Shuhai Chen

Microstructures and Mechanical Properties of Laser Penetration Welding Joint With/Without Ni-Foil in an Overlap Steel-on-Aluminum Configuration

The microstructures and mechanical properties of laser penetration welding joints with/without Ni-foil in an overlap steel-on-aluminum configuration were investigated. The interfacial structure between fusion zone and aluminum alloy without Ni-foil consists of FeAl/FeAl3. After the Ni-foil is added, the interfacial structure transforms into Ni1.1Al0.9/FeAl3, and the molten pool of aluminum alloy is expanded, which leads to the formation of the NiAl3 between Ni-foil and the molten pool. A banded structure composed of β(Fe, Ni)Al appears whether the joints are made with/without Ni-foil over the reaction zone. It was found that the Ni-foil enhanced tensile property of the joint, expanded usable processing parameters, and decreased microhardness of the intermetallic compounds. The enhancement of mechanical properties is attributed to the improvement of the toughness of the joint made by Ni-foil.

Microstructural Characteristics of a Stainless Steel/Copper Dissimilar Joint Made by Laser Welding

The microstructures and its formation mechanism of a stainless steel/copper dissimilar joint by laser welding were investigated. It was found that the two modes of joining, i.e., welding-brazing and fusion welding, depend on different processing parameters. In the welding-brazing mode, the interface between copper and the fusion zone has scraggy morphology because the molten pool is frozen by solid copper with high thermal conductivity. The interdiffusion of elements occurs in the neighborhood of the interface, which leads to the metallurgy bond of the mode. In the fusion welding mode, the liquid phase in the fusion zone undergoes not only primary but also secondary liquid separation due to the high cooling rate and high supercooling level of laser welding. Some microcracks generated in the fusion zone by thermal stress mismatch are healed by liquid copper filling.

Synthesis kinetics and thermophysical properties of La2(Zr0.7Ce0.3)2O7 ceramic for thermal barrier coatings

La2(Zr0.7Ce0.3)2O7 (LZ7C3)ceramic was synthesized by solid state reaction with La2O3,ZrO2 and CeO2 as starting materials.The synthesis kinetics,phase structure,mass loss and microstructure were studied by thermo gravimetric-different thermal analyzer (TG-DTA),X-ray difference (XRD) and scanning electron microscopy (SEM).The thermal conductivity and thermal expansion coefficient were measured by laser-flash method and pushing-rod method,respectively.XRD results showed that LZ7C3 was a mixture of La2Zr2O7 (LZ,pyrochlore) and La2Ce2O7 (LC,fluorite).The lowest synthesis temperature and time of LZ7C3 were 1400 ℃ and 5 h.There were no peaks of La2O3 when the powder granularity was about 0.82 μm in the synthesis process.The atom ratio La∶Zr∶Ce of prepared LZ7C3 powder was very close to 10∶7∶3 which was the theory value of LZ7C3.The thermal conductivity of LZ7C3 decreased gradually with the temperature increased up to 1200 ℃,and was located within 0.79 to 1.02 W/(m·K),which was almost 50％ lower than that of LZ,whereas its thermal expansion coefficient was larger and the value was 11.6× 10-6 K-1.

Expanded Lever Rule for Phase Volume Fraction Calculation of High-Strength Low-Alloy Steel in Thermal Simulation

The principle of the lever rule on the dilatation curve and its application to the corresponding differential dilatation curve were introduced in a nonoverlapped two-phase continuous cooling process. The lever rule was further expanded in the case of an overlapped two-phase process. The application of the expanded lever rule was based on the approximate symmetry treatment on the differential dilatation curve, which shows reasonably both on the theoretical calculation and in the experimental results. High-strength low-alloy steels were thermal simulated with Gleeble 3500. The transformed phase volume fractions in different cooling processes were calculated by the expanded lever rule and metallography analysis. The results showed the expanded lever rule could calculate reliable phase volume fractions as metallography analysis.

