Microstructure and mechanical properties of lost foam cast 356 alloys

Microstructure and mechanical properties of lost foam cast aluminum alloys have been investigated in both primary A356(0.13% Fe) and secondary 356(0.47%). As expected, secondary 356 shows much higher content of Fe-rich intermetallic phases, and in particular the porosity in comparison with primary A356. The average area percent and size(length) of Fe-rich intermetallics change from about 0.5% and 6 μm in A356 to 2% and 25 μm in 356 alloy. The average area percent and maximum size of porosity also increase from about 0.4% and 420 μm to 1.4% and 600 μm, respectively. As a result, tensile ductility decreases about 60% and ultimate tensile strength declines about 8%. Lower fatigue strength was also experienced in the secondary 356 alloy. Low cycle fatigue(LCF) strength decreased from 187 MPa in A356 to 159 MPa in 356 and high cycle fatigue(HCF) strength also declined slightly from 68 MPa to 64 MPa.     

Microstructure and mechanical properties of magnesium alloy prepared by lost foam casting

The microstructure and mechanical properties of AZ91 alloy prepared by lost foam casting(LFC) and various heat treatments have been investigated. The microstructure of the AZ91 alloy via LFC consists of dominant α-Mg and β-Mg17Al12 as well as a new phase Al32 Mn25 with size of about 5-50μm, which has not been detected in AZ91 alloy prepared by other casting processes. The tests demonstrate that the as-cast mechanical properties are higher than those of sand gravity casting because of chilling and cushioning effect of foam pattern during the mould filling. The solution kinetics and the aging processes at different temperatures were also investigated by hardness and electrical resistivity measurements. The kinetics of aging are faster at the high temperature due to enhanced diffusion of atoms in the matrix, so the hardness peak at 380℃ occurs after 10 h; while at the lower aging temperature(150℃), the peak is not reached in the time(24 h) considered.     

金属型铸造和消失模铸造Mg-Gd-Y-Zr合金的组织结构和力学性能

研究金属型铸造和消失模铸造Mg-10.1Gd-3.74Y-0.25zr(质量分数,%)合金的组织结构和力学性能.结果表明:采用两种铸造工艺得到的合金具有相似的铸态组织,均由а-Mg固溶体相、а-Mg+Mg<,24>(Gd,Y)5共晶相和立方状Mg5(Gd,Y)相组成:但金属型铸造合金的晶粒尺寸明显小于消失模铸造合金.经...     

Partially amorphous stainless steel coatings: Microstructure, annealing behavior and statistical optimization of spray parameters

A stainless steel powder of a mixed amorphous and crystalline structure was HVOF sprayed in an effort to produce coatings with a large glass fraction. In the first part of this work, the microstructure and annealing behavior of powder and coatings are studied. The coatings consisted of a glassy part and a martensitic part, the latter with boride, borosilicide and boro-carbide dispersions. The annealing behavior of powder and coatings is characterized by glass crystallization and martensite tempering. Annealing of the powder leads to complete microcrystallization of the glassy part, whereas annealing of the coatings eventually leads to nanocrystallization of the residual glass phase. In the second part, the effects of selected spraying parameters (oxygen-to-fuel ratio, powder feed rate, spraying distance and spraying stages) on characteristic coating properties are investigated by means of the Taguchi analysis. The oxygen-to-fuel ratio mostly affected the coating hardness and porosity. The powder feed rate had a significant effect on all the coating properties but mostly on the deposition rate and crack extension force. Spraying in stages significantly increased the deposition rate, whereas it promoted coating amorphicity. A spraying experiment under the optimum conditions determined by the Taguchi analysis, showed a good fit between the predicted and the attained property values.     

Microstructure and high-temperature wear behavior of laser clad Ni-Ti-Si ternary metal silicide coatings

Ni-Ti-Si ternary metal silicide coatings were fabricated on AISI 304 stainless steel by laser cladding process. The coatings consisted of Ni₁₆Ti₆Si₇ primary dendrite and interdendritic Fe-Ni-based solid solution γ and exhibited excellent abrasive and adhesive wear resistance under high temperature metallic dry sliding wear conditions. The excellent wear properties were attributed to the high hardness and covalent dominant atomic bond of the metal silicide Ni₁₆Ti₆Si₇. The dominant wear mechanism of the coating were delamination of the coating and material transfer from the mating surface.     

