Avoiding abnormal grain growth in thick 7XXX aluminium alloy friction stir welds during T6 post heat treatments

Friction stir welding (FSW) is becoming an essential welding technique in the aerospace industry. However, friction stir welds are usually weaker than the parent material. The dissipated heat causes a softening of the 7XXX series aluminium alloys. Post-welding heat treatments are used to restore the highest strength after welding. The reference treatment is composed of two-steps heating: a solution heat treatment typically at 470 ◦C for 30 min, quench in water followed by artificial aging 120 ◦C for 24 h. These treatments are responsible for very significant grain coarsening in the weld nugget zone known as secondary recrystallization, sometimes leading to abnormal grain growth (AGG). This is particularly true for thick plates. This secondary recrystallization phenomenon is detrimental for tensile performances. A detailed analysis of the conditions leading to secondary recrystallization during post-welding heat treatments is provided. A procedure to avoid secondary recrystallization in FSWed joints is then highlighted. Thermal cycles during welding are controlled by welding on a low thermal conductivity backing plate (i.e. stainless steel) which avoids very fine grains in the bottom of the weld nugget. This new procedure opens the path to excellent welds presenting at least the strength and ductility of the T6 base material

Understanding the ductility versus toughness and bendability decoupling of large elongated and fine grained Al 7475 - T6 alloy

7XXX aluminium alloys are widely used in aerospace components due to their high strength to weight ratio. However, first generation 7XXX alloys has a limited fracture toughness and crack propagation resistance. New generation of 7XXX alloys succeeds increasing fracture toughness, by playing on both alloying elements composition and purity level. One of the toughest alloy is the 7475 Al alloy, with a fracture toughness (KIc) that reaches up to 60 MPa√m in T6 conditions. Friction stir processing (FSP) is applied to 7475 Al rolled plates in order to generate a fine-grained microstructure. Heat treatments (HT) are performed in order to set rolled and FSPed microstructures into T6 peak-aged condition. While the T6 FSPed microstructure shows a significant fracture strain improvement in uniaxial tensile loading, this microstructure does not improve the crack propagation resistance compared to the T6 rolled material, neither on compact tension nor on bending specimens. Which mechanisms are responsible for this effect? A decoupling between ductility and toughness in 7XXX aluminium alloys is unexpected and the understanding of this phenomenon is of prime importance for mechanical design. This work reveals the competing effects of the FSPed microstructure on crack initiation and crack propagation. FSPed material delays the crack initiation under loading but the FSPed fine grains favour fully intergranular crack propagation. In comparison, the energy needed for the crack to cut through or circle around the rolled elongated grains is much higher in the rolled material. Indeed, elongated rolled grains act as crack arresters when the crack direction is perpendicular to the grain’s main axis. In this case, failure of the rolled material leads to extensive transgranular propagation, increasing the energy absorption and rising the KIc values to higher levels. This work illustrates the formation and propagation of cracks in the rolled and FSPed 7XXX alloys microstructures in order to explain the damage sequence and the competition between transgranular and intergranular failure

Micro-hardness evolution in Friction Stir Processed 7475 aluminium plate

Friction Stir Process (FSP) is an efficient way to refine microstructure of aluminium alloys and improve the damage resistance. The intense plastic deformation driven by the FSP tool leads to smaller and equiaxial recrystallized grains. However, the heat generated during the processing can strongly affect the precipitation state in case of age-hardening aluminium alloys. Considering FSP with 7XXX aluminium series is thus a compromise between grain refinement and hardening precipitation evolution. As-FSPed samples are usually softened due to dissolution and/or coarsening of the hardening phases. Studying Vickers micro-hardness mapping in the thickness of processed 7475 aluminium plates is a way of easily tracking precipitate evolution during FSP for a large number of heat treatment conditions

