Ren-Kae Shiue

The microstructural observation and wettability study of brazing Ti-6Al-4V and 304 stainless steel using three braze alloys

Both Ti-6Al-4V and 304 stainless steels (304SS) are good engineering alloys and widely used in industry due to their excellent mechanical properties as well as corrosion resistance. Well-developed joining process can not only promote the application of these alloys, but also can provide designers versatile choices of alloys. Brazing is one of the most popular methods in joining dissimilar alloys. In this study, three-selected silver base filler alloys, including Braze 580, BAg-8 and Ticusil®, are used in vacuum brazing of 304SS and Ti-6Al-4V. Based upon dynamic sessile drop test, Braze 580 has the lowest brazing temperature of 840°C, in contrast to 870°C for BAg-8 and 900°C for Ticusil® braze alloy. No phase separation is observed for all brazes on 304SS substrate. However, phase separation is observed for all specimens brazed above 860°C on Ti-6Al-4V substrate. The continuous reaction layer between Braze 580 and 304SS is mainly comprised of Ti, Fe and Cu. The thickness of reaction layer at Braze 580/Ti-6Al-4V interface is much larger than that at Braze 580/304SS interface. Meanwhile, a continuous Cu-Sn-Ti ternary intermetallic compound is found at the Braze 580/Ti-6Al-4V interface. Both Ticusil® and BAg-8 brazed joint have similar interfacial microstructures. Different from the Braze 580 specimen, there is a thick Cu-Ti-Fe reaction layer in both BAg-8/304SS and Ticusil®/304SS interfaces. The formation of Cu-Ti-Fe interfacial layer can prohibit wetting of BAg-8 and Ticusil® molten brazes on 304SS substrate. Meanwhile, continuous Ti2Cu and TiCu layers are observed in Ti-6Al-4V/BAg-8 and Ti-6Al-4V/Ticusil® interfaces.

The reliability study of selected Sn–Zn based lead-free solders on Au/Ni–P/Cu substrate

Abstract Since both Ag and In are important melting point depressants in Sn–Zn based solders, a series Sn–Zn based solders with various amounts of Ag and In additions was studied in the experiment. The melting behavior of solder alloys, wetting characteristics, coefficients of thermal expansion, microstructural evolution and long-term reliability of the selected Sn–Zn based solder on Au/Ni–P metallized copper substrate were examined. Based on the experimental result, there is little change in the melting range of Sn–Zn based solder alloys by minor addition of Ag. On the contrary, the melting point of Sn–Zn based alloys can be effectively decreased by In additions. However, the difference between solidus and liquidus temperature is broadened as the increment of In into Sn–Zn based solders. 76Sn–9Zn–15In has the lowest liquidus temperature among all alloys, and it can effectively bond the Au/Ni–P metallized copper substrate. The microstructure of 76Sn–9Zn–15In alloy soldered at 200 °C for 20 min is primarily comprised of Sn–In γ phase and needle-like ZnO 2 . Since there is no flux usage during soldering, zinc oxide cannot be avoided even the process performed under 2×10 −2 mbar vacuum environment. It is also noted that there is no interfacial reaction layer between 76Sn–9Zn–15In and Au/Ni–P metallized copper substrate after soldering. However, there is a reaction layer between 76Sn–9Zn–15In and substrate as the soldered specimen aged at 90 °C for 168 h. Its chemical composition is close to Zn-rich γ phase (NiZn 3 ) alloyed with minor Sn, In, Cu and P. For the specimen further aged at 90 °C for 336 h, there are cracks along the interface between solder alloy and electroless Ni–P layer. The oxidation of the interfacial Zn-rich γ phase plays an important role in deterioration of the bonding between 76Sn–9Zn–15In and Au/Ni–P metallized copper substrate.

A study of Sn-Bi-Ag-(In) lead-free solders

Sn-Bi-Ag-(In) solder alloys have been extensively studied in the study. The experimental results reveals that the liquidus temperatures of Sn-(1–5) Bi-(2–3.5)Ag-(0–10)In solders are between 201.7 and 225.3°C, which were higher than that of the most popular eutectic Pb-Sn solder (183°C). Additions of (5–10) wt% In into Sn-Bi-Ag solders can effectively decrease the melting point of the solder alloy. However, the gap between Ts and TL temperatures increases with the additions of Bi and In into Sn-Bi-Ag-(In) solders. Although there is no flux applied during soldering, most Sn-Bi-Ag-(In) solder alloys can well bond the Au/Ni metallized copper substrate. 94Sn-3Bi-3Ag solder demonstrates the lowest wetting angle of 45° among all test samples. Thermal expansion coefficients of both 94Sn-3Bi-3Ag and 90Sn-2Bi-3Ag-5In solders are slightly less than that of 63Sn-37Pb. Both 90Sn-2Bi-3Ag-5In/substrate and 94Sn-3Bi-3Ag/substrate interfaces demonstrate similar reaction kinetics in the experiment. The stability of the interface is greatly impaired during 90°C aging. Some locations of the electroless Ni layer break down, and new phases are formed nearby the interface during aging treatment. Initially, the growth of Ni-rich (Ni,Cu)3Sn4 phase dominates the interfacial reaction. However, the growth of Cu-rich (Cu,Ni)6Sn5 phase will dominate the reaction layer for specimens aged at 90°C for long time periods.

