Quality control is a challenge in all industries, but none more so than in metal fabrication. How do you evaluate whether a metal product is of good quality without damaging or destroying it in the process? There are a number of possible solutions to the problem, with the following four techniques the most common.
Magnetic particle inspection (MPI)
MPI only works on metals that are susceptible to magnetism, such as iron, steel, nickel, cobalt, neodymium and certain alloys. The item is exposed to a strong field or an electric current until it is lightly magnetised, at which point iron filings or a similar material suspended in a liquid are brushed onto the surface. Cracks, fissures or irregularities left by imperfect surface treatments tend to exhibit greater magnetic activity than intact areas, so these become highlighted by the liquid iron solution. The drawback of this method is that it requires specialist equipment and training to properly apply the liquid and interpret the results.
As in medical radiography, you can use X-rays to generate a photograph-like shadow of the item’s internal structure, highlighting irregularities in its density. Unfortunately, this method often struggles on objects that have intentionally irregular surfaces and it takes experience to interpret the results.
Once again, the principle is similar to that used in medicine, with the soundwaves tuned in frequency and intensity to match the substrate. An ultrasonic testing device generates sound in pulses and can detect changes in thickness and density caused either by defects or corrosive deposits, such as inside pipes or drums. Metal products readily lend themselves to ultrasound techniques, as do concrete and some composites, but the drawback is the expertise required to generate and interpret the appropriate signals. Surface irregularities are also challenging for this method.
This is one of the more traditional methods. In its modern form, two liquids are applied. The first is designed to penetrate and quickly find its way into fissures and cracks, while the second is a developer that makes the variations in penetration easy to see. Penetrant testing requires no complicated machinery to apply and relatively little training to interpret, so it is often the cheapest method. The drawback is that the defect must be accessible from the surface.
Flawless surfaces are not always the objective. The aim of many post-production operations, especially on metal items, is to produce surfaces that will accept useful subsequent treatments, such as colourants or PTFE coatings from specialists such as https://www.poeton.co.uk/standard-treatments/. Non-destructing methods are often vital to check that anodising or another procedure has completed successfully before proceeding to subsequent stages or to assembly. Competent testing reduces wastage and helps minimise per item overheads.
Materials, treatments and testing should all be aspects of a single holistic production strategy; however, choosing the right combination of methods, setting the right parameters and interpreting the test results to make sure all is going to plan is a complex process. Manufacturers should ideally consult testing specialists during their product design.