Quality PMI Testing: 5 Tips to Ensure Trusted, Consistent Results

If only life had 5 tips for quality results like PMI testing does; we would all be killing it!

Fortunately, for us metal heads, there are 5 clear-cut solutions to producing quality PMI testing results.

Positive Material Identification (PMI) testing is the analysis of materials to determine the chemical composition of a metal or alloy during alloy manufacturing, fabrication, installation or in-process inspection.

Knowing the exact composition and grade of an alloy enables suppliers, plant workers, and other responsible parties in the chain of custody of components to match alloy specifications that are chosen for their specific properties such as heat resistance, corrosion resistance, durability, etc. 

PMI is a great option for confirming the alloy ID of large batches of material, testing finished components, re-certifying materials, and evaluating materials that cannot be destroyed, or shipped to a lab.  

The right alloy with the right properties is often all that stands between safe, efficient operation and disaster.

Today, I will give you some tips on how to ensure consistent, quality results from your PMI testing equipment on a consistent basis. This approach will give you a level of confidence in your results that will also build trust with your colleagues and your customers.

Psst: This post is based on a podcast with co-host Chris Carolan. To hear this episode (and more like it), subscribe to The Manufacturing Show on Apple Podcasts, Spotify, or wherever you listen to podcasts.

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The main tip I can give to any PMI testing technician is to focus on sample preparation. We don't see it done nearly enough.

PMI Testing Tip 1: Sample Prep is Rarely Optional

All three PMI testing techniques, LIBS, OES, and XRF, are surface measurements.

That means that the quality of the surface you are testing directly impacts the quality of your test and the result you're going to get. Samples must be clean with the base metal showing and free of contamination or coating.

You may be surprised at how many samples are actually not ready to shoot as is. Just because you're seeing a result on your screen doesn't mean you're getting the right result.

Taking the time to prepare your sample properly sets the stage for your results. Shortcuts at this stage will only make the end results inconsistent and less reliable. Making sure your sample is clean and flat is the first step. 

Potential surface issues include:

• dirt 
• rust 
• corrosion 
• paint
• coatings
• moisture
• other build-up 

Sounds easy enough to handle right? I will breakdown the sample preparation into three categories:

1. Wiping
2. Grinding
3. Cutting

Wiping

A simple wiping technique will remove water, dirt, and dust. Be sure to use a clean towel so you do not add impurities, moisture, or oils to the sample surface.

Grinding

For coated, oxidized, anodized, or plated samples, the most common sample prep method for metals is grinding. 

Grinding removes the entire surface layer to expose fresh base metal and can also help with cast items. 

It’s important to choose the correct grinding materials.

Some tools that can get the job done:

• A Dremel
• Angle grinder
• Belt sander
• Disc sander
• Even sandpaper can be effective

Grinding material can contaminate your sample. Make sure you are using the correct type. 

For example, tungsten carbide, zirconium oxide, and silicon-based materials can leave a residue that contaminates the sample and changes the results by inadvertently adding unwanted impurities. 

This especially impacts aluminum alloys with low silicon and zirconium. 

So what is the recommended grinding medium? 

Generally aluminum oxide will do best. We recommend a grit between 60 and 120 for proper grinding. Make sure you have a flat spot with a dry surface. Take care not to grind at an angle as that can create an uneven testing surface that will produce inconsistent results.

Keep grinding until you see the shiny clean metal, then wipe off the grinding residue. 

Now take your shot at the clean spot. It won't disappoint!

Let’s talk patinas

Patinas on brass and aluminum can also pose a problem. This is an oxide layer that can quickly form on the surface of these materials. It will be white and dusty on aluminum, and will actually look darker on brass.  

Just like any other surface layer you are not expecting, patinas can cause problems. 

• non-repeatable results
• false-positive 
• or no matches at all. 

Cast metal: if there is a rind, you must grind! 

Lastly, cast metal has its own set of issues. You've got to grind away.

As cast metal cools, the outside layer cools at a faster rate than the inside layer. This can create a skin or rind on the surface. 

Imagine how an orange peel covers and contains the juicy fruit at the center. The peel and the fruit itself have very different compositions. If you wanted to taste the sweetness of the fruit, you wouldn’t have an accurate flavor if you only bit the peel.

Just like an orange peel, the rind or skin on a surface can have very different chemistry then the bulk of the piece you are testing.

Be sure to remove the outer layer to give yourself the confidence you are getting the most accurate result.

Cutting

Have an oddly shaped sample to test? It may be time to do some cutting. 

While yes, it takes longer than grinding, it properly exposes the base metal, which is key to assuring you have the best possible surface to test. 

Cutters can leave behind contaminants left on the cutter from previous samples, so only cut when needed.

Confidence in the quality of your result will justify the time spent prepping the sample.

PMI Testing Tip #2: Use Direct Contact

Now that you have properly prepared a sample that is clean and ready for testing, direct contact is the next step in getting the best result.

This is even more relevant when it comes to LIBS or OES technologies as they need complete contact to vaporize the metal and read the light coming back properly.

