Detailed Evolution Mechanism of Interfacial Void Morphology in Diffusion Bonding

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Similar diffusion bonding of 1Cr11Ni2W2MoV stainless steel was conducted at different bonding temperatures. The interface characteristics and mechanical properties of joints were examined, and the evolution of interfacial void morphology was analyzed in detail. The results showed that there were four typical interfacial void shapes generating sequentially: the large scraggly voids, penny-shaped voids, ellipse voids and rounded voids. The variation of interfacial void shape was dominated by surface diffusion, while the reduction of void volume was ascribed to the combined effects of plastic flow of materials around voids, interface diffusion and volume diffusion. Owing to the elimination of void from the bonding interface, the sound joint obtained could exhibit nearly full interfacial contact, and present excellent mechanical properties, in which the microhardness and shear strength of joint matched those of base material.

Introduction

Diffusion bonding is an attractive solid-state joining technology, in which two faying surfaces are joined at elevated temperature through application of pressure[1], [2], [3], [4], [5]. The temperature is usually chosen as 0.5–0.8 Tm (Tm is the melting point of the material) and the moderate pressure should be adopted to maintain that no noticeable macroscopic deformation of joined component occurs[6], [7], [8]. If the bonding parameters were chosen properly, a joint having microstructure and mechanical properties distinguishable from those of base material would be produced[9], [10], [11]. Not only similar but also dissimilar materials can be successfully joined by diffusion bonding[12], [13], [14].

During diffusion bonding, the removal of voids from the bonding interface has been a significant subject of considerable interest and has been studied for more than several decades. Many experimental researches on void shrinkage have been carried out to identify the relationship between the void shrinkage behavior and parameters such as temperature, pressure, time and surface condition. Afshan et al.[15] indicated that increasing time resulted in better void shrinkage about diffusion bonding of free cutting steels, thereby enhancing the tensile strength of joint. Vigraman et al.[16] proposed that the void shrinkage process can be greatly promoted with increasing temperature during diffusion bonding of SAE 2205 steel and AISI 1035 steel. In addition to experimental researches, a series of theoretical models have been established for predicting the void shrinkage process during diffusion bonding. These models are usually different in the assumption of void shape, including rhombic void[17], [18], lenticular void[19], elliptic void[20], cylindrical void[21], sine wave void[22] etc. Also all these void shrinkage models in the literature assumed that the voids are uniform in size, and the probabilistic feature of void sizes is not considered in these models. Nevertheless, the actual void sizes generated during diffusion bonding exhibit marked scatter[23]. Despite many efforts that have been made to investigate void shrinkage behavior via experiments and modeling, little work has been focused on the detailed evolution of realistic interfacial void morphology.

In the current study, similar diffusion bonding of 1Cr11Ni2W2MoV stainless steel was carried out at different bonding temperatures. The interface characteristics and mechanical properties of joints were examined, and the detailed evolution of interfacial void morphology was discussed. The present results can contribute to an improved fundamental understanding of the detailed evolution of realistic interfacial void morphology and may also be used as basics for future modeling void shrinkage for diffusion bonding.

Section snippets

Experimental

The chemical composition (wt%) of as-received 1Cr11Ni2W2MoV stainless steel is Fe–11.45Cr–1.58Ni–1.81W–0.44Mo–0.21V–0.16C, and the melting temperature of this steel is ~1500 °C. The dimensions of the grooved specimens to be joined are shown in Fig. 1(a). Prior to diffusion bonding, the specimen surfaces were machined to a surface roughness (Ra) of ~1.6 µm, where Ra is the arithmetic mean surface roughness measured by a C130 laser scanning confocal microscopy. The contaminated specimens were

Interface characteristics

The SEM images of interface characteristics at different bonding temperatures are shown in Fig. 2, and the higher magnification images reveal the interfacial void morphology. As seen from Fig. 2(a), the long interfacial voids with scraggly edges separated by metallurgically joined areas are clearly visible at a temperature of 1000 °C. As the temperature increases to 1020 °C, it is obvious by comparing Fig. 2(a) and (b) that the void size decreases, and the edges of interfacial voids tend to be

Conclusions

  • (1)

    As the bonding process proceeds, four typical void shapes, including the large scraggly voids, penny-shaped voids, ellipse voids and rounded voids, are observed sequentially in the bonding interface.

  • (2)

    Void shape is chiefly altered by surface diffusion, while the void volume is reduced by the combined effects of plastic flow of materials, interface diffusion and volume diffusion.

  • (3)

    Increasing temperature from 1000 °C to 1100 °C can enhance the plastic flow of materials around voids and the diffusion

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 51505386 and No. 51275416), the China Postdoctoral Science Foundation (No. 2014M562447) and the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (16-BZ-2015).

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