Overaging Treatment Research Articles

Application of Kelvin Probe Force Microscopy (KFM) to Evidence Localized Corrosion of Over-Aged Aeronautical 2024 Aluminum Alloy

The 2xxx serie aluminum alloys are characterized by good mechanical performances and low density, however they are susceptible to different forms of localized corrosion: pitting corrosion, intergranular corrosion and stress corrosion cracking. The 2024-T351 aluminum alloy is used in the aircraft industry for numerous applications such as fuselage and door skin. Corrosion damage of the material is also very detrimental for the structural integrity of the aircraft. The presence of coarse intermetallic particles, with a heterogeneous size distribution was found to be responsible for the 2024 susceptibility to localized corrosion. These particles are generally the cause of initiation sites. Presence of micro-defects in the oxide film upon coarse intermetallic particles and the galvanic coupling with the matrix contribute to the development of pitting corrosion. The over-ageing treatment (T7) is supposed to stabilize the microstructure and the mechanical properties to improve the corrosion resistance. The 2024 alloy microstructure after the T7 heat treatment remains very complex. The 2024 alloy corrosion behavior was studied in the over-ageing state for three different temperatures (150, 175 and 190 °C). During the corrosion tests in chloride-containing environment, the behavior of coarse intermetallic particles was found to be different. Thus, the 2024 samples suffer a gradual attack upon S-Al2CuMg particles and finally Al (Cu,Mn,Fe,Si) particles. The corrosion damage was studied by Atomic Force Microscopy (AFM) and Kelvin probe Force Microscopy (KFM). This technique allows simultaneous topographical and electric potential mapping to be obtained. This latest potential was shown to be correlated to the corrosion potential of the 2024 alloy. This study focuses on the variation of the KFM potential of the coarse intermetallic particles and the matrix for the over-ageing conditions (T7). Observations using optical microscope and AFM were also performed to obtain the corrosion rate for each condition. The corrosion rate was correlated to the chemical composition variation of the particles obtained by scanning electron microscope observations and EDS analyses.

Study of the Influence of the Artificial Ageing Temperature on the AA2024 Alloy Microstructure

For the last 30 years, AA2024 aluminum alloy was very used as structural material in the aerospace industry due to its low density and good mechanical strength. The phenomenon of precipitation hardening in aluminum alloys takes place at relatively low temperature and induces the precipitation of intermetallic particles composed of the main alloying elements i.e., copper and magnesium. The fundamental stage of the age-hardening process consists in the acceleration of the decomposition phenomenon of the supersaturated solid solution, resulting in the coarse intermetallic particle precipitation; stage where the mechanical properties reaches the maximum values, but at the cost of a low corrosion resistance. In this paper, the AA2024 alloy microstructure was studied during the over-ageing process. The over-ageing treatment (T7) is supposed to stabilize the microstructure and the mechanical properties to improve the corrosion resistance. The over-ageing treatment consists in a solution treatment at 495±5°C for 1 hour, quenched into cold water and artificial aged. Three different artificial ageing temperatures were studied: 150°C, 175°C and 190°C. The mechanical properties modifications were followed by Vickers macrohardness measurements. The treatment duration for each temperature (36 days for 150°C, 50 hrs for 175°C and 24 hrs for 190°C) was determined by a given macrohardness reduction. To characterize the over-aged AA2024 alloy microstructure, a statistical analysis of the surface fraction and surface density of intermetallic particles was made. The intermetallic particle dimension distribution, depending on the over-ageing temperature, was also observed. To do so, scanning electron microscope observations were carried out and image analyses were performed from backscattered electron images.

マルエージング鋼の疲労特性に及ぼす時効条件と湿度の影響

In order to investigate the effects of aging condition and humidity on fatigue properties of 18%Ni maraging steel of grade 350, rotating bending fatigue tests were carried out in relative humidity of 25% and 85%. Aging conditions selected were under- and peak-aging ones at conventional aging temperature of 753K. In addition, double aging treatments, that is, under- peak- and over-aging treatments at 673K after the peak-aging at 753K were also examined. Both of static and fatigue strengths were increased by the double aging. Although the fatigue strength was markedly decreased by high humidity in all of the steels, the decrease in fatigue strength due to high humidity was suppressed by the double aging. In all of the steels, the change in growth behavior of a crack was yielded around the crack length of a few grain sizes of prior austenite regardless of humidity. The growth rate of a crack longer than the above stated length was hardly influenced by the aging condition and humidity, while both of an initiation and its early propagation of a fatigue crack were accelerated by anodic dissolution in high humidity. These results were discussed from a point of view of the difference in precipitates depending on the aging condition.

