Precipitation Heat Treatment Research Articles

Heat Treatment Selection and Forming Strategies for 6082 Aluminum Alloy

Aluminum 6XXX alloys show high strength to weight ratios and are thus promising materials for today’s transport industry lightweight construction efforts. When considering both deformation and final mechanical properties, high ductility is interesting from the conformability point of view. On the other hand, high resistance is necessary in the automotive structural parts (which can be obtained through T6 precipitation heat treatment) but leads to reduced ductility. In order to increase aluminum alloys’ formability, warm forming is commonly applied. In contradiction to this, this article shows how the tensile deformation behavior of the 6082 T6 alloy is not affected by the temperature. In this work, the necessary formability values to obtain the parts are achieved, deforming the material under O annealed condition. But this strategy is focused on the formability perspective; therefore, the final mechanical properties do not achieve the necessary strength requirements. As a solution, the possibility of applying the T6 heat treatment after forming the parts (in annealed condition) is studied. A tensile characterization of the post-heat-treated specimens obtained from the deformed experimental part results in high flow stress levels, and thus, the strategy is validated. Nevertheless, the heat treatment leads to geometrical distortions in the final part, and thus, a last calibration step should be added to the forming process in order to obtain the desired shape.

Microstructural investigations on as-cast and annealed Al–Sc and Al–Sc–Zr alloys

Al–Sc and Al–Sc–Zr alloys containing 0.05, 0.1 and 0.5 wt.% Sc and 0.15 wt.% Zr were investigated using optical microscopy, electron microscopy and X-ray diffraction. The phase composition of the alloys and the morphology of precipitates that developed during solidification in the sand casting process and subsequent thermal treatment of the samples were studied. XRD analysis shows that the weight percentage of the Al 3Sc/Al 3(Sc, Zr) precipitates was significantly below 1% in all alloys except for the virgin Al0.5Sc0.15Zr alloy. In this alloy the precipitates were observed as primary dendritic particles. In the binary Al–Sc alloys, ageing at 470 °C for 24 h produced precipitates associated with dislocation networks, whereas the precipitates in the annealed Al–Sc–Zr alloys were free of interfacial dislocations except at the lowest content of Sc. Development of large incoherent precipitates during precipitation heat treatment reduced hardness of all the alloys studied. Growth of the Al 3Sc/Al 3(Sc, Zr) precipitates after heat treatment was less at low Sc content and in the presence of Zr. Increase in hardness was observed after heat treatment at 300 °C in all alloys. There is a small difference in hardness between binary and ternary alloys slow cooled after sand casting.

Precipitation Studies of Undeformed and ECAP-Deformed Al-4%Cu Alloys: Effects on Microstructure and Tensile Properties

The present study is an assessment of the effects of precipitation heat treatments on tensile behaviour, work hardening (WH) characteristics and microstructural evolution of an Al-4%Cu alloy deformed by equal channel angular pressing (ECAP). Two ageing temperatures were employed (170 and 100oC) and their effect on strength and WH behaviour was compared with that exerted on the same alloy, but in two different initial conditions: quenched from solution temperature and slowly cooled before anneal. Grain and precipitate sizes of samples deformed by one and four ECAP passes and heat treated as described were measured employing transmission electron microscopy (TEM). It was concluded that the lower ageing temperature gives the best combination of strength and ductility, a high WH rate and, possibly, the smaller grain and precipitate sizes. The relative participation of the various hardening mechanisms to total strength was estimated from tensile tests and hardness measurements.

Effect of Precipitation pH and Heat Treatment on the Properties of Hydrous Zirconium Dioxide

The effect of the precipitation pH and subsequent heat treatment is studied on the properties of hydrous zirconium dioxide precipitated by ammonia from nitrate solutions. Precipitation at pH ≤ 6 generates hydrous zirconium dioxide, which contains excess sorbate nitrate ions; the product precipitated at pH ≥ 7 contains excess ammonium ions. This distinction considerably affects the course of thermolysis and the morphology of products. The exotherm associated with the formation of the crystal structure of zirconia becomes more pronounced with rising precipitation pH. In addition, the samples prepared at pH ≥ 7 have a more developed surface. The morphologic and microstructural evolution of hydrous zirconium dioxide samples during thermolysis is described.

