Differential scanning calorimetry fingerprints of various heat-treatment tempers of different aluminum alloys

Heat-treatable cast and wrought aluminum alloys are widely used for structural applications in the automobile and aerospace industries. To assess and diagnose the production and quality problems related to industrial heat treatments, differential scanning calorimetry (DSC) was used as a tool in the present work to determine the thermal histories of samples that had undergone different tempers of three commonly used aluminum alloys, namely a high-pressure die-cast AlSi10Mg0.3Mn alloy, permanent-mold cast Al-Si-Cu 319 alloy, and extruded Al-Mg-Si AA6082 alloy. Various peaks detected in the DSC curves were analyzed and characterized to identify the precipitation/dissolution reactions of metastable phases, aiming to establish a “fingerprint” of each temper of the three experimental alloys. Results showed that both the number and size of exothermic peaks varied with the temper owing to distinct precipitation behaviors, providing an effective means of fingerprinting the various tempers. Meanwhile, electrical conductivity and microhardness data provided the supplementary support for the fingerprinting. The thermal histories of three experimentally heat-treated alloys were well traced and distinguished by the combination of DSC characteristics and electrical conductivity and microhardness results, promoting the DSC application in the quality control and verification of industrial heat treatments. In addition, the microstructures after the various tempers were observed to confirm the evolution of the precipitation reactions revealed in the DSC curves

Effects of La and Ce Addition on the Modification of Al-Si Based Alloys

This study focuses on the effects of the addition of rare earth metals (mainly lanthanum and cerium) on the eutectic Si characteristics in Al-Si based alloys. Based on the solidification curves and microstructural examination of the corresponding alloys, it was found that addition of La or Ce increases the alloy melting temperature and the Al-Si eutectic temperature, with an Al-Si recalescence of 2-3°C, and the appearance of post-α-Al peaks attributed to precipitation of rare earth intermetallics. Addition of La or Ce to Al-(7–13)% Si causes only partial modification of the eutectic Si particles. Lanthanum has a high affinity to react with Sr, which weakens the modification efficiency of the latter. Cerium, however, has a high affinity for Ti, forming a large amount of sludge. Due to the large difference in the length of the eutectic Si particles in the same sample, the normal use of standard deviation in this case is meaningless

Improving the elevated-temperature mechanical properties of AA3004 hot-rolled sheets by microalloying with Mo and optimizing the process route

The present work investigated the influence of Mo addition and thermomechanical process routes on microstructural evolution and elevated-temperature mechanical properties of Al–Mn–Mg 3004 alloys. Various combinations of heat treatment and hot rolling were applied to fabricate hot-rolled sheets. The results revealed that microalloying with Mo and two-step heat treatment increased the number density and volume fraction of dispersoids and decreased the volume fractions of dispersoid-free zones. The different processing routes had important impacts on microstructural evolution. The alloys processed with heat treatment followed by hot rolling had finer and better distributions of dispersoids than those subjected to hot rolling prior to heat treatment. The former resulted in higher tensile strengths at room and elevated temperatures. Among all conditions, the Mo-containing alloy subjected to two-step heat treatment followed by hot rolling exhibited the highest elevated-temperature properties and reached a yield strength of 93 MPa at 300 °C. Both the base and Mo-containing alloys subjected to two-step heat treatment followed by hot rolling showed excellent thermal stabilities up to 350 °C and almost no significant change in yield strengths after thermal exposure at 300–350 °C for 100 h

On the Room- and Elevated-Temperature Tensile Properties of Hot-Rolled 6082 Alloys Containing Thermally Stable Dispersoids