Aging Behavior in 15-5 PH Precipitation Hardening Martensitic Stainless Steel

15-5 PH stainless steel is strengthened by precipitation of copper rich phases in a low carbon body centred cubic (b.c.c) lath martensitic matrix. Microstructures developed in aerospace grade 15-5PH precipitation-hardened stainless steel with different aging heat treatments have been studied. An attempt has been made to correlate the microstructural observations with the mechanical properties in different aging regimes. The overaging treatment imparts excellent ductility with a compromise in strength in peak aged condition to overaged condition the tune of 300 MPa in both tensile and yield strength. Hardness and tensile strength showed a similar decreasing trend with increase in aging temperature.

Effect of two-step aging on recrystallised grain size of Al–Mg–Li alloy

A two-step overaging treatment was exploited in thermomechanical processing for producing fine grained Al–Mg–Li alloy sheets. The effects of each step and temperatures of overaging on the distribution of precipitates and recrystallised grain structure were investigated. The influence of preheating treatment before rolling on precipitate coarsening or dissolution was also considered. The results showed that, in contrast to rod shaped precipitates produced by single overaging at 300°C, precipitates produced by a two-step overaging tended to have globular morphologies. The globular precipitates were not deformed during the following large rolling reductions. Overaging at 200°C for 24 h then at 300°C for 24 h resulted in a distribution of globular precipitates with sizes of 1·0–1·2 μm. Preheating at 400°C or raising the overaging temperatures of each step to 300 and 400°C led to a reduction in the precipitate size. Subsequent rolling and recrystallisation resulted in a fine grain structure with an average grain diameter of approximately 8·1 μm for 200°C/24 h+300°C/24 h and 9·2 μm for 300°C/24 h+400°C/24 h overaging treated material respectively in the surface layers of the sheet.

Analysis of the cracking causes in an aluminium alloy bike frame

Two cracks were detected in the bike frame of an amateur cyclist, before the final failure of the component and after using it for around 35,000 km. The cracks were located in the joint between the bottom bracket, the chain stays, the seat tube and the down tube. This paper analyses the causes of the cracking process and comprises, basically, chemical analysis for material identification, visual inspection of the frame, microstructural analysis, microhardness measurements, mechanical characterisation through Small Punch tests and SEM (plus EDX) analysis of the fracture surface. The analysis concludes that both cracks were caused by corrosion and Stress Corrosion Cracking (SCC) processes that are justified by the differences in the solution potentials between the base material and equilibrium precipitates produced by an overageing treatment. The latter are more anodic and hence easily dissolved in a humid saline environment.

Precipitates in laser beam welded aluminium alloy AA6056 butt joints studied by small-angle neutron scattering

The volume fraction and the size distribution of strengthening secondary precipitates in laser beam welded sheets of Al-alloy AA6056 were studied non-destructively using small-angle neutron scattering. The volume fraction of precipitates was determined in the base material and the fusion zone for sheets welded in T4 and T6 tempers. Also, the effect of post-weld heat treatments on the precipitate dispersion was analyzed. Additionally, a lateral spatial resolution of 1 mm was used for small-angle neutron scattering measurements as a function of the distance from the weld line in a 6 mm thick sample welded in T6. Moreover, the effect of post-weld heat treatment on the residual stresses in the weld zone was analyzed. The results show that the precipitate distribution in the fusion zone was almost independent of the initial temper before welding. The width of the reduced hardness region adjacent to the fusion zone was found to correspond with the absence of precipitates with a size of a few nanometres that are present in T6 base material. The post-weld heat treatments (T6 or T78) induced nanometre-size precipitates in the fusion zone and the heat affected zone. Thus, the solute, and Mg in particular, was not lost totally in the fusion zone through vaporisation during the welding. Furthermore, the over-aging treatment T78 hardly changed the precipitate dispersion in the base material.

