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.”

After heat treatments, the DSC experiments were performed using a computerized differential scanning calorimeter (Perkin-Elmer DSC 8000, Waltham, MA, USA). The mass of each sample was approximately 20–30 mg. A protective atmosphere of pure argon gas with a gas flow of 60 mL/min was used. The sample was analyzed over the temperature range of 50 to 550 °C, at a constant heating rate of 10 °C/min. Three samples were analyzed for each temper. To improve the DSC resolution, a piece of high-purity aluminum (99.999%) with a weight similar to that of the test sample was placed in the reference crucible for each scan. To make the DSC results comparable between tempers, the correction and normalization of the DSC curves under a common procedure is essential, and thus the original DSC curves in the present work were corrected and normalized using a three-step procedure [6], which is shown in Figure 1 with an example in the F tempers of AlSi10Mg0.3Mn alloy. The first step was the subtraction of the instrumental baseline (Figure 1a), which was measured using high-purity aluminum with the same weight in both the sample and reference crucibles. The second step was the subtraction of the heat-effect baseline (Figure 1b), which was done using a polynomial or a Gaussian function [16]. Finally, the curve was normalized by dividing the mass of the test sample (Figure 1c). After correction and normalization, the characteristics of the DSC curve were analyzed, including the number and sizes of peaks as well as their onset/peak/end temperatures.
Figure 1. Correction process of the DSC curves: (a) subtraction of instrumental baseline; (b) subtraction of heat-effect baseline; and (c) normalization according to the mass of the sample.

Figure 2a shows the DSC heating curves of the AlSi10Mg0.3Mn alloy subjected to the F (as-cast) and SHT treatments, while Figure 2b shows those for the T5, T6, and T7 tempers. As shown in Figure 2a, three obvious exothermic peaks, ab, and c, located at temperatures of approximately 240, 330, and 430 °C, respectively, are evident for both F and SHT, and are related to the precipitation of β”, β’, and β, respectively [3,21]. On the other hand, as shown in Figure 2b, the curves for both the T5 and T6 tempers show only two exothermic peaks, b’ and c’, located at temperatures of approximately 340 and 420 °C, respectively, which are related to the precipitation of β’ and β, respectively, while only one major exothermic peak, c”, located at 370 °C, was detected for the T7 sample, which can be related to the precipitation of β [21]. In addition, three endothermic peaks (III, and III) can be observed in Figure 2, and are related to the dissolution of the specific phases. According to the literature [3,21], Peak I represents the dissolution of β”, while Peak II refers to the dissolution of β’, and Peak III for the dissolution of the β phase. The characteristics of the DSC curves corresponding to the various tempers are summarized in Table 3.
Figure 2. DSC heating curves of AlSi10Mg0.3Mn alloy subject different treatments: (a) F and SHT; (b) T5, T6, and T7.

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