During a test of the cylinder head tightness of a gas engine, water leaked, and after disassembling and inspecting the engine, cracks were found in the spark plug bushing. The material of the bushing was determined to be cast brass ZCuZn38 (H62), which is a common non-ferrous metal alloy used in various industries.
The process flow is: casting copper bar → hot pressing → machining → assembly and test.
1. Test method
The chemical composition of the cracked bushing was analyzed using an OBLF direct reading spectrometer. The bushing was also examined metallographically with a Zeiss Axio metallographic microscope, with the sample being cut using wire cutting. Microscopic observation and micro-area composition analysis of the cracks were conducted using a Zeiss EVO18 scanning electron microscope equipped with an X-ray energy spectrometer.
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 (%)
2.2 Macro observation
There are two cracks present in the bushing, which run downwards from the shoulder in the axial direction. The cracks are straight and run nearly parallel to each other. Additionally, there is evidence of green sealant at the chamfer of the shoulder. As depicted in Fig. 1, the crack extends into the interior of the shoulder and gradually becomes narrower, indicating that it started at the outer wall of the shoulder before extending downwards and inwards.
Fig. 1 Macro appearance of bushing cracks
2.3 Micro observation
An electron microscope scan reveals the presence of a layer of flocs near the outer surface of the bushing, as depicted in Fig. 2a and Fig. 2b. The entire crack fracture surface exhibits a brittle fracture, including intergranular fractures that resemble crystal sugar, some transgranular fractures, and corrosion products and small corrosion pits at the grain boundary. The corrosion products mainly consist of O, Cu, Zn, and Al, as shown in Fig. 2c and Fig. 2d.
The fresh artificial tear fracture exhibits a clear and clean parabolic dimple, with a normal fracture morphology, as shown in Fig. 2e and Fig. 2f. Additionally, laminated strips along the axial direction are visible on the original fracture surface.
Fig. 2 SEM of bushing fracture
The grind, polish, and corrosion process was carried out along the axial direction, and the crack and metallographic structure were observed, as shown in Fig. 3. The crack is irregular and continuous, with branches and sharp ends resembling a tree, which is consistent with the typical characteristics of a stress corrosion crack.
Based on these observations, we have made a preliminary determination that the crack is indeed a stress corrosion crack. The metallographic structure is composed of the α phase and a small amount of the pointy β phase. There are also prominent slip lines present in the structure, and the banded structure of the cracked bushing is clearly visible.
Fig. 3 Metallographic Structure of Cracks
2.4 Finite element analysis
The bushing is designed with an interference fit, with an interference amount ranging from 0.069mm to 0.100mm. The assembly stress of the bushing (without a spark plug) was analyzed using finite element method.
Fig. 4 displays the first principal stress cloud diagram of the bushing configuration. The bright areas represent regions of tensile stress. It can be seen that the tensile stress is high at the outer wall and the chamfer outer wall above the bushing shoulder, as well as at the inner surface of the thin neck, which is consistent with the actual position of the crack origin.
Despite the high tensile stress present on the inner surface of the bushing thin neck, there is no sealant and no conditions for stress corrosion.
Fig. 4 Cloud Chart of the First Principal Stress in Lining Configuration
3. Conclusion and Analysis
The bushing blank is formed through hot pressing at a process temperature of 650-800°C. After the pressing, the blank is left to cool to room temperature in air.
According to data, all brass materials exhibit a brittle zone between 200-700°C, and the hot pressing temperature should not be below 700°C. A lower temperature limit increases the risk of cracking, increases the difficulty of forming the blank, and leads to higher residual stress.
During the machining process, the bushing is subjected to repeated contact with the tool and forces, leading to changes in size and the creation of residual stress. Additionally, the finishing process performed on the CNC machine tool uses cutting fluid that contains additives such as S and halogens, exposing the bushing to a humid and corrosive environment for a certain period of time.
The bushing and cylinder head have an interference fit. After assembly, the pressure creates a large tensile stress on the outer wall above the bushing shoulder and the outer wall of the chamfer.
The sealant material applied to the shoulder of the bushing during assembly is made of methacrylate and contains an amino catalyst, providing a weak corrosion environment for the bushing. When the cylinder head undergoes a hydrostatic seal test, the outer wall of the bushing shoulder is subjected to stress, leading to the rapid development and expansion of cracks.
Despite the high tensile stress on the inner surface of the bushing thin neck, it does not come into contact with the sealant, and thus the conditions for stress corrosion are not favorable.
1）The cause of the liner crack is intergranular brittle cracking due to stress corrosion.
2）The cracks in the bushing occurred in large numbers within a specific time frame, and there had been no previous incidents of batch crack failure.
Based on the analysis, it was determined that there was a quality issue with the bushing in this batch. The residual stress from the bushing processing was too high, which combined with the additional tensile stress during assembly and the weak corrosion environment provided by the sealant (or cutting fluid during processing), led to stress corrosion cracking of the bushing.