Al-Zn-Mg 系発泡体の材料強度と析出挙動に及ぼす熱処理の影響

Al-Zn-Mg foams manufactured by a batch casting process were subjected to aging treatments with and without solution treatment. The effect of the heat treatments on matrix strength (micro-hardness) and precipitation behavior in the foams was investigated by means of transmission electron microscopy (TEM) and X-ray absorption fine structure (XAFS) analysis at a synchrotron radiation facility. Foam aged at 398 K for 8 h after the solution treatment showed the greatest increase in strength. Fine precipitates were dispersed homogeneously and densely, which were speculated to be metastable η′ and equilibrium η phases transformed from GP zones. It is estimated that the GP zones were formed during water-quenching after the solution treatment. Overaging treatment at 423 K for 48 h after solution treatment softened the foam past its peak strength by approximately 10 in Vickers hardness. During this treatment, η′ and η phases coarsened significantly and the number density of these precipitates decreased. It is concluded that a significant drop in the strength was suppressed during overaging by the presence of a high density of the GP zones, defined as Zn-Mg ordered phases by the XAFS analysis.

The influence of artificial aging on the microstructure, mechanical properties, corrosion, and environmental cracking susceptibility of a 7075 friction‐stir‐weld

AbstractThe influence of overaging to the T7451 temper on the microstructure, the mechanical properties, and the corrosion susceptibility of friction‐stir‐welded 7075 aluminum alloy was investigated by means of transmission electron microscope (TEM), SEM investigations, constant extension rate tests (CERT), alternate immersion tests, and potentiodynamic scans. The overaging that occurs during welding within the heat‐affected zones of the friction‐stir‐welded 7075‐O tempered plates promotes a slight formation of intragranular and grain boundary precipitates and increases the mechanical as well as the corrosion resistance properties as compared to the T7451 welded plates. The “double” overaging treatment, consisting of the T7451 temper and the thermal transient experienced by the heat‐affected zones of the 7075‐T7451 welded plates, increases the size of the intragranular and the grain boundary precipitates as well as the precipitate‐free zones. These facts decrease the mechanical and corrosion properties of the 7075‐T7451 weld. The T7451 thermal treatment applied after the welding of the 7075‐O plates promotes the high presence of small precipitates and reduces the size of the precipitate‐free zones. This fact increases the general corrosion resistance, but decreases the mechanical properties. Therefore, the corrosion as well as the mechanical properties are greatly correlated with minute changes in the microstructure, which can arise by short‐term heat exposure as for instance during welding. It was also found that the environmental susceptibility measured by means of CERTs may be influenced by “solution‐strengthening” corrosion mechanisms which increase the strain. This fact is present in weld microzones particularly susceptible to corrosion.

Influence of austenite grain size on overaging treatment of continuous annealed dual phase steels

A dual phase steel with an initial coarse microstructure was refined, by means of a thermal treatment, to study the influence of the austenite grain size reached during an intercritical annealing treatment on the martensite start temperature. Thus, the effect of the austenite grain size on a subsequent overaging treatment was also investigated. It was found that a coarser austenite grain size leads to a higher martensite start temperature and a lower amount of non-tempered martensite and/or austenite (MA constituents) volume fraction after a continuous annealing and overaging treatment. The non-tempered martensite and/or austenite volume fraction always was higher for the highest overaging temperature.

Determination of local carbon content in austenite during intercritical annealing of dual phase steels by PEELS analysis

Parallel electron energy loss spectroscopy has allowed to analyse and quantify local variations in the carbon concentration of austenite islands transformed during the intercritical annealing treatment of commercial dual-phase steels. These changes in the carbon content of different austenite regions are responsible for the different volume fractions of tempered martensite, martensite and retained austenite obtained after intercritical annealing and overaging treatment. This technique reveals how carbon distribution in austenite evolves as the transformation process advances.

Effect of heat treatments on precipitate microstructure and mechanical properties of a CuCrZr alloy

The precipitate microstructure of prime aged CuCrZr was coarsened by overaging to see if the larger precipitates could prevent the initiation of plastic flow localization in irradiated CuCrZr. A number of tensile and fracture toughness specimens of prime aged CuCrZr alloy were given overaging treatments at 873 K for 1, 2 and 4 h. A subset of these specimens were irradiated with fission neutrons in the BR-2 reactor at Mol (Belgium) at 333 K and 573 K to a dose level of ∼0.3 dpa. Both unirradiated and irradiated tensile specimens were tested at 295 K, 333 K and 573 K. Fracture toughness tests were carried out at 293, 333 and 573 K. Transmission electron microscopy was used to investigate the effects of overaging, subsequent irradiation and the effect of deformation. The results indicate that the overaging treatment of 873 K for 1 h produced a precipitate microstructure that improved the plastic instability, overall ductility and fracture toughness.

