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Testing the Metal

Bruce Morey
By Bruce Morey Senior Technical Editor, SME Media

As metals and other materials get more complex, knowing what you are getting from your supplier, or what you have in stock, is more important than ever.

The Vulcan handheld LIBS analyzer from Hitachi High-Tech collects data in one second, according to the company. Data is downloaded via WiFi and comes with an alloy library preloaded.

Materials science has opened new possibilities for designers of cars, planes and other products. Metal alloys are now as precisely engineered as they are machined. The result is longer lasting, stronger parts. But with a wider selection of materials comes risk—how can you be sure that one piece of gray metal stock is different than another? Careful warehousing procedures and paperwork only go so far.

Checking incoming material is getting even more important as supply chains grow in search of better pricing.

“Companies are looking at the growing world market and realizing that [there is sometimes a] different nomenclature for alloys,” explained James Stachowiak, technical sales manager for Thermo Fisher Scientific (Tewksbury, MA). “Sometimes the alloy material they are receiving isn’t meeting the engineering specification they require.”

To help eliminate risk, manufacturers need to know the constituent elements of metallic alloys in their inventory. They need to know a piece of stock is indeed 304 stainless and not 316 stainless—or out of specification entirely. There is even a term for it: positive material identification (PMI). Typical methods for PMI include X-ray fluorescence (XRF), optical emission spectroscopy (OES), and laser induced breakdown spectroscopy (LIBS).

If you think these are terms for bulky, cabinet sized instruments relegated to laboratories, think again.

Miniaturization and faster computers have given manufacturers a number of flexible options for PMI as well. This is especially true for the XRF class of handheld devices. These use a miniature version of an X-ray source that is pointed at a sample surface, injects an X-ray into the material and then detects an induced fluorescence with an advanced solid-state detector. Just like in optical spectrometry, the dispersion and strength of the induced wavelengths that the device detects is used to determine the elements in the piece.

“Twenty-five years ago, a portable XRF device weighed 25 pounds, used radioactive isotopes with costly replacement cycles—every three to five years—and were much slower, with lower precision and accuracy. Our latest Niton XL5 instrument weighs under three pounds with near-lab performance,” said Stachowiak.

Handhelds and Limitations

Thermo Fisher offers a number of different XRF analyzers, depending on the applications and materials an end user wants to measure. There are some limitations to any XRF device. “The lighter elements in the periodic table, starting with sodium and lighter, cannot be detected with a portable XRF device,” he explained. This includes carbon, a crucial component in many types of steel.

The Thermo Fisher Scientific Niton XL5 XRF analyzer is equipped with a 5W X-ray tube and a silicon drift detector. It can detect up to 81 elements non-destructively in real-time. Users may edit material libraries and download data via a USB connection. It weighs 2.8 lb with battery.

Performance depends on the detector. “There are two kinds of detectors in the industry. The silicon PIN-diode detector, which will detect titanium, atomic number 22 and heavier elements, and the silicon drift detector (SDD), which will detect key light elements, such as magnesium, aluminum, silicon, phosphorus and sulfur,” he said. Although OES can be used to analyze a wide range of alloys, it is a technique typically used for steels, due to its ability to analyze carbon.

However, OES is not as portable or convenient as a handheld XRF and often requires an operator to have a higher level of expertise. Mobile OES units are typically much larger and heavier, and the highest performing units are indeed large instruments used in a laboratory environment.

“To detect carbon and lighter elements, they also require sample preparation. You need to remove surface oxidation, oil, dirt or anything else that might contaminate the surface of the sample,” he said. OES does leave a small burn mark on the sample, making it a destructive test. Thermo Fisher also provides a range of such desktop or laboratory OES devices.

But that leaves wide open a large market for a portable XRF solution. There are many specialty alloys that do not require carbon analysis to determine the grade, such as nickel, cobalt or nickel/cobalt alloys, many grades of stainless steel, brass/copper alloys, titanium alloys or aluminum alloys. These are expensive, high strength alloys where misuse can cause risk of failure. Making sure they are correct is worth a test, especially if the test is convenient.