Towards high strength and high ductility 6XXX and 7XXX aluminium alloys

In high performance Al alloys, the yield stress rises up to approximately 400 MPa and 500 MPa in the T6 hardest state for the 6XXX series and 7XXX series, respectively. The main limitation is that strengthening of the Al matrix comes at the expense of a lower ductility, i.e. a lower fracture strain. In sheet bending operations, the limiting condition for formability is fracture and the fracture strain measured in a tensile test is an appropriate indicator of the formability. Bending of high-strength Al plates is thus usually restricted to moderate bend angles at room temperature. Friction stir processing (FSP) is a severe plastic deformation technique. The rotating tool leads to the formation of a nugget zone, where grain refinement is significant, as well as breakage of large second-phase brittle particles [1]. However, FSP leads to unavoidable softening of the material due to the heat generated during the process, dissolving strengthening precipitates in the matrix and leading to lower yield strength compared to T6 base material. Recent work has highlighted a procedure to restore the T6 hardening precipitation by post-FSP heat treatments on 7475 alloy [2]. Applying FSP and heat treatments on a 6056 and a 7475 alloys, this work aims at reaching higher ductility and thus bending limit at room temperature on (peak) aged conditions. Cold rolled plates base material (BM) of Al 6056 T4, 2.5 mm thick and Al 7475 T7, 12 mm thick were used. Up to 6 overlapping FSP passes were performed on 6056 alloy while maximum 2 FSP passes are applied on 7475 material. A solution heat treatment (SHT) followed by water quenching and artificial aging (AHT) have been applied on Al 7475 BM and 7475 FSPed. Only artificial ageing (AHT) was applied on both Al 6056 BM and 6056 FSPed. Macro hardness measurements were performed on the base material (BM) and FSPed material (FSPM). For Al 6056, the hardness of as-FSP material was decreased by ≈15% compared to BM in the same aged condition. For 7475 material, the hardness of FSPM + SHT/AHT material is homogeneous and equivalent to SHT/AHT 7475 BM, the T6 hardening precipitates are well restored after the post-FSP heat treatments. Tensile properties are investigated using 2 mm Ø (6056) and 6 mm Ø (7475) cylindrical specimens taken inside the nugget zone refined microstructure, parallel to the processing direction. FSP increases the fracture strain of 6056 alloy by 100% (from BM + AHT to 6 passes FSP + AHT), while the T6 7475 FSP+SHT/AHT material increases the fracture strain, by 38% and 63% for 1 pass and 2 pass FSP respectively in comparison with the T6 rolled material. This improvement is related to the refinement of the microstructure obtained by multi-pass FSP, delaying the damage mechanisms under loading. Bending tests were then performed on 2.5 mm thick coupons on both alloys and both BM and FSP microstructures. It was shown that crack initiation occurs systematically at larger punch displacement for the FSP samples and similar maximum bending force (compared to BM) for the 6056 and 7475 Al, following the fracture strain evolution observed on tensile samples. This work highlights the ductilization potential of age-hardenable aluminium alloys. In both investigated materials, i.e. Al 6056 and Al 7475, significant improvement of the ductility is obtained by the use of FSP and heat treatments. Formability is subsequently increased; the bending tests revealed the exceptional improvement of the bend angle. These results pave the way to even higher performances aluminium alloys by the local microstructural refinement demonstrated in this work. [1] F. Hannard , S. Castin, E. Maire, R. Mokso, T. Pardoen, A. Simar, Ductilization of aluminium alloy 6056 by friction stir processing, Acta Materialia 130, 2017, 121 - 136 [2] Matthieu B. Lezaack, Aude Simar, Avoiding abnormal grain growth in thick 7XXX aluminium alloy friction stir welds during T6 post heat treatments, under revie

Modifications des propriétés d’endommagement de l’aluminium 7475 en tôle de forte épaisseur par le procédé de friction-malaxage

