Search

In order to eliminate the effect of noise on the performance of the direction-of-arrival (DOA) estimation and reduce the computational complexity, a sparse representation (SR) DOA estimation method is proposed. The proposed method first utilizes the beamspace and element-space covariance differencing to eliminate noise. Afterward, it vectorizes the difference covariance matrix. In a sequence, it establishes a new SR model to complete DOA estimation. Compared to existing SR DOA estimation methods, the proposed method significantly reduces the computational complexity since the parameters to be solved in its SR cost function are regardless of the number of sources and the number of array elements. Simulation results show that in the case of the unknown number of sources and low signal-to-noise ratios (SNRs), the proposed method has high DOA resolution and estimation accuracy.

Numerical simulation was utilized to investigate the residual stresses in the multipass gas metal arc-welded joints of high-strength Al-Zn-Mg alloy plates. In order to elevate the reliability of numerical simulation, the heat source model, parameters characterizing the thermal and mechanical properties, boundary conditions, material constitutive model and finite element meshing were discussed in detail. As a novelty of this paper, the modified Arrhenius constitutive equation of the base material and the filled material was obtained according to the results of high-temperature tensile tests. Distributions of stress on the upper and bottom surface of the joint were obtained based on numerical simulation. Special attention was paid to the evolutions of local residual stress on the bottom surface of the joint, and it’s found that residual stresses on the bottom surface of the joint were almost unchanged after the weld seam approached a certain thickness. Blind-hole method and x-ray method were adopted to detect the residual stresses on the surfaces to verify the accuracy of the results obtained by numerical simulation.

Effects of Fe-to-Co concentration ratio upon the glass forming ability (GFA) of the cladding powders had been quantitatively evaluated with theories applied in bulk amorphous materials, based on which laser cladded amorphous matrix composite coatings were successfully fabricated with FeCoCrBNiSi high-entropy alloy powders. The relationship between predicted GFA, amorphous phase content and wear resistance behaviors of the coatings had been investigated. The microstructure of the coatings was uniformly composed of amorphous phase, nano-crystalline β-Co phase, γ(Fe, Ni) and Co2B crystalline phases. Coatings obtained with the lowest Fe-to-Co ratio of about 1: 1 exhibited the highest amorphous content of over 66.7%, the highest microhardness and the most outstanding high-temperature wear resistance. Amorphous phase had optimized the wear mechanism which was a combination of abrasive wear and oxidation wear, and consequently improved wear resistance of the coatings. Increment in Fe-to-Co ratio weakened the GFA of the cladding powders and led to lower content of amorphous phase, which gave birth to lower microhardness, higher fraction of oxidation wear and thus higher wear rate of the coatings. The content of amorphous phase in the coatings was proved by the present research to be a novel and reasonable criterion for GFA evaluation of cladding materials.

Pulsed plasma arc deposition (PPAD) was successfully used to fabricate the Ni-based superalloy Inconel 625 samples. The effects of three heat treatment technologies on microstructure and mechanical properties of the as-deposited material were investigated. It was found that the as-deposited structure exhibited homogenous cellular dendrite structure, which grew epitaxially along the deposition direction. Moreover, some intermetallic phases including Laves phase and MC carbides were precipitated in the interdendritic region as a result of Nb segregation. Compared with the as-deposited microstructure, the direct aged (DA) microstructure changed little except the precipitation of hardening phases γ’ and γ” (Ni3Nb), which enhanced the hardness and tensile strength. But the plastic property was inferior due to the existence of brittle Laves phase. After solution and aging heat treatment (STA), a large amount of Laves particles in the interdendritic regions were dissolved, resulting in the reduction of Nb segregation and the precipitation of needle-like δ (Ni3Nb) in the interdendritic regions and grain boundaries. The hardness and tensile strength were improved without sacrificing the ductility. By homogenization and STA heat treatment (HSTA), Laves particles were dissolved into the matrix completely and resulted in recrystallized large grains with bands of annealing twins. The primary MC particles and remaining phase still appeared in the matrix and grain boundaries. Compared with the as-deposited sample, the mechanical properties decreased severely as a result of the grain growth coarsening. The failure modes of all the tensile specimens were analyzed with fractography.