Introduction and Methods of Metallographic Examination of MIM Parts

 

Date：[2024/6/18]
 

1. Preparation of MIM parts for inspection with a display mirror

(1) Purpose: The purpose of this guideline is to establish the necessary operating procedures for preparing MIM component specimens for microscopic inspection.

(2) Parts to be inspected:

The metallographic examination of the raw parts is useful for detecting defects (cracks, bubbles, shrinkage, porosity, welding lines, etc.) in the sample. The uniformity of the powder binder mixture used in injection molding can also be checked based on the geometric shape of the parts and the type of powder used. If necessary, if the powder used is uneven, different powders (performance or shape) can be reconfigured.

Brown parts are very fragile due to the removal of adhesive. If there are defects in the parts, they are more likely to rupture along these defects. By using this method, the location of defects can be determined, and the inspection of the fracture section can provide information on the properties of defects (cracks, bubbles, etc.). Due to its rough surface, high magnification is not required.

If the binder of pre sintered parts is very soft or fragile, it will be difficult to prepare the green parts smoothly. Therefore, the parts must be removed and sintered to form a sintering neck between the particles. The sound of the parts is the same as that of metal, but their size is still very close to the size of the raw parts (shrinkage of 2% -5%). The defects in the raw parts are still very similar to those in the pre sintered parts and are relatively easy to inspect.

The metallographic examination of sintered parts is mostly used to check porosity, smoothness, and microstructure. For the metallurgical properties of MIM parts, metallographic examination is important.

(3) Sample preparation

I) Scanning Electron Microscopy (SEM)

For SEM inspection, it is often not necessary to prepare MIM parts. In electron microscopy, whole or broken parts can be used, and inspection should be conducted on the outer surface or broken section. If chemical analysis is required, especially for semi quantitative analysis, the sample should be prepared like an optical microscope and the analysis should be conducted on a polished surface.

2) Optical metallographic examination

A. Overview

Due to the fact that most parts have thin walls, metallographic examination is almost impossible without embedding the specimen in an appropriate polymer resin. The section to be inspected should preferably be the most important section of the specimen. This cross-section may be:

·Defective areas;

·Welding line;

·Segregation site;

·Any surface with expected special information.

If there is no special need for testing, the sample can be cut in the middle or at any convenient location.

B. Preparation of raw parts

The preparation of raw parts may vary greatly, depending on the properties and mechanical properties of the binder. Otherwise, the portion of the specimen to be inspected should be embedded in a cold fixed resin, preferably using a resin specifically designed for metallographic examination. After the polymerization reaction is completed, cut with a high-speed disc saw.

The following conditions were found to be valid:

High strength steel for circular saw blades

Diameter: 63 mm

Thickness: 0.3 mm

Number of teeth: 128

Cutting speed (circumference): 600 m/min

Linear speed: 0.3 mm/min

The raw sample of the parts cannot be polished. Before polishing to a smooth surface, the special agent may be removed and damaged, and the visible metal content is much lower than the actual content. The inspection should be conducted directly on the cut surface. The residual roughness hinders the use of high magnification, but it can be used to inspect and analyze the shape and distribution of the powder. During cutting, some powder particles may be removed from the polymer matrix, but the pores are clearly black and can be used for image analysis.

C. Preparation of pre sintered and sintered parts

a. Sample cutting

To prevent compression, deformation, or damage to the specimen during cutting, special care should be taken in fixing the parts. You can use a grinding wheel cutting machine, using a thin grinding wheel or a grinding wheel with diamond inlaid on the rim for cutting. When cutting, it is necessary to cool thoroughly to prevent changes and damage to the structure due to overheating.

Saws can also be used for cutting. Using a standard hacksaw will leave burrs and rough and distorted surfaces, so in future grinding, remove a few tenths of a millimeter. Similar to the fine saw used by jewelers, it may be suitable for cutting MIM specimens.

If the part is smaller than the sample mold, it may not need to be cut. If there is no specific section to inspect, the entire part can be embedded; However, this requires thorough grinding and processing.

b. Inlay sample

The most commonly used mounting method for metallographic specimens is hot pressing mounting. If the hardness of thermoplastic and thermosetting resins is high enough and the shrinkage is small, both can be used for specimen embedding. However, the parts are raw and pre sintered

The parts and some very fragile parts require cold setting samples. Curing the resin in a vacuum or pressurized atmosphere can improve the permeability of pre sintered specimens, thereby strengthening MIM parts. Additionally, due to low or zero porosity, it is easier to polish.

If the parts need to be inspected using both optical and electron microscopy methods, it is suitable to use conductive resins (such as those containing carbon powder, silver powder, or copper powder). This type of conductive inlay is also used for electrolytic etching of specimens.

