فصلنامه مواد پیشرفته در مهندسی
سال سی و نهم شماره 4 (زمستان 1399)

The purpose of the present work is to investigate the influence of the number of weld-passes on microstructure, hardness and residual stresses of composite coatings composed of Ti-Al-Si intermetallic compounds. In this regard, surface coating of pure Ti was carried out using one and two passes of tungsten inert gas (TIG) welding with an Al filler alloy (grade 4043). Phase and structural evaluations of the coatings were investigated by X-ray diffraction, optical and scanning electron microscopies. microhardness and residual stress values of the coatings were measured using ASTM E384-HV device and the Sin2ψ method, respectively. The results showed that as the number of welding passes increased or the dilution ratio decreased, the volume fraction of Ti5Si3-Al3Ti intermetallic phases within the fusion zone increased and the volume fraction of martensite phase in the heat affected zone decreased. As a result, the average hardness value of the coating increased to be about 130 % compared to that of the pure Ti substrate. The tensile residual stresses at the center line of fusion zone were 165 ± 30 and 210 ± 35 MPa for the coatings prepared in one and two welding passes, respectively.

The bare and pre-oxidized AISI 430 pieces were screen printed by copper ferrite spinel coatings. Good bonding between the coating and the substrate was achieved by the reactive sintering process of the reduced coating. The energy dispersive X-ray spectroscopy (EDS) analysis revealed that the scale is a double layer consisting of a chromia-rich subscale and an outer Cu/Fe-rich spinel. The results showed that the spinel protection layer not only significantly decreased the area specific resistance (ASR), but also inhibited the subscale growth by acting as a barrier to the inward diffusion of oxygen. ASRs of 19.7 and 32.5 mΩ.cm2, much lower than that of the bare substrate (153.4 mΩ.cm2), at 800 °C after 400 h oxidation were achieved for the bare and pre-oxidized copper ferrite spinel coated samples, respectively. Excellent, stable ASR (20.5 mΩ.cm2) was obtained with copper ferrite coating after 600 h of exposure at 800 °C. The high electrical conductivity of CuFe2O4 and its doping by Mn, the growth reduction of Cr2O3 oxide scale and the good coating to substrate adherence are proposed to be responsible for substantial improvement in electrical conductivity.

Tissue-engineering scaffolds provide biological and mechanical frameworks for cell adhesion, growth, and differentiation. Nanofibrous scaffolds mimic the native extracellular matrix (ECM) and play a significant role in formation and remodeling of tissues and/or organs . One way to mimic the desired properties of fibrous ECM is adding nanoparticles into the polymer matrix. In the current study, the uniform fibers of poly (ε-caprolactone) (PCL) enriched with different layered double hydroxide (LDH) contents (ranging from 0.1 wt.% to 10 wt.%) were successfully fabricated by electrospinning method. The LDH nano particles were randomly dispersed in the fibers, as confirmed by Energy Dispersive X-ray analysis (EDX). Scaffolds were analyzed from morphological, physical and mechanical view. Biological assessments of scaffolds in terms of cellular attachment and adipogenic differentiation of mouse adipose derived stem cells (mADSCs) were performed. The results showed that inclusion of LDH nanoparticles reduced the average fiber diameter and enhanced the tensile strength and elongation at break values of the PCL scaffold. The LDH-enriched electrospun PCL scaffolds had remarkable effects on cell adhesion. Moreover, a significant increase in adipogenic differentiation of mADSCs was observed. The PCL/LDH nanofibrous scaffolds showed great potential in application for soft tissue engineering.

This study aims at investigating changes in microstructure and strength of W alloy and Cu bimetals with varying spark plasma sintering (SPS) temperature and percentage of copper in W-Cu-Ni alloy. After SPS of W (12 wt%)-Cu (14 wt%)-Ni (3 wt%) alloy powder into consolidated discs at 1350 ° C, they were spark plasma sintered to copper discs at various temperatures. Assessment of the interface microstructure and shear strength was performed by field emission scanning electron microscpe (FESEM) and shear strength test, respectively. Results indicated SPS is successful in forming a perfect metallic bond with monolithic interface and high shear strength of about 45 MPa in Cu/W-12Cu-3Ni bimetal that is extra high quality and not reported in previous investigations.

In recent years, nanoceramics have been used in scaffolds to emulate the nanocomposite with a three-dimensional structure of natural bone tissue. In this regard, polycaprolactone biopolymer is widely used as a scaffold in bone tissue engineering. The goal of this research is to produce porous scaffolds of polycaprolactone - zeolite biocomposite with suitable mechanical, bioactive and biological properties  for bone tissue engineering applications. The nanocomposite scaffolds were synthesized by solvent casting/particulate leaching and freeze-drying approaches. Microscopic investigations showed generation of pores with an average size of 200-400μm after addition of ceramic phase. Energy dispersive X-ray analysis confirmed uniform distribution of ceramic phase in polycaprolactone matrix. FTIR results determined the binding type of zeolite nanoparticles to the polycaprolactone matrix as physical bonding. The results of mechanical tests showed the increase in young’s modulus after addition of ceramic phase (from 0.04 to 0.3 and 3 to 7 MPa, respectively). The hydrophilicity of polycaprolactone increased after adding nanozeolite and more weight loss was observed for scaffold containing 20% zeolite (53.52 6 1.6%) with an increase in the rate of hydroxyapatite formation. The results showed that the prepared scaffolds have potential for cancellous bone tissue engineering application.

In this research, effect of swarf addition on the microstructure of die cast aluminum A380 alloy and the possibility of altering the alloy structure in the metallic die has been studied. The microstructure mainly consists of the α-phase, eutectic, intermetallic compounds and porosity. Since the alloy solidifies under non-equilibrium conditions, the Scheil equation with exact amount of equilibrium distribution, analyzed by SEM-Line scan around an intermetallic phase at different mixing times as well as governing equations of thermal analysis, was used to calculate the solid weight fraction. Finally, using the thermal flux analysis in the crucible, a scientific prediction on the optimal amount of swarf addition, mixing time and temperature, was made. The shape factor at an optimum temperature of 590 °C was measured as 0.643. According to the optical microscope images of the die cast samples, the addition method (adding it to the floor or to the surface) and increasing the injection temperature have a significant effect on the solid weight fraction, morphology of the α-phases and final microstructure of the alloy.