Leijun Li

A Method for Bond Strength Evaluation for Laminated Structures with Application to Ultrasonic Consolidation

A push-pin type test is proposed as a general approach for bond strength evaluation normal to bonded areas for laminated structures. The evaluation method includes experiment and finite element (FE) simulation. The method has been successfully applied to evaluation of the bond strength of laminated structures made with ultrasonic consolidation (UC). Bond strength varying with UC process parameters has been studied. Based on the results of the simulation and the experiment, a quantitative correlation has been identified between the percent of bonded area and bond strength for UC.

Identification of Inverse Bainite in Fe-0.84C-1Cr-1Mn Hypereutectoid Low Alloy Steel

A unique dilatation trend is observed for isothermal bainite transformation in Fe-0.84 pct C-1 pct Cr-1 pct Mn steel. The dilatation is found to occur in two stages with volumetric contraction dominating the first stage, followed by volumetric expansion dominating the second stage. Through electron microscopic characterization, bainitic microstructure is identified as inverse bainite with cementite (Fe3C) nucleating first from supersaturated austenite followed by the transformation of ferrite and secondary carbides (Fe3C, Fe2C, and Fe5C2) from carbon-depleted austenite.

Effect of joint design on mechanical properties of AL7075 weldment

The effects of joint design on the mechanical properties of AL7075-T6 aluminum sheet were studied on the latest automated gas-tungsten arc-welding system. Using ER5356 filler metal, full-penetration welds were made on workpieces with various included joint angles. Testing of the mechanical properties of the joints was done in the as-welded, naturally aged, and postweld heat-treated conditions. The results show that by using crack-resistant filler, and by selecting the proper joint design and postweld heat treatment, strong, dependable welds can be produced on thin AL7075 sheet material. An elasticity model of the weld joint was established to help understand the mechanical behavior of the joints. An undermatched joint design is shown to be capable of achieving a joint strength that matches the strength of the base alloy.

Correlation Between Intercritical Heat-Affected Zone and Type IV Creep Damage Zone in Grade 91 Steel

A soft zone in Cr-Mo steel weldments has been reported to accompany the infamous Type IV cracking, the highly localized creep damage in the heat-affected zone of creep-resistant steels. However, the microstructural features and formation mechanism of this soft zone are not well understood. In this study, using microhardness profiling and microstructural verification, the initial soft zone in the as-welded condition was identified to be located in the intercritical heat-affected zone of P91 steel weldments. It has a mixed structure, consisting of Cr-rich re-austenitized prior austenite grains and fine Cr-depleted, tempered martensite grains retained from the base metal. The presence of these further-tempered retained grains, originating from the base metal, is directly responsible for the hardness reduction of the identified soft zone in the as-welded condition. The identified soft zone exhibits a high location consistency at three thermal stages. Local chemistry analysis and thermodynamic calculation show that the lower chromium concentrations inside these retained grains thermodynamically decrease their potentials for austenitic transformation during welding. Heterogeneous grain growth is observed in the soft zone during postweld heat treatment. The mismatch of strengths between the weak Cr-depleted grains and strong Cr-rich grains enhances the creep damage. Local deformation of the weaker Cr-depleted grains accelerates the formation of creep cavities.

A dilatometric analysis of inverse bainite transformation

The unique two-stage dilatation curve observed during the inverse bainite transformation of a hypereutectoid low alloy steel is analyzed to understand the transformation kinetics. A new algorithm is proposed to extract the bainitic phase fractions from the raw dilatometry data. The proposed data extraction algorithm is generic that relies only on the density of phases involved in the transformation. To verify the extracted phase fraction, a kinetics model is developed using the principles of diffusion and Johnson–Mehl–Avrami–Kolmogorov kinetics. The predicted phase fractions by the kinetics model agree fairly well with the experimental phase fraction results from dilatometry, metallography, and XRD. The two-stage transformation can be explained by the kinetics of inverse bainite as a diffusion-controlled transformation product. The transformation proceeds in a para-equilibrium mode, involving only the diffusion of carbon at the inverse bainite/parent austenite interface

Microstructural analysis of fracture in heat-affected zone of two ×70 pipeline steel weldments

