Influence of iron on the structure and strength of the Kovar to stainless steel joints by using Cu-Mn-Co-Fe brazing filler metal

doi:10.21203/rs.3.rs-3292577/v1

The paper presents the results of X-ray spectral microanalysis of dissimilar brazed joints between Kovar and stainless steel, produced by vacuum brazing with the application of Cu-Mn-Co-Fe brazing filler metals. X-ray microspectral analysis established that a two-phase structure is formed in brazed joints, which consist of two solid solutions: the primary phase is the α-Cu solid solution (based on the Cu-Mn system), which takes up the main zone of the seam. The γ-phase (Fe-Mn) crystallizes in the form of isolated discrete grains. It also contains other components of the brazing filler metal. The influence of iron on the structure and mechanical properties of the brazed joints was investigated. It was found that an increase in iron concentration leads to an increase in the quantity of the γ-phase. It was determined that increasing the iron concentration in the initial brazing filler metal from 1% to 2.5% results in the attainment of maximum strength (600 MPa) in the brazed joints, and the samples fail through the stainless steel.

Kovar

stainless steel

microstructure

strength

vacuum brazing

Cu-Mn-Co-Fe brazing filler metal

Modern engineering solutions increasingly require joining dissimilar materials in developing new components and parts characterized by specific service properties [1–2]. Producing such components promotes the realization of the advantages of each of the materials, lowering financial costs during their manufacturing and preserving their functional characteristics. Therefore, for instance, it is possible to combine metals with nonmetals, materials with different coefficients of thermal expansion, etc. These joints are used in different sectors, such as electronics, power engineering, aerospace, and automotive industries [3–6].

Kovar joints with stainless steel are becoming widely used, which allows the combining of the features of each of the materials. Different methods are applied for joining dissimilar materials, particularly Kovar – stainless steel pair, such as diffusion, electron beam, laser welding, etc. [7–11]. Brazing is a joint formation process by heating the materials being joined at a temperature below that of their autonomous melting. At heating wetting of the base material by the molten brazing filler metal and its flowing into the gap between the parts being joined with subsequent crystallization at cooling occur [12]. Brazing such a pair of materials is complicated by a significant difference in their physical-mechanical properties, particularly coefficients of linear expansion, melting temperature and different chemical composition.

Structures from stainless steels are usually brazed using nickel-based brazing filler metals, alloyed by other elements, which lower the melting temperature [13]. Application of such nickel brazing filler metals, which contain boron and silicon as depressants, ensures the required wetting of the material being brazed, which allows a significant lowering of the brazing temperature but leads to the formation of brittle phases and low-melting eutectics in the brazed seam [13–14]. An alternative for improvement of the technological properties of brazed joints can be the application of brazing filler metals based on a Cu-Mn-Co-Fe system with a solid solution structure, which does not contain boron or silicon. Unfortunately, there is not enough published data now on brazing Kovar to stainless steel. Therefore, an urgent task is conducting detailed studies in brazing dissimilar materials, particularly Kovar, to stainless steel.

This study shows the influence of iron on the seam morphology and mechanical properties of dissimilar joints of Kovar – stainless steel, which were produced by vacuum brazing with the application of Cu-Mn-Co-(1–5%) Fe brazing filler metals.

Experimental brazing filler metals were produced by nonconsumable tungsten electrode argon-arc melting on a cold copper substrate in the atmosphere of high-purity argon (argon volume fraction, % was not less than 99.993). The uniformity of alloying element distribution through the ingot volume was achieved by five-time remelting. As-cast brazing filler metals with the iron content of 1; 2.5, and 5 wt. % were used for brazing (Table 1).

Table 1

The solidus and liquidus temperatures of alloys of Cu-Mn-4.5Co-Fe system
Brazing filler metal No.	Composition, wt. %	Melting temperature, °С		Brazing temperature, °С
Brazing filler metal No.	Composition, wt. %	Т_s	Т_l	Brazing temperature, °С
1	Cu-Mn-Co-1.0Fe	912	931	961
2	Cu-Mn-Co-2.5Fe	917	957	987
3	Cu-Mn-Co-5.0Fe	923	1027	1057

Brazing experiments were conducted using experimental brazing filler metals in a vacuum furnace (SGV 2.4-2/15-IZ) with radiation heating. The brazing temperature was set to be 30°C higher than the liquidus temperature, and a soaking time of 3 minutes was applied. The working space of the furnace was maintained at a rarefaction level of 1.33×10 − 3 Pa. The heating rate during the brazing process was 18–20°C/min, while the cooling rate of the furnace was 10–15°C/min. The temperature measurement accuracy was ± 5°C. Plate samples, measuring 90×90×2 mm, were fabricated from Kovar and 12Kh18NН10T stainless steel. (Fig. 1, Table. 2).