XRF may be more forgiving, in that if you don't have 100% complete contact, it may still be able to irradiate the sample and see some energy coming back. But it's definitely a best practice to keep complete contact no matter which technology you're using.

This assures your analyzer is "seeing" the most material to provide the best reading of the light or energy that is coming back into the spectrometer.

PMI Testing Tip #3 (XRF Only): Keep testing times consistent

The next step is specifically for XRF. 

Most XRF units allow you to determine the time that you take the test manually. 

So for this first shot, you could hold the trigger for five seconds. Then for the next one, you could be holding it for 7 seconds and then, for example,15 seconds and so on.

This creates an additional variable that may affect the consistency of your results. It is generally best practice to pick one time and stick with it for all your tests on that type of alloy. 

When times can vary

An example of when you may want to choose different lengths of testing times is if you are testing, aluminum versus a steel.

The aluminum can be harder for XRF to read and it might need longer to get a good result, so make sure to familiarize yourself with how each metal reads and adjust accordingly.

If you are using the same length of testing time with the same material, you are going to get more consistent, higher quality results. 

PMI Testing Tip #4: Test Daily Check Samples

Daily check samples are always recommended when it comes to confirming the health and performance of your analyzer. 

We generally recommend once at the beginning of the day and once at the end as well if you're using the instruments daily. 

You basically have to ask yourself, how many results are you willing to throw away if that next check sample doesn't read right?

If that next check sample doesn't read right, how do you know that the analyzer was working properly through all the samples before it? 

You don't.

It's important to use your daily check samples properly to prevent this from happening.

A common misconception here is that if you are using your 316 stainless check sample or your aluminum 7075 then that will be enough to make sure all your results from all of your metals will be okay. 

Not true.

If you are not testing either steel or aluminum, in those cases, the only thing that those samples are going to reliably tell you is that the analyzer is working properly for those particular materials. 

If you test with those samples and then go on to test copper for example, if something has happened with the copper calibration, you will not be able to tell unless you're also checking with a copper standard. 

These analyzers have different calibrations for different materials, so one calibration can be good while others may not read right.

So whenever possible, it is important to match like with like when confirming the health of your analyzer. This will give you the confidence that you are getting the absolute best performance and result.

PMI Testing Tip #5: Keep Your Analyzer Healthy

The last tip I want to go over is preventative maintenance and regular checkups for your PMI testing equipment.

Handheld XRF Maintenance

Are you cleaning or changing the window when you see that it's dirty?

What about when it's been ripped or something else has changed that can affect your results?

Depending on the age of the analyzer, many manufacturers recommend or even require having an annual certification or biennial recertification completed. 

This should be based on your individual quality program as well as the manufacturer specifications. If you are using daily check samples on a regular basis and you see that the results are consistently good and reliable, you don't want to get into the habit of over-certifying the analyzer.

That being said, if your check samples aren't reading right, and you can't pinpoint why, it is time to consider recertification.

Handheld LIBS Maintenance

When it comes to LIBS it is all about keeping that window clean. 

Depending on the material you are working with and the cleanliness of your environment, we recommend taking a cleaning swab and wiping down the window every 25 to 100 shots. For most analyzers, this process takes just a few seconds to do it right.

Because of the way LIBS analyzers are built and calibrated, the calibrations are very dependable and can last a long time. This technology does not "drift" like XRF or spark OES.

Generally, there is no recommended recertification or recalibration schedule for LIBS analyzers, but again, this really depends on your quality control program.

With proper checks and standard use LIBS, analyzers can actually go years without recertification or recalibration.

Portable Spark OES Maintenance

For spark OES, a few basic rules to always keep in mind:

• everything must be clean
• keep your electrode sharp 
• argon gas shall be pure 

If your spark OES is not giving you consistent results, it is likely due to those rules being broken.

You should always give your spectrometer some time to adjust to new environments.

For example, when you leave a climate-controlled shop and head out into the field, keep the temperamental nature of these analyzers in mind. If the 2 environments are very different, you may just need to take the time for it to adjust to the new surroundings to get a reliable result.

The temperamental nature of these analyzers also leads to required annual recalibration and/or recertification. 

Unless you have an OES super user that understands the technology at a very high level, it's best to have a third party take a look. 

LIBS laser OES Maintenance

Luckily, LIBS laser OES avoids most of these issues. 

The stability, lack of moving parts, and gas as the only consumable, LIBS laser OES analyzers are virtually maintenance free.

LIBS laser OES also provides the ability to proactively monitor your components and provide true preventative maintenance. 

You can also make updates to the calibration programs and software through the cloud without having to bring someone on-site or sending the instrument to the factory or Depot.

To recap, the 5 tips to ensure trusted results from every PMI test are:

1. Sample preparation
2. Direct contact
3. Consistent test times for XRF
4. Daily check samples
5. Maintain the analyzer

Following these guidelines will ensure the best results. Have confidence in your testing and until next time, never stop testing your metal.