The microstructural evolution and room/elevated-temperature tensile properties of Al-Mg-Si 6082 alloys subjected to thermomechanical processing (homogenization, hot rolling, T6 heat treatment, and thermal exposure) were investigated. Four experimental 6082 alloys were studied, including a Mn-free base alloy and three alloys containing Mn individually and in combination with Cr + V or Mo, in which a number of α-Al(MnFe)Si, α-Al(MnFeCrV)Si and α-Al(MnFeMo)Si dispersoids were formed, respectively. The results showed that both α-Al(MnFeCrV)Si and α-Al(MnFeMo)Si dispersoids had a higher coarsening resistance compared to α-Al(MnFe)Si dispersoids. The presence of α-dispersoids hindered the formation of Mg-Si clusters, which decreased the precipitation of β″-MgSi precipitates, resulting in reductions in room-temperature strengths. During thermal exposure at 300 °C, the α-dispersoids remained thermally stable and became the predominant strengthening phase, resulting in increases of 71 to 126% in the yield strength at 300 °C relative to the base alloy without dispersoids. Among the three dispersoid-containing alloys studied, the alloy containing Mn and Mo exhibited the highest yield strength of 70 MPa at 300 °C, providing the best combination of room- and elevated-temperature tensile properties

Effect of Si Level on the Evolution of Zr‐Bearing Dispersoids and the Related Hot Deformation and Recrystallization Behaviors in Al–Si–Mg 6xxx Alloys

The precipitation behavior of Zr-bearing dispersoids is investigated in Al–Si–Mg 6xxx alloys with different Si levels (0.4, 0.7, and 1.0 wt%) at three homogenization temperatures (450, 500, and 550 °C). The hot deformation behavior is studied using uniaxial compression tests at different Zener–Hollomon parameters. The microstructure evolution during hot deformation and postdeformation annealing is evaluated using the electron backscatter diffraction technique. The results show a significant influence of the Si level and homogenization temperature on the precipitation of two types of Zr-bearing dispersoids. Si promotes the precipitation of both spherical L12–Al3Zr and elongated DO22–(Al,Si)3(Zr,Ti) dispersoids during low-temperature homogenization. However, it accelerates the transformation of Zr dispersoids from L12 to DO22 at high homogenization temperature. The flow stress is more influenced by the solid solution level and hot deformation parameters rather than by the dispersoid distribution. The fine dense L12–Al3Zr dispersoids provide higher recrystallization resistance during postdeformation annealing compared with the large elongated DO22–(Al,Si)3(Zr,Ti) dispersoids. Owing to the uniform distribution of dispersoids and limited dispersoid-free zones, the high Si alloy (1.0%) exhibits best recrystallization resistance among the three alloys studied

Precipitation Characteristics of HPVDC AlSi10Mg0.3Mn Alloy Under Different Temper Conditions

This work studied the precipitation characteristics of AlSi10Mg0.3Mn alloy produced by high-pressure vacuum die-casting in T5 and T6 conditions using transmission electron microscopy, differential scanning calorimetry and both electrical conductivity and microhardness measurements. Two different T6 tempers were used, involving partial solutionizing at 460 °C for 1 h and full solutionizing at 500 °C for 1 h, and were designated as T6P and T6F, respectively. The aging treatment was conducted at 185 °C for 4 h in all temper conditions. The results showed that conducting solution treatments resulted in the formation of α-Al(MnFe)Si dispersoids. In addition, Mg–Si rich precipitates formed and remained undissolved after partial solutionizing. The highest supersaturation of the α-Al was achieved in the as-fabricated (F) condition by the HPVDC process. The supersaturation degree decreased in both partial and full solutionizing conditions. TEM observations revealed different precipitation characteristics for T5, T6P and T6F tempers. β” precipitates just started to form in the T5 condition, whereas both β” and β’ precipitates completed their precipitation to different extents in the T6P and T6F conditions, resulting in different strengthening effects. The T6F temper yielded the highest microhardness followed by the T5 and T6P tempers

Improving the dispersoid distribution and recrystallization resistance of a Zr-containing 6xxx alloy using two-step homogenization