On the overaging behaviour of tool steel X38 CrMoV 5-3

In this study, a combination of small-angle neutron scattering, energy-filtered and conventional transmission electron microscopy and X-ray diffraction has been used to characterize the overaging behaviour of the hot-work tool steel X38 CrMoV 5-3. Hot-work tool steels are mainly used for hot-forging dies, casting dies and extruding tools where high strength is required at high service temperatures. As a consequence, thermal fatigue and softening play the most important role with regard to tool life, and they are strongly influenced by coarsening of the nanometer-sized secondary hardening carbides. The microstructural response to three different overaging treatments in the temperature range of 650–680 °C has been analyzed regarding precipitate types, particle size distributions and corresponding volume fractions. The combination of the different methods used in this study enables a very precise determination of these parameters. This allows for a profound discussion on the evolution of particles occurring in the analyzed samples.

Influence of Zinc to Magnesium Ratio and Total Solute Content on the Strength and Toughness of 7xxx Series Alloys

A systematic study of the precipitation processes in high strength Al-Zn-Mg-Cu alloys has been conducted. A series of experimental alloys was prepared with varying Zn:Mg ratio, but equivalent total solute content, to be representative of those used in applications which demand a combination of strength, fracture toughness and resistance to environmentally sensitive cracking mechanisms. Artificial ageing curves were constructed for each alloy, based upon 7xxx series duplex treatments widely used in industry. Ageing kinetics were investigated beyond peak strength into the overaged condition, since this is known to promote the most favourable balance of properties. Differential scanning calorimetry (DSC) and transmission election microscopy (TEM) observations were made on the alloys throughout the ageing process to fully understand the precipitation events occuring. For a particular overageing treatment, higher Zn:Mg ratio alloys were consistently found to be at a more advanced stage of precipitation while higher strengths were retained at low Zn:Mg ratios. Grain boundary characteristics, such as particle size and precipitate free zone width, were also influenced by Zn:Mg ratio at a given strength level. This paper provides new understanding of precipitation kinetics and the control of important microstructural features which influence the balance of properties in 7xxx series aluminium alloys.

Effect of aluminium concentration in filler alloys on HAZ cracking in TIG welded cast Inconel 738LC superalloy

The effect of concentration of aluminium in filler alloys on heat affected zone (HAZ) microfissuring in TIG welded cast Inconel 738LC (IN 738LC) superalloy was studied. Three fillers, IN 625, IN 617 and Haynes 214, with aluminium concentrations varying from 0·2 to 4·5 wt-%, respectively, were used to TIG weld cast IN 738LC alloy plates subjected to two different preweld heat treatments. One preweld heat treatment was the standard solution heat treatment at 1120°C for 2 h followed by argon quenching. The second was a novel overaging treatment developed by the present authors, termed UM treatment, involving solution treatment at 1120°C followed by air cooling and subsequent ageing at 1025°C followed by water quenching. Detailed microstructural analysis of the welds and base metal was done by optical and analytical electron microscopy. Intergranular microfissures were observed in the HAZ of all the welds, irrespective of the filler alloy and the preweld heat treatment, while no cracks were observed in the fusion zone in any of the samples. The cracks were mostly found to be associated with constitutionally liquated MC carbides, borides, sulphocarbides, γ–γ′ eutectic and γ′ precipitates. The cracking was found to increase with increase in the fusion zone hardness and the aluminium content of the fillers, i.e. it was minimum for IN 625 and maximum for Haynes 214, for a particular preweld heat treatment. Between the two heat treatments, the UM treated samples with a smaller base metal hardness, however, exhibited a considerably reduced HAZ cracking. That is, the hardness of the fusion zone as well as the base metal appears to have a significant effect on the cracking susceptibility of the welds made with different fillers.

Strain burst in creep of Mg–1Ca–0.5Zn–0.6Zr alloy

Abstract The creep behaviour and tensile properties of Mg–1Ca–0.5Zn–0.6Zr (wt.%) alloy have been studied. The alloy exhibited periodical strain bursts during creep testing at 150 °C in the peak-aged condition, while over-ageing treatment tended to eliminate the occurrence of strain burst. The strain burst was ascribed to the dynamic strain ageing (DSA) effect from the interaction between dislocations and solute atoms, which was supported by the serrated flow, yield points after static ageing, and negative strain rate sensitivity observed in tensile tests over a temperature range 150–200 °C. The high values of stress exponent of creep were interpreted in terms of breakaway of dislocations from the solute atmospheres.