Table 2

The chemical composition of base metals [15]
Grade	Chemical composition, wt.%
Grade	Fe	Ni	Co	C	Si	Mn	Cr	Ti	Al	Cu
Kovar	51.14–54.5	28.5–29.5	17–18	0.03	0.3	0.4	0.1	0.1	0.2	0.2
12Kh18N10T	67	9 − 1	-	0.12	0.8	2	17–19	0.4-1	-	0.3

Before brazing, the sample blanks were machined (polishing using a diamond wheel with a grain size of 125 µm), degreased in B-70 benzene and dehydrated using technical alcohol. Brazing filler metal (of 0.1–0.15 g weight) in as-cast condition was placed near the gap and fixed using a resistance welding machine. The gap size was not more than 20 µm.

A standard procedure prepared microsections of dissimilar brazed joints. Metallographic investigations were conducted using a scanning electron microscope TescanMira 3 LMU. Element distribution in individual phases was studied by local X-ray spectral microanalysis using an energy-dispersive spectrometer Oxford Instruments X-max 80 mm2. The sections were examined without chemical etching in BSE mode. Measurement locality was up to 1 µm. Mechanical properties were determined at room temperature using ZDM 10 Zwick-1488 testing machine.

The appearance of the brazed samples of Kovar and stainless steel (Fig. 2, а) demonstrates that the brazing filler metal easily wets both base materials. It forms a straight fillet (Fig. 2, b) and flows into the capillary gap between them. Furthermore, the filler metal penetrates through to the opposite side of the sample, resulting in the formation of a small-sized, smooth reverse fillet with a classical shape (Fig. 2, c).

Metallographic investigations were conducted to examine the microstructure of dissimilar joints between Kovar and stainless steel, fabricated using brazing filler metal No.1 (Cu-Mn-4.5Co-1Fe). The investigations confirmed the successful formation of tightly bonded and defect-free brazed seams (Fig. 3, а). Local X-ray spectral microanalysis analysis revealed that the seam exhibits a two-phase microstructure. The primary phase consists of a solid solution based on the Cu-Mn system, which also contains small amounts of nickel, iron, and cobalt (Fig. 3 c, Table 3, Spectrum 4).

The second phase, which forms within the copper solid solution in the brazed seam's peripheral and central zones, consists of discrete grains of the Fe-Mn solid solution. This second phase also contains other elements from the brazing filler metal, as indicated in Table 3 and Spectra 1 and 2. The iron concentration within these grains ranges from 37.93% to 39.27%. Although their quantity is relatively small, they occupy approximately 5% of the visible field in the section, as shown in Figure 3c. Based on the phase diagrams, this compound is likely to correspond to the γ-phase (FeMnCo) [16].

Table 3 Chemical composition of brazed seam of Kovar – stainless steel at application of Cu-Mn-4.5Co-1Fe brazing filler metal

Spectrum	Chemical elements, wt.%
	Si	Ti	Cr	Mn	Fe	Co	Ni	Cu
1	0.35	0.00	2.64	18.51	39.27	12.29	15.05	11.88
2	0.26	0.00	1.29	18.06	37.93	12.34	15.60	14.53
3	0.29	0.13	8.81	14.95	50.80	8.41	9.72	6.90
4	0.00	0.00	0.53	17.79	6.23	1.92	8.36	65.17
5	0.11	0.00	1.24	17.76	10.89	2.78	9.20	58.02
6	0.37	0.18	15.53	6.23	65.37	1.55	8.72	2.05
7	0.30	0.00	0.17	0.94	52.02	17.78	28.28	0.52

With an increase in iron concentration in the brazing filler metal up to 2.5% (No. 2), the overall appearance of the brazed sample remains practically unchanged. Full fillet areas are still formed, and the width of the brazed seam remains within the range of 12-20 µm. However, the structure of the seam undergoes specific changes (Fig. 4). It continues to be two-phase, but the morphology of the phases is altered. The quantity of the Cu-Mn solid solution is significantly reduced, while the iron concentration within it remains at a similar level to the measurements taken in the previous sample, approximately 6.39% (Fig. 4, Table 4, Spectrum 1).

