Fe-bearing Intermetallics Research Articles

Corrosion behavior of dissimilar Al–Mg–Si alloys joint welded under hybrid CMT-laser oscillation welding

As the corrosion behavior of 6xxx series aluminum alloys depends on composition and microstructure, the microstructure and corrosion behavior of 6082 and a new 6xxx aluminum alloy weld joint are investigated under a standard corrosion test in this study. These results show the asymmetrical welding joint structure and a distribution of corrosion severity across the weld joint. Substantial differences were observed in the size and distribution of Mg2Si particles and Fe-bearing intermetallics in various regions of the weld joint. It was found that the equiaxed grain zone on new 6xxx aluminum side is not most susceptible to corrosion, contrary to previous results, but the narrow partially melted line on 6082 side is. Not all corrosion induced by Fe-bearing intermetallics originates from the Al matrix, adjacent Fe-bearing intermetallics and Mg2Si could also form galvanic microcells, resulting in preferential dissolution of Mg2Si over the Al matrix.

Formation of intermetallic phases in unrefined and refined AA6082 Al alloys investigated by using SEM-based ultramicrotomy tomography

• Microstructure of AA6082 Al alloys was studied using SEM serial ultramicrotomy tomography. • The formation sequence of intermetallic phases is related to the coarsening of α-Al dendrites. • TiB 2 particles act as nuclei for the formation of β, α intermetallics and Mg 2 Si. • The grain refinement processing causes the severe interconnectivity of α-intermetallics. • Melt shearing is better to uniformly distribute and refine intermetallics than grain refiner. The spatial arrangement, distribution and morphology of Fe-bearing intermetallics in AA6082 alloys depends on the manufacturing process of the alloy and thus influences the macroscopic properties. Here, the microstructure of a near industrial scale casting AA6082 Al alloy fabricated by: (a) direct chill casting, (b) Al-5Ti-1B grain refiner addition and (c) intensive melt shearing has been investigated by three-dimensional visualization using SEM-based serial ultramicrotomy tomography. The formation sequence of phases in AA6082 alloys is generally categorized into four stages: formation of α-Al grains, Fe-bearing intermetallics, Mg 2 Si phase, and eutectic rosettes. Results of three-dimensional visualization of the microstructure indicated that TiB 2 particles not only could nucleate Fe-bearing β-intermetallics, but also could provide substrate for the formation of Fe-bearing α-intermetallics and Mg 2 Si. A further deep analysis reveals that the essential condition for the formation of secondary phases such as Fe-bearing intermetallics and Mg 2 Si phase is the build-up of a supersaturated solute front at the α-Al solid-liquid interface irrespective of the specific nucleation site. In addition, the results indicate that grain refinement processing causes the severe interconnectivity of Fe-bearing α-intermetallics. However, the intensive melt shearing is a better manufacturing process because the intermetallics are more evenly distributed and refined than with the addition of the grain refiner, thereby improving the properties of the alloy.

The Effects of Heat Treatment on the Microstructure and Tensile Properties of an HPDC Marine Transmission Gearcase

The drive for continuously improving the performance and increasing the efficiencies of marine transportation has resulted in the development of a new alloy, Mercalloy A362™. This alloy was designed to lighten Mercury Marine’s lower transmission gearcase while also improving the alloy’s recyclability. The new prototype gearcase was subjected to Mercury Marine’s standard service conditions, which resulted in the premature failure of the prototype. A previous study revealed that a large accumulation of unwanted residual stress (~120 MPa) was present in the gearcase following the high pressure die casting process. Fortunately, the T5 heat treatment reduced the magnitude of stress by approximately 50%. However, the effects that the T5 heat treatment had on the microstructure and mechanical properties of the alloy were not discussed. Thus, this research article characterizes the effects that the T5 heat treatment has on the volume fraction and morphology of the intermetallics, as well as the tensile performance of the alloy. It was found that the T5 heat treatment led to only minor increases in the volume fraction of Fe-bearing intermetallics, leading to similar tensile properties in both the as-cast and T5 condition. These results suggest that the T5 heat treatment can alleviate residual stress without significantly altering the mechanical properties of the alloy. The results from the previous stress analysis and the current study were used to optimize the manufacturing process which led to the successful introduction of the gearcase into the competitive marine industry.

