed to infer the process induced residual stress field. The reliability of the FEM simulations with respect to the induced residual stresses is assessed comparing numerical outcomes with experimental data obtained by means of the contour method. The computed stress field is transferred to a DBEM environment and superimposed to the stress field produced by a remote fatigue traction load applied on a friction stir welded cracked specimen. Numerical results are compared with experimental data showing good agreement and highlighting the predictive capability of the proposed method. Furthermore, the influence of the residual stress distribution on crack growth is evidenced.

Experimental Investigation of Heat-Treated Aluminum Profiles

Tailor Heat Treated Profiles (THTP) are profiles that exhibit local different mechanical properties optimized for a subsequent forming operation. The property distribution is realized by short term heat treatment before a forming operation. Based on the interaction of soft and hard areas the material flow can be improved and the formability can be enhanced. Prerequisite for a successful application of the technology is a comprehensive material characterization. Therefore, within this paper the influence of short term heat treatment on the mechanical properties of profiles will be presented. In particular, different heating technologies based on heat conduction and laser radiation are compared. Based on the results, a process window will be derived. All investigations were performed using the precipitation hardenable aluminum alloy EN AW 6060.

Microstructure and Crystallographic Texture Evolution During the Friction-Stir Processing of a Precipitation-Hardenable Aluminum Alloy

Friction-stir processing (FSP) has been proven as a successful method for the grain refinement of high-strength aluminum alloys. The most important attributes of this process are the fine-grain microstructure and characteristic texture, which impart suitable properties in the as-processed material. In the current work, FSP of the precipitation-hardenable aluminum alloy 2219 has been carried out and the consequent evolution of microstructure and texture has been studied. The as-processed materials were characterized using electron back-scattered diffraction, x-ray diffraction, and electron probe microanalysis. Onion-ring formation was observed in the nugget zone, which has been found to be related to the precipitation response and crystallographic texture of the alloy. Texture development in the alloy has been attributed to the combined effect of shear deformation and dynamic recrystallization. The texture was found heterogeneous even within the nugget zone. A microtexture analysis revealed the dominance of shear texture components, with C component at the top of nugget zone and the B and A(2)* components in the middle and bottom. The bulk texture measurement in the nugget zone revealed a dominant C component. The development of a weaker texture along with the presence of some large particles in the nugget zone indicates particle-stimulated nucleation as the dominant nucleation mechanism during FSP. Grain growth follows the Burke and Turnbull mechanism and geometrical coalescence.

Thermal Management in Friction-Stir Welding of Precipitation-Hardened Aluminum Alloys

Process design and implementation in friction-stir welding (FSW) is mostly dependent on empirical information. Basic science of FSW and processing can only be complete when fundamental interrelationships between the process control parameters and response variables and the resulting weld microstructure and properties are established to a reasonable extent. It is known that primary process control parameters such as tool rotation, translation rates, and forge axis force have complicated and interactive relationships to process-response variables such as peak temperature and time at temperature. Of primary influence on the other process-response parameters are temperature and its gradient in the deformation and heat-affected zones. Through a review of pertinent works in the literature and results from boundary condition experiments performed in precipitation-hardening aluminum alloys, this article partially elucidates the nature and effects of temperature transients caused by variation of thermal boundaries in FSW.

Optimizing weld quality of a friction stir welded aluminum alloy

A series of welds were completed at various processing parameters for a threaded tool in butt friction stir welded (FSWed) panels of AA 2219-T87 to determine the optimal welding parameters. The panels were sectioned transversely along the entire panel and characterized by tensile tests, hardness, and macrostructure imaging. High speed data acquisition collected the force data during the welds. X-ray diffraction was used to interpret the qualitative amount of equilibrium 2nd phases present. Variations in the mechanical properties were correlated with volumetric defects and metallurgical changes in the precipitate state. An optimal parameter window was chosen based on the UTS, hardness, changes in the weld forces, and the presence of volumetric defects. Guidelines were developed for future production welding of precipitation hardened aluminum alloys.

Thermomechanical Treatment of Aging Aluminum Alloys

The mechanical properties, corrosion resistance and fracture behavior of precipitation-hardening aluminum alloys of different alloying systems are studied after a thermomechanical treatment with deformation in a wide temperature range. Application of deformation at the temperature of optimum aging in the thermomechanical treatment is shown to have a positive effect on the strength of the aluminum alloys at a low decrease in the ductility.