Multifilamentary Nb–Zr and V–Ti superconducting alloys prepared by diffusion reactionbetween constituent pure-metal subelements

We have succeeded in making a superconducting Nb–Zr and V–Ti alloy multifilament wireusing the diffusion reaction between constituent pure-metal subelements, which can omitthe melt-and-casting step and may reduce the fabrication cost. A third elementaddition was also performed by replacing the constituent subelements with the thirdpure-metal subelement in the monofilament; Nb–Zr–V, Nb–Zr–Ti and V–Ti–Ta.Three kinds of heat treatment were investigated; (1) an alloying heat treatment(HTalloy) which is carried outat the final size (‘fs-HTalloy ’), (2) HTalloy is followed by cold-working (W) down to the final size(‘HTalloy+W ’), and (3) an intermediate precipitation heat treatment(HTprec) is carried out during the W stage coming after theHTalloy stage(‘HTalloy+W+HTprec+W’ ).The ‘HTalloy+W+HTprec+W’ treatment was found to be most effective in enhancing the critical currentdensities for both Nb–68 at.% Zr and V–60 at.% Ti–9 at.% Ta alloys, despite theHTprec treatment having slightly degraded the critical magnetic field.

The effect of time and temperature on the precipitation behavior and hardness of Al–4 wt%Cu alloy using design of experiments

The effect of time and temperature in precipitation heat treatment (artificial aging) on the hardness of Al–4 wt% Cu alloy was studied. Time was varied from 5 to 48 h and temperature was varied from 120 to 220 °C. Hardness data were obtained from standard Rockwell hardness tests using 75 randomly selected specimens. A linear statistical model, which is based on the design of experiments method, was applied to determine the importance of each variable. To optimize the process and determine the time and temperature levels at which a desired value of hardness can be achieved, a second-order regression model that captures most of the curvature in the actual relationship between hardness, time, and temperature was developed. Results showed a maximum increase of 29.3% in hardness of Al–Cu alloy when time was increased (temperature is constant), and changed between −3.6% and +21.1% when temperature was increased (time is constant). However, when both time and temperature were increased, a maximum increase of 36.0% in hardness was obtained due to a significant interaction effect between time and temperature. A contour plot was produced from the second-order regression model which showed that many time and temperature conditions at which a desired value of hardness can be achieved may exist.

Effect of the precipitation sequence on phase formation in the ZrO2-CeO2-Al2O3 system

We have studied phase formation processes during heat treatment of precipitates in the ZrO2-Al2O3 and ZrO2-CeO2-Al2O3 systems. During heat treatment of powders prepared by coprecipitation of precursors to ZrO2, CeO2, and Al2O3, α-Al2O3 is formed at higher temperatures, which is due to the formation and decomposition of T-ZrO2 and metastable Al2O3 phases. The precipitation sequence in the ZrO2-CeO2-Al2O3 system influences the lattice parameters of the forming T-ZrO2-based solid solutions because of the different degrees of Ce4+ and Al3+ substitutions for Zr4+.

On the addition of precipitation- and work-hardening in an Al–Sc alloy

Specimens of Al–0.2 wt.% Sc were solution heat treated and subjected to precipitation heat treatment. Precipitation-hardening response depended on storage time after solution heat treatment. An underaged condition and an overaged condition with equal strengths were found. Transmission electron microscopy indicates that the dislocations pass the precipitates by shearing in the underaged condition, and not in the overaged conditions. Analyses of stress–strain curves show significant differences in work-hardening behaviour between the two conditions.