Fe-bearing intermetallics transformation and its influence on the corrosion resistance of Al–Mg–Si alloy weld joints

This work investigated the transformation of Fe-bearing intermetallics and its influence on the corrosion behavior of Al–Mg–Si alloy weld joints. The results showed that the size and type of Fe-bearing intermetallics determined the pitting corrosion resistance of weldment regions. The partially melted zone (PMZ) suffered most serious corrosion attack, because of the formation of coarse needle-like β-AlFeSi. The heat-affected zone (HAZ) immediately behind the PMZ (HAZ-Ⅰ) experienced a complete transformation of β-AlFeSi into α-AlFeSi, but in the rest of the HAZ (HAZ-Ⅱ) was not sufficient for this transformation. Therefore, the HAZ-Ⅰ was more resistant to pitting corrosion than HAZ-Ⅱ.

An Alternative to the Recycling of Fe-Contaminated Al

Iron is considered an unavoidable impurity in Al–Si alloys, since it is collected during melting and casting operations. This is particularly critical when scraped and recycled materials are used. Excessive iron can reduce the mechanical properties. This is explained by the formation of Fe-bearing intermetallic particles, since the alloy properties are deeply related to their type, size, and amount within the microstructure. The effects of Ni addition on the microstructure of impure Al–Si alloys have not been established so far. As such, the present investigation examines Fe- and Fe/Ni-containing Al–9 wt% Si alloys. Two directionally solidified (DS) alloys are investigated: the Al–9 wt% Si–0.5 wt% Fe (nonmodified) and Al–9 wt% Si–0.5 wt% Fe–0.5 wt% Ni (Ni-modified) alloys. The focus is to determine solidification conditions (i.e., cooling rate, TR; and growth rate, VL) as well as Fe- and Fe/Ni-containing Al–9Si alloys that will yield particular volume fractions, sizes and shapes of Fe-bearing intermetallics. Considering a certain limit of the dendritic microstructure scale (i.e., λ1 > 100 μm), it is shown that the ultimate tensile strength of the Al–9Si–Fe–Ni alloy is higher than that of the Al–9Si–Fe alloy and quite close to that of an Al–9Si alloy, that is, the beneficial effect of Ni addition on providing lower fraction and more compacted Fe-bearing intermetallics has counterbalanced the deleterious effect of the Fe contaminated aluminum on the tensile strength. The elongation-to-fracture, however, only approaches that of the Al–9Si alloy for the smallest λ1 values, which are associated with the highest solidification cooling rates.

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 (β-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 μm), 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 °C) and after overaging, the influence of sludge is less relevant, probably due to a softening of the α-Al matrix and a reduction of stress concentration related to Fe-bearing intermetallics.

Influence of Al grain structure on Fe bearing intermetallics during DC casting of an Al-Mg-Si alloy

207mm diameter direct chill (DC) cast billets of 6063 aluminium-magnesium-silicon (Al-Mg-Si) alloy were produced with various different primary aluminium (α-Al) grain structures including feathery-dendrites, equiaxed-dendrites and equiaxed-globular morphologies. To control the α-Al grain structure (grain morphology and grain size) an intensive shearing melt conditioning technique and Al-5Ti-1B grain refiner were used. For the first time, due to the variety of controlled microstructures produced in the DC billets, it was possible to study and determine the role of the α-Al grain structure on the iron (Fe) bearing intermetallics (Fe-IMCs). The size, shape and three dimensional (3D) inter-connectivity of the Fe-IMCs were observed to be affected by the modification of the primary α-Al grain morphology and grain size. Although both “αc-AlFeSi” and “β-AlFeSi” phases are present in all billets, β-AlFeSi phase is dominant in the billets with grain refiner addition while αc-AlFeSi is dominant in the billets without grain refiner. This suggests that the addition of Al-5Ti-1B grain refiner plays a more significant role in the intermetallic phase selection than the primary α-Al grain morphology or grain size.