The position of the sample in the resin must be completely fixed and recorded. If the resin used for sample setting is transparent, the position can be verified and, if necessary, corrected before complete polymerization. If the resin is opaque, the most important thing is to ensure that the position of the specimen is correct and that it remains in the correct position before pouring the resin. For this purpose, special stabilizers can be used.

c. Grinding

In order to eliminate burrs and structural changes caused by cutting, a layer of considerable thickness on the cutting surface should be removed by grinding before final polishing. Experience can be helpful in determining how thick to remove. After grinding in the world, inspecting the surface at low magnification is sometimes useful for identifying the results. It can also be ground all the way to specific areas or defects of the sample.

Grinding is a key task in sample preparation. Improper grinding may result in some pores being sealed due to plastic deformation or filled with debris from grinding.

After each operation, especially after grinding, the sample should be thoroughly cleaned. After cleaning with tap water, it is appropriate to use isopropyl alcohol for ultrasonic cleaning.

d. Final polishing

Optical microscope metallography requires a flat mirror to surface polished surface. Each metallographic sample needs to be polished and finally ground with diamond powder paste with a particle size of 6.3 μ m. To prevent contamination of the disc, special care should be taken. If the abrasive particles are small, they may remain in the pores and fall on the next disc. If the particles are larger than the pores, they may block the pores and scratch the surface of the test when removed. It is important to thoroughly clean the specimen between each operation during grinding. Electrolytic polishing cannot be used as it will affect the edges of the pores.

(4) Inspection

1) Optical microscope

Firstly, inspect the unexposed sample at low magnification to select the surface for further inspection. The sample should first be tested for pore size. It should be pointed out the uniformity of pore size and pore distribution in the matrix. Then, the cleanliness of the sample should be checked. Classification of pores and inclusions will be specified in other materials.

The inspection was conducted at a magnification of 100 times. Especially when it is suspected that the preparation of metallographic specimens is incorrect, in order to identify between pores and inclusions and to observe pores better, higher magnification can be used. To determine if the pore size is correct, diamond powder can also be used for polishing separately.

For different purposes, different etching methods can be used, such as:

·Display microstructure;

·Identify specific phases;

·Identify inclusions;

·Verify that the pores do not contain burrs or unrelated particles.

Etching is carried out using chemical or electrochemical methods and chemical reagents, which is the same as conventional metallographic examination. Table 6 presents a list of these reagents.

Table 6 Most commonly used etchants

2) Scanning electron microscopy

SEM is very useful as an auxiliary inspection tool. It can provide supplementary information on the surface of parts and is an important tool for determining the composition of inclusions. It can be considered for identifying corrosion by-products and conducting chemical analysis of materials.

3) Image analysis

Especially for describing pores (size measurement, number of pores per unit surface, size distribution, etc.), image analysis software is highly recommended. Specify its application in the data related to pore classification.

2. Microscopic Analysis of MIM Parts

(1) Overview

For the purpose of metallographic examination, there are some differences between the preparation of specimens produced by injection molding (MIM) and the commonly used metallographic specimens. See 6. l above.

(2) Optical microscope inspection

1) Equipment

A high-quality optical microscope capable of magnifying approximately x 50, x I00, x 200, and x 500 times. It requires a microscope ruler for measuring dimensions. In order to manually measure dimensions and record, photographic equipment is required. When using image analysis, digital images from the camera can be obtained and processed using specialized software in the computer.

2) Porosity

Firstly, check the porosity on the unexposed sample; If necessary, then perform another test on the etched sample.

A. Testing of pore distribution

Verify pore distribution at low magnification (x 50). If the porosity is quite uniform, it can be inspected and measured on the surface. If the density of pores in certain parts of the surface is different, further testing should be conducted separately in each representative part. The general signs of pore distribution should be recorded (such as at 0 Lower porosity near the surface with a thickness of 6 mm.

B. Characterization of porosity

The characterization of porosity should be carried out at x 100 times. The surface to be inspected should be selected with typical representativeness and good optical quality (smoothness, polishing state, no scratches, etc.). To ensure that there are no closed pores during sample preparation, it may be beneficial to inspect the same area after supplementary polishing.

The characterization of porosity should include the following data:

The pore shape is evaluated based on the shape of most pores. Or it can be referred to as "circular or spherical pores" or "irregular pores". When the pores are irregularly shaped, specific descriptions of the actual pore shape can be added as needed.

The average size of pores - The average pore diameter is obtained by analyzing images or measuring and calculating the average size of a sufficient number of visible pores (>10% of the total).