Two-step homogenisation was applied to a 6xxx alloy containing Zr to enhance the characteristics of Zr-bearing dispersoids and recrystallization resistance. The two-step homogenisation treatments were composed of a first step at 400 °C for 48 h and a second step at 500 °C for 2 and 5 h and compared with single-step homogenisation conducted at 500 °C for 2 and 5 h. The dispersoid microstructure was characterised using optical microscopy and scanning and transmission electron microscopies. The thermomechanical simulator Gleeble 3800 was used to conduct the hot compression tests at 350°C/1.0s−1. To study the recrystallization resistance, post-deformation annealing at 500 °C for 1 h was performed on the deformed samples. The grain structure after deformation and annealing was characterised based on the EBSD technique. The results showed that compared to single-step homogenisation, the dispersoid characteristics were significantly improved using two-step homogenization, where the number density of L12-Al3Zr dispersoids increased by 75–145% while their size decreased by 9–25% and the distribution of the DO22-(Al, Si)3(Zr,Ti) dispersoids became more uniform. The improved characteristics of Zr-bearing dispersoids and the narrower dispersoid-free zones produced by the two-step homogenization significantly improved the recrystallization resistance with a reduction in the recrystallized area fraction reached 85% when compared with single-step homogenisation

Development of thermal-resistant Al–Zr based conductor alloys via microalloying with Sc and manipulating thermomechanical processing

Thermal-resistant Al–Zr based conductor alloys were developed using microalloying with a low level of Sc (≤0.10 wt%) and two thermomechanical processing routes, in which the cold wire drawing was conducted before and after the aging treatment. The mechanical properties, electrical conductivities, and thermal-resistant properties of several alloys were investigated and evaluated according to the IEC standard of thermal-resistant aluminium conductors. The evolutions of the precipitates and grain structure during processing were also studied. The microalloying with Sc resulted in the precipitation of a large number of fine Al3(Sc,Zr) precipitates, which provided substantially high strength of 188–209 MPa, representing 73–88% improvement compared to the Sc-free base alloy, while maintaining excellent electrical conductivity of 57.4–59.9% IACS. Moreover, the Sc-containing alloys exhibited outstanding thermal-resistant properties, where the maximum strength reduction was limited to ≤6.0% after thermal exposures at 310 and 400 °C. The best combinations of mechanical properties and electrical conductivities of the Sc-containing alloys were obtained after aging at 350 °C for 48 h, following solutionizing at 600 °C for 8 h. Both processing routes yielded comparable precipitation strengthening and strain hardening, and consequently comparable mechanical and electrical properties, where the maximum differences in the strength and electrical conductivity between both routes were 12 MPa and 1.4% IACS, respectively. The excellent combinations of mechanical, electrical, and thermal-resistant properties made the developed alloys promising candidate materials for four standard grades of thermal-resistant aluminium conductors, while taking advantage of an affordable material cost and using conventional thermomechanical processes

Nucleation and transformation of Zr-bearing dispersoids in Al–Mg–Si 6xxx alloys

The nucleation and transformation of L12-Al3Zr and DO22-(Al,Si)3Zr dispersoids in Al–Mg–Si 6xxx alloys were studied using interrupted quenching and transmission electron microscopy. Spherical L12-Al3Zr dispersoids precipitated preferentially along  〈001〉Al in early stages of nucleation, coinciding with the same sites and orientation of β′ precipitates that dissolved during heating. Two nucleation mechanisms were suggested to explain this preferable precipitation. At a relatively low heat treatment temperature (400 °C), the L12-Al3Zr dispersoids were predominant, and no transformation occurred. With further increase in temperature to 550 °C, the L12-Al3Zr dispersoids started to transform into DO22-(Al,Si)3Zr. At high temperatures, elongated DO22-(Al,Si)3Zr dispersoids, which were formed through the transformation of pre-existing L12 dispersoids rather than the direct precipitation from the supersaturated aluminum solid solution, became the dominant phase

Effect of Rare Earth Metals on the Microstructure of Al-Si Based Alloys

The present study was performed on A356 alloy [Al-7 wt %Si 0.0.35 wt %Mg]. To that La and Ce were added individually or combined up to 1.5 wt % each. The results show that these rare earth elements affect only the alloy melting temperature with no marked change in the temperature of Al-Si eutectic precipitation. Additionally, rare earth metals have no modification effect up to 1.5 wt %. In addition, La and Ce tend to react with Sr leading to modification degradation. In order to achieve noticeable modification of eutectic Si particles, the concentration of rare earth metals should exceed 1.5 wt %, which simultaneously results in the precipitation of a fairly large volume fraction of insoluble intermetallics. The precipitation of these complex intermetallics is expected to have a negative effect on the alloy performance