Influence of Grain Refiner Addition on the Precipitation of Fe-Rich Phases in Secondary AlSi7Cu3Mg Alloys

The effect of grain refiner addition on the precipitation of Fe-rich compounds in secondary AlSi7Cu3Mg has been studied. Thermal and metallographic analysis techniques have been used to examine the formation of Fe-bearing intermetallics. The results show that grain refinement with AlTi5B1 can alter the precipitation sequence of Fe-rich phases. In non-grain refined alloy and grain-refined alloy with AlTi10, two different Fe-rich phases, α-Al15(Fe,Mn)3Si2 and π-Al8Mg3FeSi6, form, whereas, with AlTi5B1 addition, the β-Al5FeSi phase also tends to crystallize, nucleating on TiB2 particles. It has also been demonstrated how at high cooling rates, the β-Al5FeSi-involved reactions occur more preferentially, when compared to α-Al15(Fe,Mn)3Si2 phase. This crystallization behaviour is explained in terms of phase diagram relationship and the nucleation kinetics of competing α-Al15(Fe,Mn)3Si2 and β-Al5FeSi phases.

Intermetallic Phase Formation in Explosively Welded Al/Cu Bimetals

Diffusion couples of aluminum and copper were fabricated by explosive welding process. The interface evolution caused by annealing at different temperatures and time durations was investigated by means of optical microscopy, scanning electron microscopy equipped with energy dispersive spectroscopy, and x-ray diffraction. Annealing in the temperature range of 573 K to 773 K (300 °C to 500 °C) up to 408 hours showed that four types of intermetallic layers have been formed at the interface, namely Al2Cu, AlCu, Al3Cu4, and Al4Cu9. Moreover, it was observed that iron trace in aluminum caused the formation of Fe-bearing intermetallics in Al, which is near the interface of the Al-Cu intermetallic layers. Finally, the activation energies for the growth of Al2Cu, AlCu + Al3Cu4, Al4Cu9, and the total intermetallic layer were calculated to be about 83.3, 112.8, 121.6, and 109.4 kJ/mol, respectively. Considering common welding methods (i.e., explosive welding, cold rolling, and friction welding), although there is a great difference in welding mechanism, it is found that the total activation energy is approximately the same.

Evolution of Fe Bearing Intermetallics During DC Casting and Homogenization of an Al-Mg-Si Al Alloy

Abstract The evolution of iron (Fe) bearing intermetallics (Fe-IMCs) during direct chill casting and homogenization of a grain-refined 6063 aluminum-magnesium-silicon (Al-Mg-Si) alloy has been studied. The as-cast and homogenized microstructure contained Fe-IMCs at the grain boundaries and within Al grains. The primary α-Al grain size, α-Al dendritic arm spacing, IMC particle size, and IMC three-dimensional (3D) inter-connectivity increased from the edge to the center of the as-cast billet; both α c-AlFeSi and β-AlFeSi Fe-IMCs were identified, and overall α c-AlFeSi was predominant. For the first time in industrial billets, the different Fe-rich IMCs have been characterized into types based on their 3D chemistry and morphology. Additionally, the role of β-AlFeSi in nucleating Mg2Si particles has been identified. After homogenization, α c-AlFeSi predominated across the entire billet cross section, with marked changes in the 3D morphology and strong reductions in inter-connectivity, both supporting a recovery in alloy ductility.

Controlling the Formation of Iron-Bearing Intermetallics in Wrought Al Alloys by Melt Conditioned DC (MC-DC) Casting Technology

With the increasing use of recycled aluminium alloys from the end-of-life products more and more iron is accumulated into the compositions of alloys. Sometimes, recycling causes the iron levels to increase beyond the set target levels for down-stream processing. The only way to deal with this impurity currently in industry is to increase the primary aluminium added to the furnace to dilute the melt and re-add all other elements or cast it for re-melting or extrude it for products that is not surface finish critical or required higher corrosion resistance. Formation of small well dispersed spherical a- or small b- Fe-bearing intermetallics, which can be homogenised for shorter times and has no negative effect on downstream processing, would be promising even if the iron levels are above the targeted compositional limits. In the present paper, fine and dispersed Fe-bearing intermetallics have been achieved by Melt Conditioned DC (MC-DC) casting technology, instead of coarser Fe-bearing intermetallics forming network like morphology in the DC castings with grain refiner additions (DC-GR). This suggests feasibility of an increased tolerance of iron levels by melt conditioned DC casting technology.

