Cu-base-FP - analysis of copper alloys

‘Copper-base’ (Cu-base) is a term used to describe a range of alloys where Cu is the principal component. For different applications, the physical and chemical properties of Cu are adjusted by alloying with elements such as Sn, Zn, Ni Al, and Pb. Cu-based alloys are used in the manufacture of coinage, bells, a wide range of mechanical equipment and in electrical wiring.

During the production process fast chemical analysis is needed to minimize errors in production and to maximize production yield. Close tolerance specifications, combined with tight control of expensive alloy additions and tramp elements, demand the highest levels of accuracy and precision possible in analysis. 

Zetium Metals edition

Introduction

‘Copper-base’ (Cu-base) is a term used to describe a range of alloys where Cu is the principal component. For different applications, the physical and chemical properties of Cu are adjusted by alloying with elements such as Sn, Zn, Ni Al, and Pb. Cu-based alloys are used in the manufacture of coinage, bells, a wide range of mechanical equipment and in electrical wiring.

During the production process fast chemical analysis is needed to minimize errors in production and to maximize production yield. Close tolerance specifications, combined with tight control of expensive alloy additions and tramp elements, demand the highest levels of accuracy and precision possible in analysis. A variety of analytical techniques are pushed to their limits for speed, precision and stability. For X-ray fluorescence analysis, system stability, sensitivity and fast sample handling are strong points in favor of the technique.

To address the special needs of the copper industry, the Metals edition of the Zetium spectrometer can be configured with the Cu-base-FP package. Consisting of an application template and a suite of 22, carefully selected reference samples, this package covers the analysis of brass (chill cast), leaded brass, manganese brass (chill cast), aluminium bronze (cast), phosphor bronze (chill cast), leaded bronze (chill cast), bronze (chill cast) and cupro nickel (chill cast). PANalytical’s Fundamental Parameter (FP) model is employed in the calibration regression and has the flexibility necessary to cope with such a wide variety of materials. The Cu-base- FP application template ensures the optimal setup of the Metals edition of the Zetium spectrometer for this type of analysis.

Preparation of samples

The Cu-base samples and standards were re-surfaced with a fine surface-milling machine to obtain a quality surface for analysis. This method is suitable for a variety of materials of varying hardness, such as the hard aluminium bronzes and the softer high copper alloys.

Measurement conditions

The total measurement time of the analytical program was 128 seconds, using the Kβ1 line. Line overlap corrections were made for the following interferences: As on Mg, Cu on P, Fe on Co, Sn on Pb, and Sn on Sb. To simplify installation, the SuperQ software for the Metals edition of the Zetium spectrometer contains a template for the Cu-base-FP application and calibration only requires measurement of the Cu-base setup samples.

Calibration

In the calibration, the Fundamental Parameter model was used to calculate the matrix correction term, M, in the calibration formula:

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The calibration details are shown in Table 1 and calibration plots (Figures 1-4) give a graphic illustration of the accuracy of the Cu-base-FP application. The ‘root mean square’ (RMS) factor (effectively 1 standard deviation) and the K-factor are an indication of the calibration quality, indicating the magnitude of the differences between the certified and measured concentrations. Lower values for RMS and K indicate more accurate calibrations (see side text). In these types of wide range calibrations the value of the FP model is clearly demonstrated. With the FP model, the corrections for the interelement effects are optimized for each of the calibration standards and the routine samples, making it possible to calibrate concentration ranges from 0 to 50 percent or more.

Table 1. Calibration results and detection limits for the Cu-base-FP application

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Accuracy

The accuracy of the Metals edition of the Zetium spectrometer using the Cu-base-FP application for major and minor element analyses in coppe alloys is very good. This is illustrated in accuracy overview plots for a number of elements (Figures 5 & 6), which compare certified and measured values for 11 Certified Reference Materials (CRMs) of widely varying composition (Table 2). These CRMs were measured as routine samples against the Cu-base-

FP application and did not  form  part of the original calibration set. With the exception of Pb in the lead bronzes, the Cu-base-FP results match the certified values closely. It is well known that metallic structure and element distribution along grain boundaries can vary from one alloy  type to another and these metallurgical differences can affect some elements more strongly than others. Lead is added to bronze to improve its workability, however it is not homogeneously distributed within the alloy and during  sample  preparation the lead tends to smear over the sample surface. This results in the analytical differences observed. In this instance it is advisable to set up a dedicated application for lead bronze.

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Table 2. Analytical accuracy: comparison of certified and measured values for 14 elements in 11 CRMs of various types. For the certified concentrations, values in ( ) are for information only and those in [ ] are determined by difference from 100 wt% . 

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Precision and instrument stability 

The precision of the Metals  edition of the Zetium spectrometer with the Cu-base-FP package was determined by short- and long-term repeatability measurements of an Al bronze CRM (32X ALB6) analyzed as an unknown sample against the Cu-base-FP calibration (Table 3 and Figure 7). The results were obtained without drift correction.

Comparison of the RMS value (1 σ standard deviation) with the counting statistical error (CSE) shows the excellent precision of the method and the outstanding stability of the Metals edition of the Zetium spectrometer. Theoretically, the  CSE is the minimum possible error and the similar magnitude of the RMS and CSE values shows that errors originating from the instrument are negligible.

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Table 3. Analytical precision derived from repeated measurements of an Al bronze CRM (32X ALB6)

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Components typeset in bold were present in the spectrometer used to obtain the data in this note.

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Conclusions

Elemental analysis by XRF is the principal method for process and quality control for the production of a wide variety of copper-based alloys. The Metals edition of the Zetium spectrometer with Cu-base-FP is an excellent solution for the analysis of copper alloys, such as bronze, aluminium-bronze, phosphor bronze, leaded bronze, brass, alloyed brass, and leaded brass. It has been demonstrated that the analyses are accurate and precise and that the stability of the Metals edition of the Zetium spectrometer is excellent, resulting in highly consistent data over time. The Metals edition of the Zetium spectrometer has been configured to meet the basic needs of the industry, however, it can be further upgraded to enhance performance in terms of speed  and precision. Upgrade  features  include  increased  power,  continuous loading and direct loading for extra speed of analysis, a sealed Xe counter for lower limits of detection, and Omnian for standardless analysis. Integration of the Metals edition of the Zetium spectrometer into laboratory automation setups is supported with the universal automation interface software and optional sample transfer features.

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