Convenience, Simple Training

Dr. Michael Hull, applications scientist for Olympus (Webster, TX) believes, along with others interviewed for this article, that their more sophisticated customers are seeking speed along with the convenience of a handheld XRF device. “They want a reading in two to five seconds, not 30 seconds,” he said. Today’s users are examining a large volume of product and speed is essential.

Handheld XRFs, such as the Olympus Vanta analyzer, can be safely used in a shop environment. Makers like Olympus are intent on making them easy to use with easy data transfer.

They also need a device that operators with little training can use. Today’s shop floor operators wear many hats and becoming an expert in X-ray spectroscopy might be asking a bit much. “The interface needs to have a very shallow learning curve,” he said. “They need to set up with ease and then lock it down so that the interface cannot be accidentally altered by lower-level users.” That means simple, intuitive and reliable.

Beyond that, data management is vital as well, especially where manufacturers are using it in high volume situations. “The user interface needs to follow the general consumer technology trends and look like an app on a smartphone with a touch screen,” he said. “It needs to be just as easy to use. The data needs to be transferred easily via wireless or a USB stick.”

The Vanta analyzer is Olympus’ flagship handheld XRF. Like many devices in its class, it features wireless LAN, Bluetooth and USB connections, as well as an 800 × 400 WVGA display. Vanta analyzers provide alloy composition chemistry and grade identification in as little as one to two seconds for materials, according to the company. The Vanta analyzer also collects down to magnesium in the periodic table.

Handheld XRFs are starting to migrate from shipping, receiving and warehousing to in-process manufacturing. “However, as we move into [Industry 4.0] or smart manufacturing, we have lots of customers that are looking for in-line or automated solutions,” explained Hull.

In response, Olympus offers the FOX-IQ in-line XRF analyzer that is primarily for metal rod, tube and bar material. The system is engineered to endure high levels of vibration, electromagnetic and acoustical noise, dust and moisture. It measures down to titanium elements and heavier on the periodic table. Designed to operate 24/7, the FOX-IQ XRF system performs fully automated, in-line analysis.

“It measures product that is relatively clean and consists of fixed shapes and sizes,” Hull said. “This is in situations where you already have equipment moving it down the assembly line. The FOX-IQ is relatively easy to integrate, with a probe head and shoebox-sized analyzer with an XRF source and detector in that shoebox. You bolt that on in-line with the existing rod tube bar analysis equipment.”

Other Tools, Other Applications

The advantages of an XRF device is clear to Mikko Järvikivi, product business development manager, HHXRF, LIBS and mobile OES for Hitachi High-Tech Analytical Science (Westford, MA). “An XRF leaves no mark, which means you can use it for finished goods and where an unblemished surface is needed,” he said. This ranges from medical implants to aircraft turbines. Hitachi also offers an XRF, the X-MET8000, which also measures down to magnesium.

According to Järvikivi, there is a growing need for all types of analysis, especially metal grades that contain nickel, chromium and molybdenum. “These materials are getting more expensive, so if you examine the specifications of a steel that requires 8-10% nickel, steel makers are providing steels on the low-end of the specifications,” he explained. Operating on the edge means it is easier to miss the recipe by accident, making the material unfit for its purpose.

But what about those applications where the limitations of XRF demand another solution, especially in measuring carbon? “The ultimate performance tool is optical emission spectroscopy (OES). We have several products in this range. These can examine difficult elements in steel, including carbon, hydrogen, phosphorus and sulfur. OES gives you by far the most accurate reading,” he said. “Carbon affects most properties of steel, especially weldability.”

Olympus offers the FOX-IQ in-line XRF analyzer, aimed at classifying rod, tube and bar metals material in a production setting. It is an automated analyzer designed to easily integrate into production systems.