A wettability study of Cu/Sn/Ti active braze alloys on alumina

Active brazing is one of the ideal ways to make metal/ceramic joints. The active braze alloy contains active element(s), such as: Ti, Zr, Cr... etc., reacting and wetting the ceramic surfaces during brazing. Therefore, a strong chemical bonding can be formed after brazing. Cu base active braze alloys are alternatives among active braze alloys. With the aid of additional melting point depressant, Sn, in Cu-Ti alloys, the intermetallic phase in the active braze can be changed. However, its ability to braze structural ceramics, e.g. alumina, needs further study. The purpose of this research is concentrated on the wettability study of the Cu/Sn/Ti alloy on polycrystalline alumina. Based on the experimental results, the minimum Ti content is 6 wt pct in order to effectively wet alumina. Volume fraction of the intermetallic phase in the braze will be greatly increased if the Ti content in the alloy is increased to 12 wt pct. According to sessile drop test results, 70Cu-21Sn-9Ti demonstrates the best wetting ability on alumina. Meanwhile, the Sn content in Cu/Sn/Ti alloy should be less than 21 wt pct in order to maintain proper wettability of the braze. In addition, Cu/Sn/Ti alloys have both lower wetting angle on alumina and lower thermal expansion coefficients than commercial Ticusil® braze.

Microstructural evolution of brazing 422 stainless steel using the BNi-3 braze alloy

The 422 stainless steel (422SS) is one of the typical martensitic stainless steels with both excellent creep strength and corrosion resistance up to 650°C. Its application includes steam turbine blades, high temperatures bolting ... etc. Repair welding of 422SS is one of the most common methods to fix the turbine blade. However, repair brazing of surface shallow cracks, e.g., less than 1 mm in depth, is an alternative way to fix such blades. The microstructural evolution of brazing 422SS with BNi-3 braze alloy using both infrared and furnace brazing was performed in the study. Based on the experimental results, BNi-3 cannot effectively wet 422SS substrate below 1025°C. As the brazing temperature increases above 1050°C, comprehensive wetting can be obtained in 1200 sec. For the infrared brazed specimen with a short brazing time, the cooling path starts from the formation of a BNi3 phase in the molten braze, subsequently forms a Ni-rich phase, and finally a eutectic phase is solidified from the residual eutectic liquid. The microstructure of the furnace-brazed specimen is similar to that of infrared brazed specimen, but the interfacial reaction zone is significantly increased in furnace brazing. There are Kirkendall voids in the braze close to the interface between BNi-3 and 422SS, and the size of Kirkendall porosity is increased with increment of the brazing time and/or temperature. The homogenization treatment of the brazed joint at 900°C results in growth of both the interfacial reaction zone and porosity.

Mechanical properties of modified 9Cr–1Mo steel welds with notches

Abstract Notched specimens of a modified 9Cr–1Mo steel and its laser welds were extensively evaluated by various methods. Impact, notched tensile and fatigue crack growth tests were employed on specimens tempered at different temperatures. Notched tensile specimens were also tested in gaseous hydrogen, in addition to laboratory air. The results indicated that the deterioration in impact toughness of the weld metal was pronounced for tempering below 680xa0°C, in contrast to a narrow tempering range around 540xa0°C for the base metal. The coarse-grained weld metal was more susceptible to hydrogen embrittlement (HE) than the fine-grained base metal for specimens tempered at 250 and 540xa0°C as verified by the loss of notched tensile strength (NTS) in hydrogen. For specimens with low impact toughness and high NTS losses, the unstable crack growth which was characterized by quasi-cleavage on fracture surfaces and abrupt change in FCGRs was observed for Δ K beyond certain values. In general, the Paris gradient of the weld metal was steeper than that of the base metal with the same tempering treatment, but the discrepancy became insignificant for specimens tempered at 750xa0°C. For specimens tempered at 750xa0°C, not only the impact energy but also the HE resistance could be significantly increased. It was suggested that modified 9Cr–1Mo welds should be tempered in the neighborhood of 750xa0°C for improved mechanical properties.

Hydrogen embrittlement susceptibility of laser-hardened 4140 steel

Abstract Slow strain rate tensile (SSRT) tests were performed to investigate the susceptibility to hydrogen embrittlement of laser-hardened AISI 4140 specimens in air, gaseous hydrogen and saturated H2S solution. Experimental results indicated that round bar specimens with two parallel hardened bands on opposite sides along the loading axis (i.e. the PH specimens), exhibited a huge reduction in tensile ductility for all test environments. While circular-hardened (CH) specimens with 1 mm hardened depth and 6 mm wide within the gauge length were resistant to gaseous hydrogen embrittlement. However, fully hardened CH specimens became susceptible to hydrogen embrittlement for testing in air at a lower strain rate. The strength of CH specimens increased with decreasing the depth of hardened zones in a saturated H2S solution. The premature failure of hardened zones in a susceptible environment caused the formation of brittle intergranular fracture and the decrease in tensile ductility.