Influences of pin profile on the mechanical and microstructural behaviors in dissimilar friction stir welded AA6082–AA7075 butt Joint

Abstract This paper investigates the effect of tool pin profile and post weld heat treatment on the mechanical and microstructural aspects of friction stir welded dissimilar precipitation-hardenable aluminum alloys between 6xxx (Al–Mg–Si) and 7xxx (Al–Zn–Mg). It was found that the tool with a conical probe with three grooves produces higher temperatures than the tool with a square frustum probe. The mixture of materials in the weld nugget of joints welded by the tool with the square frustum probe was more uniform. Of the natural aging times studied, the stir zone of all welds showed the highest kinetics of microhardness and strength recovery; its effect increased considerably in the heat input per unit length decreased.

Numerical Modeling of AA2024-T3 Friction Stir Welding Process for Residual Stress Evaluation, Including Softening Effects

In the present paper, a numerical finite element model of the precipitation hardenable AA2024-T3 aluminum alloy, consisting of a heat transfer analysis based on the Thermal Pseudo Mechanical model for heat generation, and a sequentially coupled quasi-static stress analysis is proposed. Metallurgical softening of the material is properly considered and included in the calculations by means of the Myhr and Grong model, implemented as a user subroutine in ABAQUS. Numerical outcomes are compared with experimental results, highlighting the intriguing predictive capabilities of the model for both temperatures and residual stresses. The contour method is employed to map the longitudinal residual stress distribution on a transverse cross section of the joint. The influence of the applied boundary conditions and of the release of the clamping system on residual stresses is also assessed.

Process Window for Friction Stir Lap Welding of Aluminum Alloy 6061

Precipitation-hardenable 6xxx series aluminum alloys are incorporated in many structural components with due consideration of their good combination of properties including a relatively high strength, outstanding extrudability and excellent corrosion resistance. Accordingly, AA6061 has been identified as a very good candidate material for structural lightweighting of transportation vehicles. However, the weldability of aluminum alloy (AA) 6061 by means of conventional technologies such as GMAW and GTAW methods is limited by sensitivity to solidification cracking. In this respect, friction stir welding (FSW) presents a tremendous potential for assembly of aluminum structures for the transportation industry due to the low heat involved that can mitigate crack formation and, thus, translate into improved mechanical performance of the assembly. In this work, FSW of 3.18 mm thick AA6061-T6 sheets in the lap joint configuration was investigated. This configuration is considered to be more challenging for assembly by FSW than the butt joint type due to the orientation of the interface with respect to the welding tools and the necessity to break the oxide layer on two aluminium alloy planar surfaces. Weld trials were performed to examine the influence of the FSW tool geometry and process parameters on the welding defects, microstructure, hardness and bend performance. Unacceptable material expulsion and/or significant thinning in one of the two overlapped sheets were produced under most conditions. A set of FSW tool geometries leading to a viable process operational window under which the risk of defects could be mitigated and/or eliminated was identified in this study.

Load induced inhomogeneous plastic deformations in welded aluminium joints

In this work, the influence of varying heat input related to different welding processes (gas tungsten arc welding (GTAW), gas metal arc welding (GMAW) and electron beam) on the structure and property of the weld near heat-affected zone was investigated using a cold-formed and a precipitation-hardened aluminium alloy. Multiple tensile tests on welded samples were performed in order to comprehend the correlation between heat-induced local softening and dissipation of precipitations and the local elastic and plastic deformation behaviour under tensile loading. During these testings, the transient deformation was measured using an optical 3D deformation measuring system based on grey-scale pattern recognition. Additionally, material characterizations, such as two-dimensional hardness measurements using the instrumented indentation test, were carried out. Both aluminium alloys showed a different strain distribution under tensile loading. In addition, finite element (FE) models were developed to describe the plastic deformation behaviour. In finely meshed models, the distribution of the local yield strength was set corresponding to measurements using instrumented indentation. That way, the differences between models which include a softening area and those that do not could be shown.

Postponing Crack Nucleation in 2024 Aluminium Alloy by Dynamic Precipitation from the Supersaturated State

Recently a novel strategy to improve the fatigue resistance of precipitation hardened aluminium alloys has been proposed, which is based on dynamic precipitation in partially aged material. In this paper, the effect of mean stress and alternative temperature treatments that can enhance the high cycle fatigue resistance through the mentioned mechanism were investigated. The material used is an under-aged 2024 aluminium alloy, which showed superior fatigue life compared to the peak aged condition. The recorded behaviour was observed to be more effective at lower stress amplitudes and lower R values. As the dynamic precipitation may not be able to keep up with the damage evolution, dedicated experiments were conducted to insert periods of controlled healing in a stress free state.