Table 4 Chemical composition of brazed joint of Kovar – stainless steel produced with application of Cu-Mn-4.5Co-2.5Fe brazing filler metal

Spectrum No	Chemical elements, wt.%
Spectrum No	Si	Ti	Cr	Mn	Fe	Co	Ni	Cu
1	0.00	0.00	0.74	17.68	6.39	1.71	7.49	65.99
2	0.25	0.00	5.15	20.25	46.13	11.25	9.46	7.52
3	0.29	0.00	3.64	17.71	45.81	12.69	13.89	5.98
4	0.33	0.19	9.85	16.28	53.58	7.43	8.00	4.34
5	0.21	0.00	4.94	20.34	46.18	10.56	9.63	8.13
6	0.47	0.23	17.81	1.31	70.59	0.44	8.89	0.26
7	0.16	0.00	0.23	0.36	54.08	17.68	27.49	0.00
8	0.19	0.00	4.81	18.68	42.67	9.39	10.08	14.18

An increase in iron concentration from 37.93% to 45.81-53.58% is observed within the grains of the γ-phase (Fe-Mn) compared to the previous sample. The cobalt concentration, on the other hand, remains practically unchanged and is the same as in the previous sample when the filler metal contained 1% iron. In some regions of the seam, these γ-phase grains crystallize closely together, giving the impression that they merge and form a conglomerate of larger size (Fig. 4). Such characteristics of the brazed seam formation may result from the interaction between the molten brazing filler metal (Cu-Mn-4.5Co-2.5Fe) and the solid base metal during brazing at higher temperatures.

Comparative analysis of the structure of brazing filler metal No.2 in its initial state and after spreading over the base metal (Kovar) leads to the conclusion that there are morphological changes (Fig. 5, а, b).

The observed differences in the microstructure can be attributed to variations in cooling conditions. During the brazing filler metal production, a high cooling rate of 17°C/s is achieved through argon blowing. This rapid cooling leads to the crystallization of a solid solution based on copper and manganese (γ-phase). However, during spreading and brazing in a larger vacuum chamber, the cooling rate decreases to 0.25°C/s. Additionally, the direct contact between the liquid brazing filler metal and the solid base metal promotes their interaction, increasing iron concentration within the filler metal and forming an iron-based phase. This iron-based phase is enriched in manganese (Fe-Mn) and contains other elements.

Based on the results of X-ray spectral microanalysis, it can be observed that there is a consistent trend of increasing iron concentration during the formation of the γ-phase after spreading and brazing of dissimilar joints between Kovar and stainless steel (Fig. 6).

It is important to note that when applying a brazing filler metal with 5 wt.% of iron in its initial state, the width of the brazed seam increases by approximately 3-4 times compared to previous samples. This significant increase in width suggests a strong interaction between the brazing filler metal and the base metals (Fig. 7, а, b, c, Table 5) at higher temperatures.

Table 5 The chemical composition of Kovar – stainless steel joint, produced with application of Cu-Mn-4.5Co-5Fe brazing filler metal

Spectrum No	Chemical elements, wt.%
Spectrum No	Si	Ti	Cr	Mn	Fe	Co	Ni	Cu
1	0.23	0.00	6.36	11.42	46.44	9.78	14.89	10.87
2	0.14	0.00	0.45	15.62	5.74	1.77	8.12	68.17
3	0.11	0.00	2.92	11.91	45.00	12.30	18.65	9.10
4	0.25	0.00	2.27	9.10	46.53	13.92	18.86	9.08
5	0.40	0.21	17.46	1.27	71.11	0.00	9.18	0.37
6	0.23	0.00	3.62	13.26	29.48	7.05	11.72	34.64
7	0.31	0.00	0.19	0.20	53.02	17.95	28.32	0.00

Proceeding from the results of X-ray spectral microanalysis, the same phases remain to be the brazed seam components: Cu-Mn solid solution and discrete oval-shaped inclusions of an iron-based phase (45.00-46.53 %), which are enriched in nickel (14.89-18.86 %) and other elements. It should be noted that their quantity increases significantly, but iron, cobalt and manganese concentration practically does not change (Fig. 7, b, c, Table 5).

As one can see from the obtained investigation results, the morphological structure of the brazed seams is different, and it essentially depends on the iron concentration in the initial brazing filler metal. At the application of brazing filler metal with 1 % iron, a considerable quantity of copper-based solid solution and isolated discrete grains of Fe-Mn solid solution are observed in the brazed seam. When applying the brazing filler metal with 2.5 % iron, the brazed seam is almost unvisualized, and the fraction of copper-based solid solution becomes much smaller (Fig. 4).