Les alliages d’aluminium de la série 7000 sont largement utilisés dans l’industrie aéronautique pour leur excellent rapport entre limite d’élasticité et masse. Le procédé de friction-malaxage (FSP pour Friction Stir Processing) est une technique intéressante dans le but de modifier localement la microstructure de ces alliages. En effet, la rotation de l’outil FSP entraîne une déformation plastique de la matière et son échauffement par frottement. Par conséquent, un raffinement de la taille des grains est observé dû à une recristallisation sans passer par l’état liquide. Cependant, la chaleur générée lors du procédé peut aussi entraîner la dissolution des précipités de durcissement qui confèrent les caractéristiques mécaniques de la tôle. L’objectif de ce travail est donc de raffiner la taille des grains sans affecter significativement l’état de précipitation antérieur au FSP afin de décorréler les effets de la taille des grains et de l’état de précipitation structurelle sur les propriétés d’endommagement. Une analyse micrographique révèle que l’outil FSP génère, après son passage dans la matière, une zone appelée noyau (NZ pour nugget zone), présentant une taille de grains raffinée et une dissolution partielle des précipités de durcissement. Cette zone noyau est entourée par une zone affectée thermo-mécaniquement (TMAZ pour thermo-mechanically affected zone), présentant des grains déformés mais de taille comparable au métal de base et présentant également une dissolution partielle des précipités. Autour de ces deux premières zones, la zone affectée thermiquement (HAZ pour heat affected zone) présente des grains similaires au métal de base. La HAZ présente cependant des précipités de durcissement sur-vieillis. Des essais de dureté indiquent que les trois zones sont adoucies par rapport à l’état initial. Des traitements thermiques sont ensuite appliqués au matériau afin de restaurer l’état de précipitation antérieur au FSP. Un vieillissement artificiel à 120°C permet d’améliorer les propriétés de limite d’élasticité et de dureté des zones NZ et TMAZ. La HAZ, par contre, doit faire l’objet d’une mise en solution des précipités à 510°C afin de revenir à un état sous-vieilli avant une éventuelle maturation à 120°C. Cette mise en solution est cependant néfaste pour le raffinement des grains de la NZ car la température élevée facilite la croissance anormale de certains grains. Une optimisation des paramètres FSP et des temps de traitement thermique est effectuée afin d’obtenir des éprouvettes possédant des microstructures significativement différentes mais des limites d’élasticité et des capacités d’écrouissage similaires. Références [1] F. Hannard, S. Castin, E. Maire, R. Mosko, T. Pardoen, A. Simar, Ductilization of aluminium alloy 6056 by friction stir processing, Acta Materialia, 130, pages 121-136, 201

Ductilization by multi-pass Friction Stir Processing of thick 7475 aluminium alloy

The exceptional tensile strength of 7XXX aluminium alloys is due to a fine hardening precipitation. In peak-aged T6 state, the yield strength reaches more than 500MPa. However, heterogeneous precipitation occurs along grain boundaries. This induces a softer precipitate free zone (PFZ) due to depletion of alloying elements. Industrial 7475 aluminium alloy plates are obtained by rolling. The elongated grain length may reach 1 mm. Under loading, impurities like iron-rich particles (IM) break or detach with the matrix. These first micro-cracks will propagate along the grain boundaries following the heterogeneous precipitates inside the PFZ. Finally, trans-granular failure cuts the remaining ligaments of material by void coalescence and shearing. Both inter-granular and trans-granular modes play a role in the damage mechanism, as showed by Ludtka et al [1]. According to Rometsch et al [2], the presence of PFZ and heterogeneous precipitates cannot be removed by solution heat treatments. However, by applying Friction Stir Processing (FSP), a refined microstructure is obtained, leading to a new distribution of PFZ and heterogeneous precipitates in the material. Investigations on 10mm thick 7475 plate are performed from 1 to 4 passes. Further heat treatments can homogenize the precipitation and restore the yield strength of the initial rolled material. FSPed materials show a significant enhancement in ductility compared to the rolled one at the same yield strength. Up to 60% true fracture strain is obtained, improving by 150% with respect to the rolled material in the hardest state. The number of passes plays a crucial role on the ductility improvement. FSP breaks the IM particles into fragments. Crack initiation can however be affected if the fragments form clusters due to insufficient FSP passes. Investigations are performed to converge to the optimal number of passes to obtain the best distribution of IM particles. References [1] G. Ludtka and D. Laughlin, Metall.Trans. A, 13A (1982) 411-425 [2] P. Rometsch, Y. Zhang, S. Knight, Trans. Nonferrous Metals Soc. China, 24(2014) 2003-201

Ductility and Formability Enhancement by FSP in 7XXX Al alloys

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Understanding the influence of WAAM deposition strategy on mechanical properties and damage mechanisms of 5183 and 2219 Al alloys

