Need of Metallurgical Studies in FSW

While considering Friction Stir Welding (FSW), most of the joints fabricated by using similar or dissimilar metals. It's easy to understand why to place such a preference for metallurgy. One of the significant benefits of friction stir welding is its ability to join dissimilar metals having different melting points. This process heavily depends on strong metallurgical knowledge to attain. 

There is a lot to learn about a metal by merely holding it in hand and analyzing it with the naked eyes. Still, there are ample below the surface that needs special examining equipment and knowledge to evaluate.

When should metallurgy employ?

In almost all cases, a researcher defines the stage at which a metallurgical examination needs to conduct. It explains the properties, behaviour and internal structure of the friction stir weldments because welding involves the heating and cooling of metals, and the heating and cooling of metals affect the mechanical properties of the resulting part. Moreover, metallurgical engineers can determine the cause of failure and make recommendations to prevent future reoccurrences.  As a result, the metallurgical lab works on the requirement of the researcher or metallurgical engineers who initially fabricate the friction stir weldment.

Understanding the importance of metallurgical studies in friction stir welding. Let us know how to prepare the specimen, how to conduct the metallurgical examinations, and what are all types of equipment and standards required for this process.

Metallographic specimen preparation or metallurgical sample preparation involves the following steps

Metallographic Specimen Preparation is a crucial step in performing reliable metallurgical testing. This type of testing often consists of evaluating the microstructure of materials through the use of optical magnification or scanning electron microscopy (SEM). It is essential that all microscopy sample preparation, including SEM sample preparation, be completed carefully and accurately for image clarity.

Selecting a representative sample of the material to accurately characterize the microstructure or the features of interest is a significant first step. For example, grain size measurements are performed on transverse sections, whereas general microstructure evaluations are performed on longitudinal sections. Therefore, the researcher needs to provide information about the orientation or the rolling direction of the test specimen before sample preparation begins.

Sectioning of the test sample is performed carefully to avoid altering or destroying the structure of the material. Thus, if an abrasive saw is used, it is vital to keep the sample cool with lubricant or coolant. However, no matter how carefully abrasive sawing or electric discharge machining is performed, a small amount of deformation occurs on the sample surface. This deformation must be removed during subsequent sample preparation steps.

After the specimen is sectioned to a convenient size, it is mounted in a plastic or epoxy material to facilitate handling during the grinding and polishing steps. Mounting media must be compatible with the sample in terms of hardness and abrasion resistance.

The next sample preparation step is grinding with a water-lubricated abrasive wheel to achieve a flat, smooth, and scratch-free surface. This step is required to remove the surface damage that occurred during sectioning. The grinding procedure includes the use of a series of progressively finer abrasive grits.

The polishing step in metallographic specimen preparation removes the last thin layer of the deformed metal for a smooth reflective surface. It leaves an adequately prepared sample ready for examination of the unetched characteristics, such as inclusion content or any porosity that may exist.

The final step that might be used is etching in an appropriate acidic or basic solution to bring out the microstructural details of the test sample. This step reveals features such as grain boundaries, twins, and second phase particles not seen in the unetched sample.

Macroscopic Examination

Macroscopic examination, also called Macro Test or Macro Examination, evaluate the quality and consistency of a test sample using only low or no magnification. Macro analysis of metals can be used to assess quality through the evaluation of a sample's macrostructural features, which may include grain flow, porosity, and cracks.

Test Methods / Standards

• ASME Sect. IX

• ASTM E340

• ASTM E381

Macro examination of specimens exposed to passivation, salt spray, case depth, and another testing, as well as the macroscopic examination of welds, are frequently performed. A weld cross-section examination will reveal internal discontinuities, weld profile, the extent of penetration, and more. The macrostructural properties of a weldment can be important in weld procedure qualification or welder qualification.

Microstructure Analysis 

During Microstructure Analysis of metals and alloys, a Microscopic Examination is conducted to study the microstructural features of the material under magnification. The properties of a material determine how well it will perform under a given application, and these properties are dependent on the structure of the material.

The team of highly qualified metallurgical engineers at Laboratory Testing Inc., in the Philadelphia, PA (USA) area, routinely perform a microscopic examination of metallic materials. Their microstructure analysis services range from pure determination of parameters such as grain size or coating thickness to full evaluation of abnormalities and failure mechanisms such as inclusions, segregation, and surface layers.

Test Methods / Standards 

• ASME Sect. IX

• ASTM A247

• ASTM A262

• ASTM E112

• ASTM E1268

• ASTM E2142

• ASTM E3

• ASTM E407

• ASTM E45

• ASTM E562

• ASTM E883

• ASTM E92

Microhardness Testing 

Microhardness Testing is a method of determining a material's hardness or resistance to penetration when test samples are very small or thin, or when small regions in a composite sample or plating are to be measured. It can provide precise and detailed information about surface features of materials that have a fine microstructure, are multi-phase, non-homogeneous, or prone to cracking.

The microhardness test can measure surface to core hardness on carburized or case-hardened parts (case depths). This also helps to find the surface conditions such as grinding burns, carburization or decarburization.

Test Methods / Standards

• AMS 4081

• AMS 4083

• ASME Sect. IX

• ASTM B578

• ASTM E1077

• ASTM E384

Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy (SEM) is a test process that scans a sample with an electron beam to produce a magnified image for analysis. The method is also known as SEM analysis and SEM microscopy and is used very effectively in microanalysis and failure analysis of solid inorganic materials. Electron microscopy is performed at high magnifications, generates high-resolution images, and precisely measures minimal features and objects.

The signals generated during SEM analysis produce a two-dimensional image and reveal information about the sample, including external morphology (texture), chemical composition, when used with the EDS feature, and orientation of materials making up the sample.

The EDS component of the system is applied in conjunction with SEM analysis to determine elements in or on the surface of the sample for qualitative information. It also measures elemental composition for semi-quantitative results and identifies foreign substances that are not organic in nature and coatings on metal.

The Takeaway

By now, you likely understand the power of metallurgy and the value it can bring to a friction stir welding projects. 

If you wish to know more about Friction Stir Welding or requires consulting, feel free to contact.