Effect of V addition on the nano-size spherical particles growing on the Fe-bearing intermetallics and silicon phases of gas atomized hypereutectic Al–20Si–5Fe alloys

In this study, rapidly solidified hypereutectic Al–20Si–5Fe–XV (X=0, 0.5, 1 and 1.5) alloys were fabricated by gas atomization in Ar atmosphere. Microstructural and spectroscopic analyses were performed using SEM, SEM – EDS and XRD measurements. Experimental results showed that addition of V to gas-atomized Al–20Si–5Fe alloys led to formation of a refined microstructure composed of finer Si and Fe-bearing intermetallics. Possible refining mechanism of Si phases is the IIT because of Rv:RSi is 1.38 close to the value of 1.65 which is the ideal ratio of radius for the impurity-atom to Si-atom (Ri:RSi). The reason of formation finer of δ-Al4FeSi2 phase with addition of V is that vanadium addition leads to an increment in the thermodynamic stability and a reduction in the free energy of δ-Al4FeSi2 phase. Therefore, finer and modified morphology became more favorable for δ-Al4FeSi2 phase in terms of thermodynamic. Nanosize Si particles growing on the surface of Fe-bearing intermetallics and micro size polygonal Si phases were observed. There was no effect of V on the number of fine silicon particles growing up on the micro size polygonal Si phases. However, during solidification, addition of V led to an increase in the number of fine silicon particles growing on the Fe-bearing intermetallics. Microhardness measurements showed that there was an increase in the microhardness values of Al–20Si–5Fe alloys with addition of V, in general.

Formation of novel flower-like silicon phases and evaluation of mechanical properties of hypereutectic melt-spun Al–20Si–5Fe alloys with addition of V

In this work, rapidly solidified hypereutectic Al–20Si–5Fe–XV (X=0, 0.5 and 1) alloys were fabricated by melt spinning under vacuum. Microstructural and spectroscopic analyses were performed using SEM, TEM, TEM-MAPing, TEM-EDS and XRD measurements. Mechanical properties of the alloys were determined using DSI measurements. Experimental results indicated that addition of 0.5wt% V to melt-spun Al–20Si–5Fe alloys induced formation of a novel flower-like Si phase. And addition of higher amount V (1wt%), caused formation of refined Si phases and mostly hindered formation of Fe-bearing intermetallics. Observations along with manuscript strongly indicate that V modifies the Si phases by the impurity induced twinning (IIT). Changes in the dynamic microhardness of the samples were mainly determined by the size of Si phases. Addition of vanadium led to quite lower elastic modulus in the vanadium added alloys compared to base alloy.

Twin Roll Casting of Al-Mg Alloy with High Added Impurity Content

The microstructural evolution during twin roll casting (TRC) and downstream processing of AA5754 Al alloy with high added impurity content have been investigated. Strip casts with a high impurity content resulted in coarse α-Al grains and complex secondary phases. The grain size and centerline segregation reduced significantly on the addition of Al-Ti-B grain refiner (GR). Coarse-dendrite arm spacing (DAS) “floating” grains are observed in the impure alloy (IA) with higher volume in the GR strips. Two-dimensional (2D) metallographic analysis of the as-cast strip suggests that secondary phases (Fe-bearing intermetallics and Mg2Si) are discrete and located at the α-Al cell/grain boundaries, while three-dimensional (3D) analysis of extracted particles revealed that they were intact, well interconnected, and located in interdendritic regions. Homogenizing heat treatment of the cast strip breaks the interconnective networks and modifies the secondary phases to a more equiaxed morphology. During rolling, the equiaxed secondary phases align along the rolling direction. X-ray diffraction (XRD) analysis suggests that α-Al(FeMn)Si and Mg2Si are the predominant secondary phases that are formed during casting and remain throughout the downstream processing of the GR-IA. The high-impurity sheet processed from TRC resulted in superior strength and ductility over the sheet processed from small book mold ingot casting. The current study has shown that the TRC process can tolerate higher impurity levels and produce formable sheets from the recycled aluminum for structural applications.