Further increase of iron content in the brazing filler metal by up to 5% and increased brazing temperature significantly influence the brazed seam morphology. The results of the conducted X-ray spectral microanalysis investigations showed that at the formation of the brazed seam structure, active processes of interdiffusion of the component elements of the brazing filler metal and the base metal take place, in particular, iron, cobalt, nickel, chromium and manganese. The heating temperature and the chemical composition of both the base metal and the brazing filler metal primarily influence the observed diffusion processes. These factors contribute to establishing a concentration gradient at the interface between the brazing filler metal and the base metal, affecting the solidification structure of the brazed seam. The diffusion processes promote the appearance of an iron-based phase, which forms as discrete individual grains, the quantity of which is significantly increased compared to the previous samples (Fig. 7, b, c).

Obtained data of local element distribution agree well with the results of brazed seam mapping (Fig. 8, а, b, c, d, e, f), which visually complement the information on individual phase formation.

A feature of mapping results is a homogeneous distribution of manganese in the brazed seam (Fig. 8, f), which can be explained by the fact that manganese is a component element of copper-based solid solution [17], as well as the iron-based phase, which forms as individual grains. Cobalt and chromium are concentrated in the phase, forming on the iron base.

It is essential to highlight that despite the base metal remaining in the solid state during brazing, the structure and chemical composition of the initial brazing filler metal and the resulting brazed seam differ significantly. This difference arises due to the diffusion processes that commence when the brazing filler metal enters the liquid phase [18].

Furthermore, the investigation results from the study provide evidence that the structure and morphological characteristics of the brazed seams in dissimilar joints of Kovar and stainless steel are influenced not only by diffusion processes but also by the iron concentration in the initial brazing filler metal.

At the same time, an increase of iron concentration in the brazing filler metal by up to 5 % negatively influences the mechanical properties of brazed joints (Table 6), which is attributable to the structural features of brazed seams.

The highest strength of 600 MPa is achieved in overlap brazed samples when using a brazing filler metal containing 2.5% iron. These samples fail in the base metal, specifically the stainless steel, with only minor plastic deformation. (Fig. 9, b).

Table 6 The strength (at 20 °С) of brazed joints of Kovar – stainless steel, depending on Fe content in the brazing filler metal

Brazing filler metal No.	Composition, wt.%	τ_з, MPa	σ_в,MPa	Fracture site
0	Cu-Mn-4,5Co	413-450	-	seam
1	Cu-Mn-4,5Co-1Fe	480-495	-	seam
2	Cu-Mn-4,5Co-2,5Fe	-	580-635	12Kh18N10T
3	Cu-Mn-4,5Co-5Fe	339-387	-	seam

Indeed, in this case, it is evident that the strength of the brazed seam exceeds that of the stainless steel after the thermal brazing cycle. Based on the obtained results, it can be concluded that the presence of iron within specific ranges in the initial brazing filler metal contributes to the strengthening of the solid solutions and improves the mechanical properties of the brazed seam.

The structure of dissimilar brazed joints of Kovar and stainless steel was investigated in this study. The joints were produced by vacuum brazing with radiation heating using brazing filler metals from the Cu-Mn-Co-Fe system. X-ray spectral microanalysis revealed that the primary phase in the brazed seams is an α-Cu solid solution based on the Cu-Mn system. The γ-phase (Fe-Mn) solidifies as discrete grains and contains other brazing filler metal component elements. The quantity of the γ-phase increases with an increase in iron concentration.

Mechanical testing demonstrated that an increase in iron concentration up to 5 wt.% in the brazing filler metal negatively affects the mechanical properties of the brazed joints. This is attributed to the structural features of the brazed seams, particularly the considerable presence of the γ-phase. The overlap brazed joints produced with a brazing filler metal containing 2.5% iron exhibit maximum strength of 600 MPa. These samples fail in the stainless steel base metal with minimal plastic deformation.

Acknowledgment This work was financially supported by the by the National Academy of Sciences of Ukraine. Follow the decision of the Department of Physical and Technical Problems of Materials Science of the National Academy of Sciences of Ukraine (grant number 0118U100528).

Competing interests The authors declare no competing interests.