A study of the galvanic corrosion behavior of Inconel 718 after electron beam welding

In this paper, the galvanic corrosion behavior of as-received, solution-annealed and precipitation-hardened Inconel 718 sheets after electron beam welding was studied in 3.5 wt% NaCl solution at 30 °C. An easy and reliable specimen-preparation method of a weld for electrochemical testing is proposed. Microstructures of the welds were investigated with an optical microscope and a transmission electron microscope integrated with an energy-dispersive X-ray spectrometer for chemical composition analysis. Experimental results show that galvanic corrosion could take place in an Inconel 718 weld when it is freely corroded or anodically polarized in 3.5 wt% NaCl solution. Furthermore, differences in corrosion potential between the fusion zone, heat-affected zone and base metal in an Inconel 718 weld is the key factor for galvanic corrosion. Due to a relatively active corrosion potential and a very small exposed area compared to those of the adjacent heat-affected zone and base metal, severe attack of the fusion zone could be observed in the as-received and solution-annealed welds when they were anodically polarized at 2.0 V for a few seconds. However, galvanic corrosion of solution-annealed weld can be prevented through post precipitation heat treatment, which resulted in only a small difference in corrosion potentials between the fusion zone, heat-affected zone and base metal.

Synthesis and characterization of aluminum alloy 7075 reinforced with silicon carbide particulates

In this study 7075 matrix alloy is chosen and SiC particles having average particle size of 29 μm was incorporated into the alloy at the liquid state before vertical pressure casting. Four different additions of SiCp were made and the weight fractions used are 10%, 15%, 20% and 30%. Composites were processed by vertical pressure/squeeze casting machine developed at Middle East Technical University (METU) under 80 MPa pressure. The mold is specially designed to produce specimens for tensile and three point bend tests. Both as-cast and heat treated aluminum composites were examined and the T6 heat treatment was applied. Three point bend tests were performed to reveal the fracture strength of the aluminum composites. The 10 wt% SiCp aluminum matrix composites showed the maximum flexural strength both for the as-cast (450 MPa) and heat treated conditions (588 MPa). The maximum flexural strength increased by about 40 MPa (10%) for the as-cast and 180 MPa (44%) for heat treated composites. Hardness tests were performed to determine the maximum value. For the as-cast specimens the hardness values increased from 133 to 188 Vickers (10 kg) with an increase in SiCp content from 0 to 30 wt% and for the heat treated specimens the hardnes values increased from 171 to 221 Vickers (10 kg). The peak hardness values were obtained for the 24 h precipitation heat treatment.

Recovery of Cobalt in Sulfuric Acid Leaching Solution Using Oxalic Acid

ABSTRACT Oxalic acid was used for recovering cobalt from leaching solution of spent lithium ion battery by precipitation method. Crushed powders containing LiCoO2 of spent lithium ion battery were obtained by crushing, magnetic separation and screening process. These powders were dissolved in 2 M H2SO4 solution with H2O2 as a reducing agent. After dissolving Co and Li components, the solution of oxalic acid was added. Co(II) in leaching solution could be removed in the form of insoluble oxalates. The precipitate formed was crystalline, compact and easily filtered. With the increase in oxalic acid, the rate of cobalt precipitate increased to 90% at 1:4 mole ratio of Co(II) and oxalic acid concentration. When cobalt ion was precipitated into cobalt oxalate, nickel and copper ions were co-precipitated but there were no co-precipitates of Li, Al and Fe. The amounts of Ni and Cu in the solutions were below 1%. After heat treatment of precipitates at above 250°C, cobalt oxalate was changed into cobalt oxide.

Synthesis of nanocrystalline hydroxyapatite by using precipitation method

In this investigation, hydroxyapatite powder has been synthesized from the calcium nitrate hydrated and di-ammonium hydrogen phosphate solution by precipitation method and heat treatment of hydroxyapatite powders. In order to study the structural evolution, the Fourier transform infrared spectroscopy (FTIR), the X-ray diffraction (XRD) and simultaneous thermal analysis (STA) were used. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to estimate the particle size of the powder and observe the morphology and agglomeration state of the powder. Results show that hydroxyapatite nanocrystalline can successfully be produced by precipitation technique from raw materials. Hydroxyapatite grain gradually increased in size when temperature increased from 100 to 1200 °C, and the hydroxyapatite hexagonal-dipyramidal phase was not transformed to the other calcium phosphates phases up to 1200 °C.