However, using an OES to measure a sample requires some tradeoffs. OES requires an electrical spark to induce spectrometry, leaving a burn mark on the sample. OES systems are also heavier, though Hitachi High-Tech has responded with a human transportable unit that weighs about 30 lb.

Hitachi High-Tech also offers a handheld LIBS unit with its Vulcan units. “LIBS is very fast; you squeeze the trigger and you can identify a metal in one second,” said Järvikivi. While a laboratory grade LIBS unit is accurate enough to detect lighter elements, the handheld LIBS unit Hitachi offers can grade stainless steels, low-alloy steels, tool steels and nickel alloys, with an option for cobalt, copper, lead, tin, titanium, zinc, aluminum, and magnesium alloys. LIBS also leaves a small mark on the surface of the sample. “We think a handheld LIBS is ideal for 100% PMI of incoming material,” he said.

Coatings Analysis

Coatings analysis is just as important as bulk PMI since many engineered parts are coated with different materials to improve their performance. Hitachi High-Tech offers two technologies for coatings analysis. XRF analyzes coating thickness and composition. The other method, electro-magnetism, measures thickness, according to Matt Kreiner, product business development manager of coatings analysis for Hitachi High-Tech Analytical Science.

“XRF is used primarily to measure metal or metallic plating in the range of nanometers and micrometers applied to various substrates, including metals, plastics and ceramics,” he said. The electromagnetic thickness gages use either magnetic induction or eddy current techniques to measure organic coatings such as paint and resin, as well as anod-ized layers in the range of micrometers and millimeters on metal substrates.

“Coating a material can improve the part’s corrosion resistance, heat resistance, wear resistance or electrical contact,” said Stachowiak from Thermo Fisher. “A handheld XRF can save manufacturers a lot of money, for example, if they are applying too much or too little of a specified coating over their materials. Using too much of a coating increases material costs, where using too little could affect the desired performance or life of their products.”

Automotive manufacturing and the aerospace segment in particular are adding more coatings to their repertoire of materials, making non-destructive coatings analysis on the manufacturing line increasingly important. According to Stachowiak, Thermo Fisher’s handheld XRF devices are easily calibrated for a wide range of coatings applications.

While coatings may be thin, precisely controlling that thickness determines both cost and fitness for purpose, according to Robert Weber product manager for Fischer Technology (Windsor, CT). “Chrome plating, for example, needs to be thick enough to withstand wear but not too thick, both because you might be wasting material and also because it might not fit in its assembly,” he said. Many suppliers are now being required to certify the thickness and alloy content of their coatings, especially in automotive, according to Weber. This makes them responsible if anything is amiss.

The advantage in using a handheld XRF for coatings identification is that most coatings tend to be made of the heavier elements, he said. Fischer Technology’s XAN500 XRF device measures elements including titanium and heavier on the periodic table, according to Weber, which includes typical plating alloys containing chromium, nickel, copper and zinc.

“The fundamental parameter approach determines elements and calculates thicknesses,” he said. “The XAN500 does not need a preloaded library of materials.” This means the XAN500 measures a wider range of alloys and thicknesses and does not need a return-to-factory representative to create new library entries.

“An important coating for automotive and aerospace applications is zinc-nickel, a replacement for cadmium, once widely used but now recognized as toxic,” Weber said. He noted that Fischer Technology’s XAN500 handheld XRF can both detect the thickness as well as the precise elemental make-up of the coating, both of which manufacturers need to know. An additional advantage of using a handheld XRF is that it can measure larger parts that might not fit into the cabinet of benchtop XRF (or OES or LIBS) systems.

The XAN500 can be mounted in a measurement box and become a desktop unit, allowing precise, repeatable measurements on small parts like nuts and bolts. It can also be integrated into the control system of a production line for 100% inspection and monitoring, though Weber noted that this is not yet a common application of the benchtop adaptation. Nevertheless, he sees automation as the future, both in general and for PMI and coatings analysis in particular. “We are starting to mount our units onto six-axis robots for measurements in-process in some specialized industries,” he said.

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