Why Is the Spark Plug Bushing Made of Brass Cracked?

When a gas engine was tested for cylinder head tightness, water leakage occurred, and cracks were found in the spark plug bushing after disassembly and inspection.

The bushing material is cast brass ZCuZn38 (H62), which is a nonferrous metal alloy widely used in industry.

The process flow is: casting copper bar → hot pressing → machining → assembly and test.

1. Test method

OBLF direct reading spectrometer was used to analyze the chemical composition of the cracked bushing;

The Zeiss Axio metallographic microscope is used for metallographic examination, and the sample is cut by wire cutting.

The Zeiss EVO18 scanning electron microscope equipped with an X-ray energy spectrometer was used to conduct microscopic observation and micro area composition analysis of the cracks.

2. Test results

2.1 Chemical composition test

Use direct reading spectrum to detect the chemical composition of the bushing.

See Table 1 for the results.

Comply with the requirements of GB/T 1176-2013 Cast Copper and Copper Alloys.

Table 1 Chemical composition (mass fraction) of bushing (%)

elementCuZnAlPSnSbFeConclusion
Prototype61.538.150.078<0.0010.036<0.0010.124qualified
GB/T1176—201360.0~63.0rest<0.15

2.2 Macro observation

There are two cracks in the bushing, which extend downward from the bushing shoulder in the axial direction.

The cracks are straight and nearly parallel, and there is green sealant left at the shoulder chamfer.

The crack extends to the inside of the shoulder and gradually narrows, as shown in Fig. 1, indicating that the crack originated at the outer wall of the shoulder and extended downward and inward.

Why Is the Spark Plug Bushing Made of Brass Cracked? 1

Fig. 1 Macro appearance of bushing cracks

2.3 Micro observation

Through electron microscope scanning, it can be seen that there is a layer of flocs near the outer surface of the bushing, as shown in Fig. 2a and Fig. 2b.

And it can be seen that the whole crack fracture surface shows brittle fracture: crystal sugar like intergranular fracture, a small amount of transgranular fracture, and there are corrosion products and small corrosion pits at the grain boundary.

The corrosion products mainly contain O, Cu, Zn, Al, as shown in Fig. 2c and Fig. 2d.

The parabola dimple of fresh artificially torn fracture is broken, and the dimple is clear and clean, with normal fracture morphology, as shown in Fig. 2e and Fig. 2f.

Laminated strips along the axial direction can be seen on the original fracture surface.

Why Is the Spark Plug Bushing Made of Brass Cracked? 2

Fig. 2 SEM of bushing fracture

Grind, polish and corrode along the axial direction, and observe the crack and metallographic structure, as shown in Fig. 3.

The crack is irregular and continuous, with forks and sharp ends in the form of tree branches, which is consistent with the typical characteristics of stress corrosion crack.

Therefore, we preliminarily determined that the crack is a stress corrosion crack.

The metallographic structure is α phase+a small amount of pointy β phase, and there are obvious slip lines in the structure, and the banded structure of the cracked bushing is obvious.

Why Is the Spark Plug Bushing Made of Brass Cracked? 3

Fig. 3 Metallographic Structure of Cracks

2.4 Finite element analysis

The bushing is an interference fit with an interference amount of 0.069~0.100mm.

The bushing assembly stress (without spark plug) is analyzed by finite element method.

Fig. 4 shows the first principal stress cloud diagram of the bushing configuration.

The bright area is the tensile stress area.

It can be seen that the tensile stress at the outer wall and chamfer outer wall above the bushing shoulder and the inner surface of the thin neck is large, which is consistent with the actual crack origin position.

Although the tensile stress on the inner surface of the bushing thin neck is large, there is no sealant and no stress corrosion condition.

Why Is the Spark Plug Bushing Made of Brass Cracked? 4

Fig. 4 Cloud Chart of the First Principal Stress in Lining Configuration

3. Conclusion and Analysis

After the bushing blank is formed by hot pressing at the process temperature of 650~800 ℃, it is placed in air and naturally cooled to room temperature.

According to the data, all brass have a brittle zone between 200~700 ℃, and the hot pressing temperature should not be lower than 700 ℃.

The lower limit temperature is low, and the material tends to crack;

The difficulty of blank forming increases, and the residual stress is large.

In the process of machining, the bushing is frequently contacted with the tool and subjected to force, and the size changes, which will inevitably produce a certain amount of residual stress;

The cutting fluid used in the finishing of CNC machine tools contains additives such as S and halogens, which makes the bushing in a humid and corrosive environment for a certain period of time.

There is an interference fit between the bushing and the cylinder head.

After the pressure assembly, a large tensile stress is generated on the outer wall above the bushing shoulder and the outer wall of the chamfer;

The sealant base material coated on the shoulder of the bushing during assembly is methacrylate, which contains amino catalyst, providing a weak corrosion working environment for the bushing.

When the cylinder head is subjected to the hydrostatic seal test, the outer wall of the bushing shoulder is stressed, and cracks occur and expand rapidly;

Although the tensile stress on the inner surface of the bushing thin neck is large, it does not contact the sealant, and the stress corrosion condition is poor.

4. Conclusion

1) The mechanism of liner crack is stress corrosion intergranular brittle cracking.

2) The cracks of the bushing occurred intensively in a certain month, and there was no batch crack failure before.

Through the above analysis, it was determined that there was a batch quality problem with the bushing.

The residual stress during the processing of the bushing was too large, which led to the stress corrosion cracking of the bushing under the additional tensile stress after assembly and the weak corrosion environment of sealant (or cutting fluid during processing).

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