Influence of Pre-straining and Heat Treatment on the Yield Surface of Precipitation Hardenable Aluminum Alloys

Abstract Precipitation hardenable aluminum alloys are some of the most important lightweight materials. However, their range of applications in comparison to conventional deep drawing steels is limited by the low formability. Therefore, a new and innovative approach to enhance the formability of aluminum alloys in multistage forming operations was invented at the Institute of Manufacturing Technology, called intermediate heat treatment (IHT). Based on a short-term, laser-assisted heat treatment between two forming steps, it is possible to locally adapt the mechanical properties and realize an optimized strength distribution. For the successful application of the technology, the influence of the heat treatment on the mechanical properties has to be analyzed. Concerning the simulation of a multistage forming process, in particular, the yield surface of the material is very important. Within this paper, the influence of a combined pre-straining and a subsequent short-term, laser-assisted heat treatment on the yield surface will be presented.

Laser Shot Peening to Improve Surface Properties of Precipitation Hardened Aluminum Alloy Al-6061-T6

Laser Shot Peening to Improve Surface Properties of Precipitation Hardened Aluminum Alloy Al-6061-T6

Effects of Chromate and Molybdate Ions On Scratch Repassivation Behavior of Precipitation Hardened Aluminum Alloys

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Identification of Load Induced Inhomogeneous Plastic Deformations in Aluminium Welds by Diffraction Methods

Cold formed and precipitation hardened aluminium alloys welded with arc welding and electron beam welding processes have been investigated to analyse the local deformation behaviour under static and cyclic loads. Therefore lateral strain measurements with different techniques have been carried out in the surrounding of the weld seam. The strain measurements were combined with micro hardness measurements in order to determine loading induced local softening and hardening. These investigations were combined with X-ray diffraction experiments which should provide information about the residual stress condition and additionally about load induced changes of the hardening condition by the analysis of the FWHM-values of the diffraction lines. Additionally in-situ diffraction experiments using synchrotron radiation where carried out to analyse the deformation behaviour of particular welds under varying static loads. The paper gives an overview about the experimental procedures and important results and their consequences for the evaluation of such aluminium welds.

Ultrasonic Welding of Aluminum Alloys to Fiber Reinforced Polymers

To realize multi‐material structures, e.g., out of light metals and fiber reinforced polymer (FRP) composites suitable joining methods are required. The ultrasonic metal welding technology was applied to produce high strength joints between different aluminum alloys and carbon fiber reinforced polymers (CFRPs) in the framework of research unit 524 of the German Research Foundation. The bonding mechanisms were characterized by light optical and scanning electron microscopy (SEM). The performed analysis have shown a bonding zone with an intensive contact between the metallic surface and the load bearing carbon fibers of the CFRP as a result of the ultrasonic welding process. Using precipitation hardening aluminum alloy AA2024 tensile shear strengths of up to 58 MPa could be achieved. In comparison to established joining techniques ultrasonic metal welding can be an interesting alternative to realize dissimilar joints for the automotive or aircraft industry.

Effects of Chromate and Molybdate Ions on Scratch Repassivation Behavior of Precipitation Hardened Aluminum Alloys

Scratch depassivation of AA2024-T351, AA7075-T6, and 99.999% aluminum in molybdate (MoO4 2-) and chromate (CrO4 2-) containing NaCl solutions was used to examine electrochemical repassivation kinetics over a range of potentials and inhibitor concentrations. Single frequency impedance measurements were taken in the capacitive region of frequency response. Capacitance measurements enabled the observation of the effects of inhibitor additions on oxide growth as protective oxide films formed on the scratched electrode surface. Chromate was found to suppress scratched electrode current transients at high potentials on both AA2024-T351 and AA7075-T6. Capacitance measurements revealed the rapid growth of a thick, protective oxide when concentrations as low 5 mM CrO4 2-E were in solution. Molybdate did not suppress transient current density in AA2024-T351 or AA7075-T6. In some cases the addition of molybdate increased the total charge passed after the scratch. However, capacitance measurements in the presence of molybdate indicate the rapid growth of a passive film.