The abundance of pores - The abundance of a pore is determined by the ratio of the number of pores in the measured area to the total area measured. When using image analysis, this value is generally called "count/area". Additionally, dividing the area of pores by the measured area may be important.

a) The research area; b) Calculate the smaller area; c) Non rescue pore

C. Examples of manual measurement and calculation.

If the area being studied, the actual method of calculation and measurement is to divide the surface area into several smaller areas, such as in (b), and then calculate the number of pores in one or several areas. When the pore is a part of two different areas, only the lower right part of the area should be calculated. In (c), the average diameter of non spherical pores is determined by the average value between the minimum size (a) and the maximum size (b).

D. Verification of porosity

To display the microstructure of the material, it is necessary to etch the sample. As before, when testing the sample with the same magnification, it should display the same shape of pores, size, and deformation.

3) Cleanliness

Cleanliness is characterized by the performance, quantity, and range of inclusions. Inclusions are essentially metallic or non-metallic impurities that are different in composition from the matrix material and are unrelated. Microscopes often cannot distinguish between inclusions and pores. Sometimes, a magnification higher than x 500 times or higher can indicate whether the pores are true pores or inclusions. The first measure that can be taken in case of suspicion is to thoroughly clean the sample with ethanol and ultrasound stirring. Using specialized reagents and applying them to inclusions will result in a different color from the pores and matrix.

The most suitable method to determine inclusions is to use scanning electron microscopy (SEM). This guideline specifically discusses the application of SEM in the following section.

After determining the inclusions, characterize them using the same method as for pores. Sometimes, due to the small differences in shape, color, and appearance between pores and inclusions, it is difficult to analyze them using images. If the number of inclusions is not too large, manually selecting inclusions may be a feasible solution to this problem.

4) Microscopic organization

The microstructure inspection of MIM parts can only be carried out using sintered parts. The program is the same as materials used for other sources, with the only difference being that most MIM materials have small and uniform porosity (see Figure 22).

5) Etching

The chemical reagents used for etching are shown in Table 7 and can be selected from them. Be particularly careful because it can erode the porosity of the material. If the liquid remains in the pores, it may escape during inspection, affecting the quality of the image and even corroding the equipment, especially on flipped microscopes.

6) Inspection

In order to verify the porosity (see above), after the first inspection at x 100 times, in order to identify different phases and for metallographic observation, an appropriate magnification should be selected.

7) Measuring grain size

If necessary, the grain size can be determined. For MIM materials, the procedure described in standard ISO 643 is also applicable.

8) A. Ferrite

Special attention should be paid to s-ferrite. This phase is widely used in MIM processes for stainless steel (such as 316L). When stainless steel is heated to a sufficiently high temperature, it will appear at grain boundaries. This color has a very light light blue color. The presence of ferrite should be reported, and the relative abundance of this phase can be determined.

9) Defects

A. A defect is an accident that may affect the appearance, shape, or performance of a part. This includes defects in injection molding (welding lines, incomplete filling, sinking, etc.), material heterogeneity, voids, and cracks. Holes are much larger than pores, and some defects with longer dimensions greater than 100 μ m are systematic or accidental. The system is much easier to inspect and identify than occasional defects.

B. The inspection and characterization of defects on the surface of a green or sintered part can be visually observed. Finally, a magnifying glass or microscope can be used for inspection. According to the nature and size of defects, several non-destructive testing methods can be used for inspection. Both perspective testing and magnetic testing methods are effective for imaging surface cracks in magnetic materials.

When defects are located inside, it is difficult to detect them unless there is suspicion of defects or internal defects causing external deformation of the appearance. If the internal voids and cracks are large enough, they can be located using X-ray or ultrasonic depth automatic recorders. Minor defects smaller than 1 mm can be detected using X-ray microscopy tomography.

For defects in green and brown parts, destructive testing can be conducted by breaking or cutting the parts. Inspect the damaged and cut sections to obtain information on the location of defects. Defects can also be inspected under a microscope. Unless the location of the defect is inferred and the size of the defect is large or elongated within zero, it is unreliable for the specimen to fracture through the defect.

(3) Scanning Electron Microscopy (SEM)

1) Equipment

SEM is a very useful equipment for inspecting materials. It uses an electron beam to excite the material being tested and generates a video image of the surface using the secondary electrons emitted by the material. Due to the depth of the focal length, by zooming in, it is possible to accurately determine the contour of this image and the indicated surface. The typical magnification of SEM is from x 20 to x20000 times.

By using an energy dispersive spectrometer (EDS) attached to SEM and utilizing the energy of X-rays emitted by materials, the composition of local materials can be analyzed.

2) Inspection and analysis

In order to induce the discharge of charges using an electron beam, the sample tested by SEM must be conductive. If the sample itself cannot conduct electricity (polymer from raw parts or embedded samples), it must be metallized before inspection. A thin vacuum coating of carbon or gold is commonly used.