ABSTRACT In the previous study, different crack propagation behaviours (ductile fracture and brittle cleavage fracture) were observed in two ×70 pipeline steel weldments (13.4 and 17.8-mm-thick) during single-edge notched bend testing. To further understand these two fracture behaviours, detailed microstructures of the base metal (BM), fine-grained heat-affected zone (FGHAZ), and coarse-grained heat-affected zone (CGHAZ) of these two ×70 pipeline steel weldments have been analysed. The results show that the initial structure of the two pipe BMs and different welding cooling rates owing to different thicknesses contributed to structural variations of the correlated sub-regions of the HAZ. For both weldments, the FGHAZ close to the BM has the highest fraction of the high-angle grain boundaries, the finest grain size, the lowest local strain levels, and the highest fraction of recrystallised ferrite grains. The CGHAZ of the 17.8-mm-thick pipe welds exhibits the lowest toughness with the highest hardness, a high frequency of deformed grains, the highest local strain level, and the highest density of preferred {100} cleavage planes than the other sub-regions in the HAZ. The high density of the {100}<011> texture components in the HAZ may cause the cleavage micro-cracks to propagate toward the BM at an approximate 45° angle to the original crack plane during bending tests.

The Microstructure and Pitting Resistance of Weld Joints of 2205 Duplex Stainless Steel

Abstract2205 duplex stainless steel (DSS) was welded by submerged arc welding. The effects of both heat input and groove type on the ferrite/austenite ratio and elemental diffusion of weld joints were investigated. The relationships among welding joint preparation, ferrite/austenite ratio, elemental diffusion, and pitting corrosion resistance of weld joints were analyzed. When the Ni content of the weld wire deposit was at minimum 2–4% higher than that of 2205 DSS base metal, the desired ratio of ferrite/austenite and elemental partitioning between the austenite and ferrite phases were obtained. While the pitting sensitivity of weld metal was higher than that of base metal, the self-healing capability of the passive film of weld metal was better than that of the base metal when a single V-type groove was used. Furthermore, the heat input should be carefully controlled since pitting corrosion occurred readily in the coarse-grained heat-affected zone near the fusion line of welded joints.

Effect of simultaneous addition of ferroniobium and ferrotitanium on properties of hardfacing

ABSTRACT The slag-free self-shielded flux-cored wire with simultaneous addition of ferroniobium (Fe-Nb) and ferrotitanium (Fe-Ti) was developed to fabricate the iron-based hardfacing alloys. The transfer coefficients of Nb and titanium of slag-free self-shielded flux-cored wire were 91.2 and 63.8%, respectively. The changes in microstructures indicate that Nb and Ti addition shifted the carbon concentration in the remaining liquid to one corresponding to the near eutectic state owing to the formation of (Nb, Ti)C which consumed carbon. The wear loss of the hardfacing alloy with 18 wt-% Fe-Nb and 6 wt-% Fe-Ti addition was the smallest among all the alloys owing to the formation of reinforced uniform quadrangle-shaped (Nb, Ti)C carbides in the refined microstructure and the highest hardness.

Homogenisation of austenite during non-equilibrium heating in hypereutectoid steels

ABSTRACT The homogenisation (Stage III) of austenite during non-equilibrium heating of hypereutectoid steels has been characterised for the first time through dilatometric strain measurements and electron microscopic techniques. It has been found that the homogenisation temperature (Ach) is significantly higher than the cementite dissolution temperature (Acm). Though there is a complete conversion of low-temperature constituents to austenite above the Acm, a significant segregation of carbon in the austenite is observed below the Ach. Performing heat treatment with an incomplete austenitisation between the Acm and the Ach temperature can affect the subsequent on-cooling phase transformations. It is proposed that the Ach temperature should be used as the full austenitisation temperature in hypereutectoid steels.

Effect of Process Parameters on Pulsed-Laser Repair of a Directionally Solidified Superalloy

Directionally solidified nickel-based superalloy GTD-111 was repaired using pulsed Nd:YAG laser powder deposition (PLPD). A test matrix was created and tested to link deposit and defect formation to process parameters. Epitaxial solidification of the deposit was achieved. The grain size was uniform throughout multiple layers of deposit and appeared to be of cellular dendritic morphology. The crack formation is associated with the “stray” grains, which seem to be controlled by the geometry and overlap of the deposits. The formation of stray grains tended to be suppressed in deposits with shallow and wide profiles with smooth toe transitions into previous beads. Geometric discontinuity and formation of stray grains were found to contribute significantly to the formation of microfissures at the weld toes. Multiple pulses were found to remelt and heal the microfissures in the previous spot deposit. With optimized process parameters, crack-free, multiple-layered, deposits were achieved for tip repair (top-down orientation) of GTD-111 blades with René 80 and IN625 filler metals.