Author contributions

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Maksymova Svitlana, Kovalchuk Petro and Voronov Vitalii. The first draft of the manuscript was written by Kovalchuk Petro and Maksymova Svitlana and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Liangliang Z, Honggang D, Peng L, Shuai L, Baosheng W, Yueting M, Libing H, Chao L, Jiachen L, Yuesen Y (2023) Vacuum brazing TiAl intermetallic to K4169 alloy using amorphous filler metals Ti56.25–_xZr_xNi25Cu18.75. J mater sci. technol. 154:217-231. doi.org/10.1016/j.jmst.2022.12.073
Srinivas V, Singh, AK, Reddy M (2018) Vacuum brazing of dissimilar tubular component of AA2219 and AISI 304 by a low melting Al-18Ag-20Cu-5Si-0.2 Zn braze alloy. J Mater Process. Technol 252:1–12. https://doi.org/10.1016/j.jmatprotec.2017.09.005.
Qiao GJ, Wang HJ, Gao JQ, Jin ZH (2005) Brazing Al2O3 to Kovar Alloy with Ni/Ti/Ni Interlayer and Dramatic Increasing of Joint Strength after Thermal Cycles. MSF 486–487:481–484. https://doi.org/10.4028/www.scientific.net/msf.486-487.481
Fang YJ, Jiang XS, Mo DF, Song TF, Shao ZУ, Zhu DG, Zhu MH, Luo ZP (2018) Microstructure and Mechanical Properties of Electron Beam-Welded Joints of Titanium TC4 (Ti-6Al-4V) and Kovar (Fe-29Ni-17Co) Alloys with Cu/Nb Multi-Interlayer. Adv. Mater. Sci. Eng. 2018:1-11. doi.org/10.1155/2018/2042871
Sun Z, Ion JC, (1995) Laser welding of dissimilar metal combinations. J Mater Sci. 30:4205‒4214. DOI:10.1007/BF00361499
Xiao R, Dong P, Chen K (2009) Laser beam welding of dissimilar materials. ICALEO 2009 - 28th International Congress on Applications of Lasers and Electro-Optics (CALEO):660-665.
Mai TA, Spowage AC (2004) Characterisation of dissimilar joints in laser welding of steel–kovar, copper–steel and copper–aluminium. Mater Sci Eng A 374:224–233. doi: 10.1016/j.msea.2004.02.025
Fadhali MMA, Zainal SJ, Munajat Y, Jalil A, Rahman R (2010) Laser Welding Characterization of Kovar and Stainless Steel Alloys as Suitable Materials for Components of Photonic Devices Packaging. AIP Conf. Proc. 1217:147-152. doi.org/10.1063/1.3377802
Sun Z, Moisio T, (1994) Melting ratio in laser welding of dissimilar metals. J. Mater. Sci. Lett. 13:980–982. doi.org/10.1007/BF00701444
Xin C, Jiazhen Y, Li N, Liu W, Du J, Cao Y, Shi H (2016) Microstructural evolution during the brazing of Al₂O₃ ceramic to kovar alloy by sputtering Ti/Mo films on the ceramic surface. Ceram Int42:12586-12593. doi.org/10.1016/j.ceramint.2016.04.094
Baghjari SH, Gholambargani M, Akbari Mousavi SAA (2019) Application of the Pulsed Nd: YAG Laser Welding to Investigate the Effect of Laser Beam Position on Weld Characteristics of AISI 420 Stainless Steel to Kovar Alloy. Lasers Manuf. Mater.Process. 6:14–25. https://doi.org/10.1007/s40516-018-0078-y
Ermolaev GV, Kvasnitsky VV, Kvasnitsky VF, Maksymova SV et al (2015) Metal brazing Textbook. NUK, Mykolaiv
Maksymova SV, Voronov VV, Kovalchuk PV (2017) Brazing filler metal without boron and silicon for brazing of heat-resistant nickel alloy. The Paton Welding J. 8:12-17 https://doi.org/10.15407/tpwj2017.08.02
Schilm J, Goldberg A, Partsch U, Dürfeld W, Arndt D, Pönicke A, Michaelis A (2016) Joining technologies for a temperature-stable integration of a LTCC-based pressure sensor. J Sens. Sens. Syst. 5:73–83.
Steel and Alloy Grader. http://www.splav-kharkov.com Accessed 19.04.2023.
Zakharov AM (1990) State diagrams of binary and ternary systems. Metallurgy, Moscow.
Massalski TB (1990) Вinary Alloy Phase Diagrams, American Society for metals. (Ohiо: Metals Park: ASM International: СD).
Maksymova SV, Khorunov VF (2015) Diffusion processes and formation of structure of brazed joints on titanium aluminides. Int. J. Appl. Phys. 2:(1)24-29

Influence of iron on the structure and strength of the Kovar to stainless steel joints by using Cu-Mn-Co-Fe brazing filler metal

Status:

Version 1

Abstract

Figures

1. Introduction

2. Investigation methods and materials

3. Discussion of the obtained results

4. Conclusions

Declarations

References

Status:

Version 1