Analyzing Aluminum Alloys

Aluminum is the world's third most common element and makes up 8% of the earth’s crust. It is the most abundant metal found on earth. Its excellent versatility makes it the most commonly used metal after steel. Pure aluminum is resistant to corrosion, soft, ductile and exhibits excellent electrical conductivity.

Standard applications of aluminum include conductor cables and foil, but it can also be alloyed with Si, Mg and other elements to achieve the greater level of strength necessary for other applications. When aluminum and its base materials are being produced, quick and precise elemental analysis is very important.

This article describes the capability of the Metals edition of the Zetium XRF spectrometer for the analysis of Al-Si and Al-Mg alloys.

Instrumentation and Measurement Conditions

The Zetium XRF is a fully integrated wavelength dispersive XRF spectrometer. The Metals edition of the Zetium spectrometer is equipped with a SuperQ analysis software and an X-Y sample handler. Its default configuration includes the Ge111, PE002, LiF220 and LiF200 analysis crystals. The PE002 crystal was scaled up to a PE002-curved crystal to achieve better intensities and resolution for Si analysis. To perform high-resolution testing of Mg a PX8 multilayer crystal was added.

A 300 µm collimator was used to determine all elements other than Mg, which was determined using a 700 µm collimator. The tube settings varied between 40 kV to 60 mA for Ti, 24 kV to 100 mA for Mg and Si, and 60 kV to 40 mA for Ni, Mn, Cu, Fe and Zn. A 27 mm collimator mask was used to quantify the samples. The overall measurement time took 64 seconds and the SuperQ FP algorithm was used to make matrix corrections.

Sample Description and Preparation

Two sets of certified reference materials (CRM) were used: one for Al-Mg alloys (CKD 242 – 246) and the other for Al-Si alloys (CKD 236 – 241). Prior to measurement the samples were allowed to resurface on a Herzog lathe.

Calibration Results

Figures 1 and 2 show the calibration curves for Mg and Si. Table 1 shows the calibration details for all the elements.

Calibration graph for Mg

Figure 1. Calibration graph for Mg

Calibration graph for Si. The blue line is the combined calibration, the green line is for Al-Mg alloys and the red line for Al-Si alloys.

Figure 2. Calibration graph for Si. The blue line is the combined calibration, the green line is for Al-Mg alloys and the red line for Al-Si alloys.

Table 1. Calibration details

Element Concentration range (wt%) K-factor RMS (wt%) LLD (ppm, 100 s)
Mg 0.2 - 7.5 0.022 0.032 4.4
Si 0.3 - 11.8 0.085 0.230 3.1
Low Si (Al-Mg alloys) 0.3 - 3.1 0.014 0.019
High Si (Al-Si alloys) 7.5 - 11.8 0.031 0.090
Ti 0 - 0.23 0.007 0.003 0.9
Mn 0 - 0.85 0.009 0.007 0.4
Fe 0 - 1.14 0.016 0.014 0.5
Cu 0 - 1.51 0.023 0.015 0.6
Ni 0 - 1.65 0.017 0.018 0.3
Zn 0 - 1.14 0.044 0.021 0.5

The Zetium XRF is a fully integrated wavelength dispersive XRF spectrometer. Unlike the RMS value, the K-value is free from the calibrated concentration range, and hence, can reliably indicate the quality of calibration. Generally, K-values of 0.02 – 0.03 suggest good calibration standards.

Excluding Si and Zn, it is possible to acquire precise calibrations for all of the elements. High K-values of 0.085 and 0.044 were observed for Si and Zn respectively. It is a common knowledge that element distribution and metallic structure along grain edges can differ from alloy to alloy.

Certain elements are affected by these metallurgical variations more than others meaning independent calibrations may be needed for the most severely affected alloys. In Figure 2, this can be distinctly observed for Si. When high (Al-Si alloy) and low (Al-Mg alloy) range calibrations are indicated, as shown in Table 1, the accuracy of calibration is greatly enhanced. During measurement, the right calibration can be automatically chosen by using the SuperQ’s Automatic Program Selection (APS) option.

Accuracy

The methods accuracy was ensured by the measurement of a CRM routine sample. The results for the Al-Mg alloy, CRM CKD 244, are given in table 2. Apart from Zn, where a major difference can be seen, the values for all elements are near to the certified values.

In Zinc the variation is the result of the metallurgical variations between two different types of alloys, and can be corrected by choosing an alloy-specific calibration for Zn with the APS feature.

Precision

One sample was measured 20 times in succession, to test the system and the precision of the method. Table 2 shows the results, and Figure 3 shows a graphical representation for Si.

Table 2. Repeatability results (20 consecutive times) for Si-Al alloy CKD 240

Element Mg Si Ti Mn Fe Cu Ni Zn
Average (wt%) 0.65 10.19 0.073 0.27 0.45 0.69 0.91 0.045
RMS (wt%) 0.003 0.01 0.001 0.001 0.002 0.002 0.002 0.0003
RMS (rel% 0.44 0.10 1.04 0.49 0.41 0.25 0.24 0.60
CSE (wt%) 0.003 0.01 0.001 0.001 0.002 0.002 0.002 0.0003

Repeatability results for Si

Figure 3. Repeatability results for Si

It was found that for almost all elements, relative errors are less than 1%, which suggests excellent instrument stability and repeatability. The same equivalent value for all elements was seen when the total RMS values when evaluated against the CSE values. This means for a repeated measurement under these settings, this value remains at the lowest error possible.

Conclusion

This article has demonstrated how the Metals edition of the Zetium XRF spectrometer can be effectively used for quick and precise assessments of Al alloys. The experimental results obtained are highly accurate, making the Metals edition of Zetium suitable for Al alloy process control. Independent calibration lines can be created for each type of alloy to perform more accurate assessment of a wide range of Al alloys. Using the APS feature of the SuperQ software, the correct calibration can be selected automatically.

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

For more information on this source, please visit Malvern Panalytical.

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