Fabrication and characterization of sputtered NiTi shape memory thin films

Sputtering conditions to produce nearly equiatomic NiTi films were studied. Two heat treatment methods, substrate heating (550 °C) and post-sputter heat treatments (solution heat treatment at 700 °C and precipitation heat treatment at 400 °C), were used to crystallize NiTi films. The NiTi films were analysed for their atomic composition, crystal structure, transformation temperatures, residual stress and Young's modulus. NiTi films produced by rf magnetron sputtering were slightly titanium rich. With appropriate post-sputter heat treatment, NiTi films with phase transformation temperatures within the human body temperature range (34–42 °C) were produced. The films also demonstrated two-way shape memory training capability.

Optimisation of precipitation for controlling recrystallisation of wrought Fe 3Al based alloys

A Fe–26Al–5Cr (at.%) single-phase (α:A2/B2/D0 3) alloy and two-phase (α+TiC) alloys with different amounts of TiC particles have been hot rolled at 800 °C and the kinetics of static recrystallisation have been studied. In the alloys with a high amount of TiC, needle-like TiC of more than 1 μm in length formed during cooling after homogenisation in the α single-phase region and coarsened during hot rolling. The large particles cause particle stimulated nucleation (PSN) and hence accelerate recrystallisation. In order to accomplish both strengthening by precipitates and inhibition of recrystallisation that deteriorates room-temperature ductility, a thermo-mechanical treatment consisting of hot deformation with a low amount of precipitates and a subsequent heat treatment for further precipitation is proposed. This process is difficult to carry out in the (Fe–26Al–5Cr)–TiC system due to the high precipitation temperature of TiC. The precipitation temperature is significantly decreased by replacing TiC by VC or MoC.

ＣＭＳＸ４のクリープ強度におよぼす時効熱処理の影響

In single crystals of Ni-based superalloys, the γ' precipitate size has been shown to affect both the tensile strength and the creep behavior at intermediate temperatures. The microstructure of Ni-based superalloys contains primary cuboidal γ' particles that precipitate during aging treatment, and cooling γ' particles that precipitate during cooling from the aging temperature. In the Ni-based superalloy containing a high volume fraction of γ' precipitates, the change in the morphology of the secondary γ' precipitates in a matrix channel has been found to affect the creep strength. In this study, three kinds of aged specimens were prepared to investigate the influence of the morphology of γ' precipitates on the strengths of CMSX4 alloy. Slow furnace cooling from the aging temperature was employed to obtain a clean matrix channel completely devoid of secondary γ' precipitates. In order to clarify the effect of secondary γ' precipitates, the strengths of the furnace-cooled specimens were compared with those of the forced-airquenched specimens containing numerous superfine γ' precipitates in the matrix channel, and those of the standard two-step-aged specimens. The purpose of the present investigation is to gain a better understanding of the effect of precipitation heat treatment on the material strengths at an intermediate temperature, where the secondary γ' precipitates remain undissolved during creep deformation.

On the relationship between microstructure, strength and toughness in AA7050 aluminum alloy

The effect of process parameters such as quench rate and precipitation heat treatment on the compromise between the toughness and the yield strength of AA7050 aluminum alloy (AlZnMgCu) are investigated, as well as the anisotropy of this compromise in the rolling plane. Fracture toughness is experimentally approached by the Kahn tear test. The microstructure is studied quantitatively in detail by a combination of scanning electron microscopy, transmission electron microscopy and small-angle X-ray scattering, and the relative fractions of the various fracture modes as a function of microstructural state are quantitatively determined on scanning electron microscopy images. Toughness is confirmed to be minimum at peak strength, and lower for an overaged material than for an underaged material of the same yield strength. A lower quench rate is shown to result in an overall reduction of toughness, and in a reduced evolution of this toughness during the aging heat treatment. The overall toughness is also lowered when the main crack propagation direction is parallel to the preferential elongation direction of the coarse constituent particles (rolling direction). The competition between intergranular and transgranular fracture is explained in terms of the modifications of the work hardening rate, and of grain boundary precipitation. The evolution of fracture toughness is qualitatively explained in terms of evolution of yield stress, strain hardening rate, grain boundary precipitation and intragranular quench-induced precipitates.