Relaxation of the residual stress in 6061Al-15 vol.% SiCw composites by isothermal annealing

Abstract The residual stress (RS) in extruded 6061Al-15 vol.% SiC w composites manufactured by a powder metallurgical route has been studied by neutron diffraction. Several precipitation states were investigated, going from a fully hardened state, T6, to an over-aged (OA) condition. For comparison, the unreinforced powder metallurgy 6061Al alloy was also studied. The end precipitation stages T6 and OA were obtained after appropriate heat treatments. The T6 condition involves a quenching step, which provokes a macro-RS state. Instead, the OA treatment (at 300 °C) does create relaxation at intermediate times and leads to total relaxation of the axial deviatoric component and to partial relaxation of the hydrostatic term of the stress in the composite. Total relaxation of both deviatoric and hydrostatic terms occurs in the unreinforced alloy. Furthermore, it is found that relaxation of the macro-RS occurs more rapidly in the unreinforced alloy than in the composite. These results are in agreement with the idea that discontinuously reinforced metal matrix composites (DRMMCs) have a more stable microstructure than the corresponding unreinforced alloys. On the other hand, no decrease of the deviatoric micro-RS developed in the composite and a small reduction of the hydrostatic one occurs when treating from the T6 to the OA condition. This implies that the geometry of the reinforcing particles plays an important role and the geometrically necessary dislocations do not disappear with heat treatment.

Optimization of the strain aging resistance in aluminum killed steels produced by continuous annealing

The influence of manganese, boron and carbon content and of the coiling temperature on the strain aging resistance of continuous annealed Al killed steels was investigated, using five different steel compositions and three coiling temperatures. The material was produced and processed in laboratory and the continuous annealing cycle was performed using a thermomechanical simulation machine. In boron added steels, it was found that the coiling temperature has no influence on the strain aging resistance. On the other hand, in boron free steels of similar composition, coiling temperatures equal to or higher than 720 °C were required to reduce the amount of nitrogen in solid solution at room temperature after the overaging treatment and thus to increase the aging resistance. However, an increase in the Mn content of the boron free steels produced the same results observed in boron added steels. This effect is discussed in terms of the influence of the Mn content on the kinetics of AlN precipitation in these steels.

Effect of Aging Treatment on the Mechanical Properties of C-250 Maraging Steel by Flow Forming

The technique of forward flow forming has been used to produce a long, thin walled tube made of C-250 maraging steel. The forward flow forming can save raw material, increase strength, and reduce the production process time. Because the work hardening effect on solution-treated C-250 using flow forming is minimal, the flow-formed tube requires an additional heat treatment to obtain higher hardness and strength. With the direct aging treatment, low elongation values are obtained, making this treatment unsuitable for the engineering design. It was found that the 540 °C/6 h/AC over-aging treatment results in better strength and elongation values. The strengthening phase of the flow-formed C-250 maraging steel was found to be the intermetallic compound of Fe3Mo.

Influence of thermomechanical treatments on the production of rheocast slurries by partial melting

This work deals with the influence of thermomechanical treatments on the production, by partial melting, of rheocast structures of Al–3.35wt.%Cu alloy. Two types of treatments are performed: recrystallization and partial melting (RAP), in which the alloy is submitted to cold work at room temperature prior to heating to the semi-solid state; and overaging treatment (OAT), where the alloy is submitted to homogenisation and controlled precipitation of second phase followed by deformation at room temperature and heated to temperatures above Tsolidus. The influence of initial grain size, degree of cold work and overaging conditions in the resulting microstructure is investigated. Results are presented in terms of Cu distribution, solid fraction, globule diameter and shape factor. For both processes there are substantial reductions in the globular primary phase size compared to initial grain sizes, this reduction being more significant for rheocast produced via the OAT than the RAP route. The degree of previous deformation only influences the final globule diameter of the rheocast structure obtained by the OAT process, while initial grain size influences globule diameter in both processes. Changes in shape factor with process parameters show that different mechanisms are operative during rheocasting, such as recrystallization coarsening and coalescence; the predominance of one of them depending on the initial microstructure of the material. In general, OAT produces more rounded and smaller globules than RAP, mainly from higher deformed and coarser raw material, due to the predominance of recrystallization.