Wire arc additive manufacturing (WAAM) of aluminium alloys is a rising technique for manufacturing large and complex parts. Based on the advanced GMAW-variants with dynamic wire feeding linked to a controlled current and voltage waveform, even alloys considered as less or even unweldable can be successfully deposited by WAAM. This study focuses on both weldable and less weldable 5183 and 2219 respectively. First, this work investigates the influence of different deposition strategies for thin and thick wall manufacturing of 5183 Al on the component properties, since different strategies imply different heat concentrations and dissipation. Microstructure, mechanical strength, and damage mechanisms are then related to the printing deposition path. Computed tomography scans are performed in order to capture the crack formation and propagation under tensile loading of the deposited 5183 Al. Second, 2219 alloy is deposited, and the influence of deposition strategy is similarly investigated for thick wall manufacturing using weaving techniques. As 2219 Al is a heat-treatable alloy, post-WAAM heat treatments are performed in order to restore peak-aged strength. The influence of heat treatment is quantified in terms of microstructural changes. The as-deposit and heat-treated 2219 alloys are then fully characterized for measuring the mechanical properties and for identifying the damage mechanisms in both as-deposit and peak-aged conditions. Tomography scans are obtained before and after straining the material, highlighting the influence of defects on crack propagation. The found results will be used in ongoing research related to the development and characterization of components of 7xxx Al alloy manufactured by WAAM

Waam van aluminium legeringen met hoge sterkte. WAALU-Onderzoeksproject

Aluminium Legeringen. Aluminium en zijn legeringen krijgen aandacht uit diverse industriële sectoren zoals automobiel, ruimtevaart, defensie, scheepvaart, spoorwegen, en dit wegens hun uitzonderlijke eigenschappen zoals een laag soortelijk gewicht, hoge specifieke sterkte, hoge thermische geleidbaarheid en uitstekende anticorrosie-eigenschappen. Zuivere aluminium legeringen hebben lage mechanische eigenschappen. Daarom wordt aluminium gelegeerd met elementen zoals Cu, Mg, Mn, Zn en Li om aan de eisen van de industrie te voldoen. Warmte behandelbare legeringen zoals de 2xxx, 6xxx, en 7xxx reeksen krijgen hun hogere sterkte door precipitatie harden, terwijl de overige legeringen een hogere sterkte krijgen door koud vervorming. In de recente literatuur over WAAM (Wire Arc Additive Manufacturing) van aluminium legeringen worden vooral de 2xxx, 4xxx, en 5xxx reeksen vermeld, wegens de goede lasbaarheid van deze materialen. Beperkte literatuur is beschikbaar over de 6xxx en 7xxx reeksen.Alliages d'aluminium L'aluminium et ses alliages suscitent l'intérêt de divers secteurs industriels tels que l'automobile, l'aérospatiale, la défense, les chemins de fer, et ce en raison de leurs propriétés exceptionnelles telles qu'une faible densité, une résistance spécifique élevée, une grande conductivité thermique et d'excellentes propriétés anticorrosion. Les alliages d'aluminium pur ont de faibles propriétés mécaniques. C'est pourquoi l'aluminium est allié à des éléments tels que Cu, Mg, Mn, Zn et Li pour répondre aux exigences de l'industrie. Les alliages pouvant être traités thermiquement, tels que les séries 2xxx, 6xxx et 7xxx, acquièrent une plus grande résistance grâce au durcissement par précipitation, tandis que les autres alliages acquièrent une plus grande résistance grâce à la déformation à froid. La littérature récente sur la fabrication additive par arc de fil (WAAM) des alliages d'aluminium mentionne principalement les séries 2xxx, 4xxx et 5xxx, en raison de la bonne soudabilité de ces matériaux. Les séries 6xxx et 7xxx font l'objet d'une littérature limitée

Reinforced concrete walls detailed with shape memory alloys: recent experimental and numerical investigations

The substitution of reinforcing steel with shape memory alloys has been offered as a solution for reducing residual displacements in reinforced concrete (RC) walls. This paper presents some recent experimental and numerical research efforts conducted by the authors and colleagues in the iMMC at UCLouvain focusing on the seismic performance of RC walls detailed with shape memory alloys. A summary of an experimental campaign is provided, which involved testing two large-scale planar RC walls. Some results from numerical simulations using state-of-the-art finite element modelling are also presented. Current and future research will focus, among others, on developing a more robust connection between the shape memory alloy and conventional steel rebars for implementation in concrete structures