Production of rheocast slurries by partial melting through alternative thermomechanical treatments

The production of alloys with rheocast structure, where the primary phase consists of globularized particles, by partial melting, involves grain recrystallization and secondary phases melting. In the process known as SIMA (Strain Induced Melt Activation), the raw material is cold deformed by a two-step rolling, making it little efficient. This work investigates the production of thixotropic material, by alternative thermomechanical treatments, of an Al-3.35wt%Cu alloy. Two treatments, which avoid the hot deformation step of SIMA, are performed: Recrystallization and Partial Melting (RAP) and Overaging (OAT) processes. In the first, the as-cast, dendritic, alloy is cold deformed at ambient temperature and then heated to a constant temperature in the mushy zone, to obtain the rheocast structure. In the second, the alloy undergoes a solution and precipitation heat treatment, before cold-deformation, with the aim of obtaining second phase precipitated particles with size and interparticle spacing favourable to the increasing of the nucleation rate of recrystallized grains, during heating of the alloy. It was found that the two routes produce globularized structures, with grain reduction of 85 % and 90 % for RAP and OAT, respectively, with respect to initial grain sizes. In general, OAT process resulted in more rounded and smaller globules than RAP.

Structure and Properties of Rapidly Solidified P/M Samples of Al-Mn-Cr Alloys

Rapidly solidified P/M materials of Al-Mn-Cr ternary alloys were prepared containing 6∼8 mass% Mn and 1∼3 mass% Cr. The effect of alloy composition on precipitation hardening was studied with an aim of obtaining precipitation hardened materials having acceptable ductility. The critical composition for observing precipitation hardening was determined to be 8.8 mass% Mn+Cr; above this value hardening increased with increasing the Mn+Cr content. The greatest hardening was observed for Al-7Mn-3Cr alloy. In these alloys, CrAl 7 was formed under rapid solidification and precipitation of MnAl 6 occurred during hot extrusion in alloys of lower Cr content. Finally, G phase ((Mn,Cr)Al 12 ) was formed during heating of hot-extruded materials, causing hardening when the amount of G phase formation is sufficiently large. For the as-extruded P/M materials, the highest tensile strength at room temperature (480MPa) was obtained for Al-7Mn-3Cr, with 7% elongation. Elongation of the as-extruded P/M materials remained unchanged up to 473K but increased appreciably at 673K. However, precipitation hardened P/M materials showed poor ductility so that the tensile strength decreased with aging time. Increases in the elastic modulus were observed after precipitation heat treatment.

Optical spectroscopy of oxygen precipitates in heavily doped p-type silicon

Results are presented on the photoluminescence (PL) characterizationof heavily doped p+ Czochralski silicon, which has been subjected to atwo-step, oxygen precipitation heat treatment. It will be shown that thepresence of oxygen precipitates gives rise to the D1, D2 and D5 lines, wherethe energy of the D1 line shifts to lower values for a stronger degree ofprecipitation. The occurrence of these PL features is also a function of theboron concentration in the p+ material. The PL results are compared withFourier transform infrared absorption data and with transmission electronmicroscopy results. From this, it is concluded that PL has a good potential foruse in the assessment of oxygen precipitation in heavily doped silicon.

Modelling of solidification and heat treatment for the prediction of yield stress of cast alloys

A model has been developed for the prediction of yield stress of cast alloys. It consists of integrated unit step models of the physical metallurgy of solidification, solid-state transformations, structural hardening and micro-mechanisms. Calibration of the precipitation and hardening unit step models is conducted based on experimental data available in the literature for aluminium–copper alloys. Applications are then carried out to study the effects of material and process parameters such as grain size, alloy composition, solidification cooling rate, duration and temperature of the homogenisation and precipitation heat treatments, as well as quenching rate after homogenisation. The effect of the residual volume fraction of eutectic after completion of the solidification and homogenisation treatments is clearly revealed as the main parameter controlling a yield stress knock-down factor.