Investigation of microstructural evolution and electrical properties for Ni-Sn transient liquid-phase sintering bonding

Ni/Ni-Sn/Ni sandwiched simulated package structures were successfully bonded under low temperature and low pressure by Ni-Sn transient liquid-phase sintering bonding. The results show that, after isothermally holding for 240 min at 300 °C and 180 min at 340 °C, Sn was completely transformed into Ni3Sn4 intermetallic compounds. When the Ni3Sn4 phases around Ni particles were pressed together, the porosity of the bonding layer increased, which obviously differed from the normal sintering densification process. With further analysis of this phenomenon, it was found that large volume shrinkage (14.94% at 340 °C) occurred when Ni reacted with Sn to form Ni3Sn4, which caused void formation. A mechanistic model of the microstructural evolution in the bonding layer was proposed. Meanwhile, the resistivity of the bonding layer was measured and analyzed by using the four-probe method; the microstructural evolution was well reflected by the resistivity of the bonding layer. The relationship between the resistivity and microstructure was also discussed in detail.

High temperature resistant Ni-Sn transient liquid phase sintering bonding for new generation semiconductor power electronic devices

This paper presents the microstructural evolution of a Niekel-Tin(Ni-Sn) TLPS process. Ni-Sn power mixture was assembled in a Ni/Ni-Sn/Ni sandwiched structure and sintered in a vacuum furnace using different times at 340°C. The results show that after 180min with slight pressure of 0.1MPa the majority of Sn and Ni in the joint have completely formed intermetallic compounds(IMC) and the jointing layers mainly consists of Ni3Sn4, but not dense enough. With the increase of processing time, a dense microstructure can be observed after 300min. Microstructural characterization revealed the formation joints governed by the interdiffusion of the main constituents. The differential scanning calorimetry(DSC) profiles also show that the melting endothermic peak corresponding to Sn is negligible after 180min at 340°C and the melting event corresponding to the Ni3Sn4 occurs at approximate 798.9°C. Compared to the traditional solding technique, a higher temperature resistance bonding joint can be achieved using the Ni-Sn TLPS bonding at lower bonding temperature.

Method for Assessing Grain Boundary Density in High-Strength, High-Toughness Ferritic Weld Metal

A method for measuring peak values on the maxlength-area fraction curve and the perimeter-area fraction curve with morphological photos using Image Pro Plus 6.0 Soft for assessing grain boundary density in high-strength, high-toughness ferritic weld metals is developed. Results show the sizes of the peak values have a tough relationship with grain boundary densities in that a larger peak value stands for a larger grain boundary density. As ferrite transforms into a certain orientation relationship, this semi-empirical method provides handy references for judging the sizes of effective grain boundary densities.

New bainite kinetics of high strength low alloy steel in fast cooling process

Based on Kolmgorov-Johnson-Mehl-Avrami analysis, a new bainite kinetics of high strength low alloy steel in fast cooling process was developed by utilizing different experimental methods. Upper bainite transformation morphological evolutions at a cooling rate of 8.3 K/s were directly observed by laser scanning confocal microscopy. This qualitative analysis suggests that bainite packet is more suitable to give a one-dimensional growth model if it is considered as a transformation unit. The nucleation rate of bainite packets in fast cooling process is assumed to give an a priori item. One-dimensional growth model with constant growth rate which is assumed as a function of cooling rate is adopted as well. Thus, the developed new bainite kinetics is simple in expression and contains an adjustable parameter and an empirical parameter. Experimental results show upper bainite and lower bainite transformations in fast cooling processes. Their referential phase volume fractions are calculated by the expanded lever rule on the first derivative dilatometer curves. For the similar transformation mechanisms, upper bainite and lower bainite are considered to give the same kinetics. With considering the Nakamura s equation, the bainite kinetics is fitted with experimental data. Results show that bainite volume fractions and bainite transformation rates can be expressed precisely by the newly developed bainite kinetics.

Effects of germanium additions on microstructures and properties of Al–Si filler metals for brazing aluminum

Abstract A series of Al–Si–Ge filler metals were studied for brazing aluminum. The microstructures and properties of the filler metals were investigated systematically. The results show that the liquidus temperature of Al–Si–Ge filler metals drops from 592 to 519 °C as the content of Ge increases from 0 to 30% (mass fraction). As the content of Ge increases, bright eutectic Ge forms. However, as the Ge content exceeds 20%, the aggregation growth of the eutectic structure tends to happen and coarsened primary Si–Ge particle forms, which is detrimental to the properties of alloys. The Al–10.8Si–10Ge filler metal has good processability and wettability with the base metal Al. When this filler metal is used to braze 1060 aluminum, the complete joint can be achieved. Furthermore, the shear strength test results show that the fracture of brazed joint with Al–10.8Si–10Ge filler metal occurs in the base metal.