Influence of Grain-Refiner Addition on the Morphology of Fe-Bearing Intermetallics in a Semi-Solid Processed Al-Mg-Si Alloy

The three-dimensional morphologies of the Fe-bearing intermetallics in a semisolid-processed Al-Mg-Si alloy were examined after extracting the intermetallics. αc-AlFeSi and β-AlFeSi are the major Fe-bearing intermetallics. Addition of Al-Ti-B grain refiner typically promotes β-AlFeSi formation. β-AlFeSi was observed with a flat, plate-like morphology with angular edges in the alloy with and without grain refiner, whereas αc-AlFeSi was observed as “flower”-like morphology in the alloy with grain refiner.

Effects of Co addition on Fe-bearing intermetallic compounds and mechanical properties of AlSi20Cu2Ni1Fe0.7–1 alloys

Cobalt is generally considered as the element that cannot only neutralize the effect of iron, but also promote the high-temperature mechanical properties of Al–Si alloys. The effects of Co on the size and morphology of Fe-bearing intermetallics have been studied in two alloys: Al–20Si–2Cu–1Ni–0.7Fe and Al–20Si–2Cu–1Ni–1.0Fe. The effect on tensile strength was also investigated. In all cases, Co is proved to be a very effective element in reducing the size and improving morphology while increasing volume fraction of Fe-bearing intermetallic compounds. The Fe-bearing compounds presented as long acicular β-Al5(Fe, Ni)Si phases when Co was 0% while turned into Chinese script, granular, rod-like or dendritic α-Al15(Fe, Co, Ni)3Si2 phases when Co:Fe ratio was 1.0, 1.3, 1.5, respectively. Co is also a very effective element in promotion of tensile strength of both alloys. The maximum tensile strengths at room temperature and elevated temperature (350°C) of alloys with 0.7% Fe were respectively 275 and 101MPa, while the corresponding data were 296 and 120MPa respectively for alloys with 1.0% Fe. Compared with the minimum tensile strengths that obtained when Co was 0%, they respectively increased by 13%, 28%, 24% and 35%. The optimum Co:Fe ratio in these two alloys studied is 1.3.

Solidification behaviour of an AA5754 Al alloy ingot cast with high impurity content

AbstractIn view of the recycling of aluminium scrap for automotive sheet application, we have investigated the solidification behaviour of AA5754 alloy containing additional amounts of impurity elements such as Si, Fe, and Cu. Ingot casts with a high impurity content resulted in coarse α-Al dendrites and complex-shaped secondary phases. A large volume of coarse Chinese script and needle-type Fe-bearing intermetallic phases were observed to form at the centre of an ingot. In addition to the grain-boundary eutectic, spherically shaped rosette-type eutectic phases were observed within the Al grain in the high-impurity alloy. The more uniform size distribution of the Fe-bearing intermetallics observed in the Al–Ti–B grain refined alloy is attributed to the presence of a large fraction of α-Al grain boundaries which distributes the eutectic liquid where the Fe-bearing intermetallic forced to nucleate and grow.

The Sequence of Intermetallics Formation during the Solidification of an Al-Mg-Si Alloy Containing La

A mechanism by which La modifies the Fe-bearing intermetallics in a 6xxx series Al alloy is examined using calorimetry. Thermal events accompanying the formation of various phases are identified by establishing a correspondence between them and microstructures of as-cast alloys. It is concluded that a previously suggested surface adsorption mechanism is unlikely to be operative. During solidification, the addition of lanthanum results in the formation of the La(Al,Si)2 phase and the depletion of the remaining melt of Si. It is hypothesized that the decreased Si/Fe ratio in the melt caused by the presence of La favors the formation of the α-AlFeSi phase, which is less detrimental to the formability of the alloy than the β-AlFeSi phase.

Effect of iron-intermetallics on the fluidity of 413 aluminum alloy

The effect of Fe content on the fluidity of 413 aluminum alloy has been investigated in both un-modified and Mn-modified states by vacuum fluidity test. Increasing Fe decreases the fluidity due to the formation of β-Al5FeSi and α-Al15(Fe, Mn)3Si2-intermetallic compounds. The effect of melt holding time was also studied. The fluidity of Mn-modified alloys decreases drastically with increasing holding time. Agglomeration of α-intermetallics and subsequent sludge formation is the main cause of fluidity descending. The fluidity of un-modified alloys is, however, less sensitive to holding time. Applying enough superheat and holding time, corresponding to segregation factor of the alloy, is an effective way to increase the fluidity of Mn-modified alloys by the dissolution of Fe-bearing intermetallics.