Showing papers by "H.F. Zhang published in 2022"

Improved strength with good conductivity in Cu–Cr–Zr alloys: Determinant effect of under-aging treatment before rolling and aging

Abstract: To achieve high strength and good electrical conductivity, thermo-mechanical treatments are usually applied for precipitation strengthened Cu–Cr–Zr alloys. However, the dislocation-precipitate interaction is still unclear for these alloys during deformation and post-aging, which hinders a further improvement in their mechanical-electrical properties. In the present study, to investigate the effect of precipitates’ size and their orientation relationship with the Cu matrix on the dislocation accumulation during rolling and post-aging, a thermo-mechanical treatment was fulfilled by adjusting the pre-aging state of the alloys. Samples with four precipitate states (solid solute, under-aged, peak-aged, and over-aged) were subjected to rolling and post-aging, respectively. The results showed that, the under-aged sample with less precipitates accumulated the similar deformed structure as the peak-aged one during rolling and relieved the negative effect caused by aging recovery during post-aging, retaining more deformed structure. Therefore, the ultimate tensile strength reached to 588 MPa, ∼30 MPa higher than those with other pre-aging treatments. Additionally, the re-dissolution and refinement of precipitates caused the abnormal decrease of electrical conductivity during rolling, and the post-aging treatment restored the electrical conductivity to the normal level (81% IACS). The results indicate that a proper under-aging pre-treatment before rolling helps to obtain better mechanical properties, which is feasible for large-scale industrial production and can be extended to other thermo-mechanical treatment forms.

Effect of static spheroidization on superplasticity of fine lamellae in nugget of a friction stir welded Ti-6Al-4V joint

Abstract: The spheroidization of the lamellar structure can greatly contribute to the superplasticity of the nugget zone (NZ) of Ti alloy welds, which is the key to achieve the integral superplastic forming of welds for the fabrication of large-scale complex components. However, the spheroidization process is complex and costly since it cannot be obtained generally, unless the lamellae suffers from a large deformation. In this study, the static spheroidization was achieved for the fine lamellae structure in the nugget of a friction stir welded (FSW) Ti-6Al-4V joint, particularly by the annealing without any deformation. The special α/β interface obeying a Burgers orientation relationship (BOR) after FSW was first time directly observed, whose effect on the spheroidization was discussed. A new static spheroidization mechanism with the gradual coalescence of the adjacent lamellae was discovered, which we named as “termination coalescence”. There was a slower coarsening rate in the lamellar structure than in the classical equiaxed one, due to the BOR in the lamellae, although both of them exhibited a volume diffusion character during annealing. Consequently, the similar superplasticity can be achieved for the base material and NZ after annealing. This study can provide a new way to the spheroidization and a theoretical basis for the integral superplastic forming of welds during production.

Welding behavior of an ultrahigh-strength quenching and partitioning steel by fusion and solid-state welding methods

Abstract: How the quenching and partitioning (Q&P) steel with an ultrahigh strength (>1 GPa) behaves during the welding process is a considerable issue for its practical application, but the systematical evaluation on the welding behavior of this new steel has not yet been concerned. Here the structure-property relationships of three typical Q&P 1180 steel joints were comparatively studied by using laser welding (LW), tungsten inert gas (TIG) welding, and friction stir welding (FSW). Weld metal (WM) region and heat-affected zone were generated in the welded joints and their widths were mainly related to the welding linear energy (LW < FSW < TIG welding). During LW and TIG welding, the parent metal (PM) was totally destroyed and coarsened in the WMs whereas ultrafine microstructures with high hardness were obtained after FSW by means of the low peak temperature and severe plastic deformation. The TIG welding produced serious material softening, while the softening degree could be alleviated via LW and FSW. Excellent tensile strength as high as that of the PM was achieved in both the LW and FSW joints by means of the suppressed material softening as well as small soft zone width. However, dramatic losses of strength and ductility were found in the TIG welded joint owing to the premature strain localization and fracture in the soft zone.

Uncertainty and Sensitivity Analysis of Building Integrated Photovoltaics

Abstract: The performance of building-integrated photovoltaics (BIPV) shows high variations due to several factors, including design model uncertainty, installation mode, dirt/soil effects, aging factors, and manufacturing issues. This paper explores the uncertainty of BIPV outputs from the perspectives of both model uncertainty and parameter uncertainty using the EnergyPlus program. The sampling-based Monte Carlo method is implemented to conduct the uncertainty analysis of BIPV outputs. The meta-model global sensitivity analysis (Bayesian adaptive spline surfaces) is used to obtain important factors affecting BIPV outputs due to its high computational efficiency. The results indicate that both model and parameter uncertainty has significant influences on PV outputs. The combined remaining effect, power rating, and model uncertainty are three important factors influencing PV electricity. Therefore, these factors should be carefully chosen or adjusted to provide a reliable estimation of PV outputs.