Microstructure evolution and corrosion behavior of Fe-Al-based intermetallic aluminide coatings under acidic condition

Two Fe-Al-based intermetallic aluminide coatings were fabricated on 430-SS (Fe-Cr) and 304-SS (Fe-Cr-Ni) substrates by pressure-assisted solid diffusion bonding with coating on pure Fe as control. The microstructure and intermetallic phases of the coatings were characterized by SEM, EDS and EBSD. A network of Cr₂Al₁₃ with matrix of Fe₄Al₁₃ was formed by inter-diffusing of Al with the substrates. The corrosion behavior of intermetallic coatings was investigated in 0.5 mol/L HCl solution by mass-loss, OCP, Tafel plot and EIS. It was found that corrosion resistance was greatly enhanced by dozens of times after the addition of Cr and Ni compared with that on pure Fe. The presence of cracks in the coating on 430-SS provided a pathway for corrosion media to penetrate to the substrate and accelerated the corrosion rate. Moreover, the corrosion product was analyzed by XRD, demonstrating that the addition of Cr and Ni facilitated the formation of more corrosion resistant phases, and therefore improved corrosion resistance.     

Microstructure,corrosion behavior,and surface mechanical properties of Fe oxide coatings on biomedical ZK60 Mg alloy

The Mg‐5.5Zn‐0.6Zr (in wt%, ZK60) alloy has been surface modified by dual Fe&O ion implantation and deposition (II&D) under different O₂ fluxes from 0 to 40 sccm. The microstructure is investigated by glancing angle X‐ray diffraction, atomic force microscope, and scanning electron microscopy. The results show that the modified layers, with a gradient microstructure of outer deposition region and inner implantation region, are composed of α‐Fe + Fe&Mg mixture, FeO/Fe‐rich oxide + Fe&Mg mixture, and α‐Fe₂O₃/Fe‐rich oxide + Fe&Mg mixture for 0, 10, and 40 sccm O₂ fluxes, respectively. The electrochemical and immersion tests in 37°C Hank's solution indicate an improvement in corrosion behavior under 0 and 10 sccm O₂ fluxes, but a deterioration in corrosion resistance under 40 sccm O₂ flux. In addition, the nanoindentation tests suggest that the dual Fe&O II&D simultaneously enhances the surface hardness and elastic modulus due to the formation of Fe and its oxide coatings.     

Status quo and development trend of lost foam casting technology

Lost foam casting（LFC）technology has been widely applied to cast iron and cast steel.However,the development of LFC for Al and Mg alloys was relatively slower than that for cast iron and cast steel.The application of LFC to Al and Mg alloys needs more effort,especially in China.In this paper,the development history of LFC is reviewed,and the application situations of LFC to Al and Mg alloys are mainly discussed.Meanwhile,the key problems of LFC for Al and Mg alloys are also pointed out.Finally,the prospects for LFC technology are discussed,and some special new LFC technologies are introduced for casting Al and Mg alloys.In future,the development trends of green LFC technology mainly focus on the special new LFC methods,metal material,coating,heat treatment,new foam materials as well as purification technology of tail gas,etc.     

Porosity of aluminum alloy in lost foam casting process

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅｌｏｓｔｆｏａｍｃａｓｔｉｎｇ (ＬＦＣ) ｐｒｏｃｅｓｓｉｓｃａｌｌｅｄａｓｔｈｅ 2 1ｓｔｃｅｎｔｕｒｙｔｅｃｈｎｏｌｏｇｙ .Ｉｔｓｆｏｒｍｉｎｇｉｓｄｉｆｆｅｒｅｎｔｆｒｏｍｔｈａｔｏｆｔｈｅｃｏｎｖｅｎｔｉｏｎａｌｅｍｐｔｙ ｃａｖｉｔｙｃａｓｔｉｎｇｍｅｔｈｏｄ[1,2 ].ＦｏｒａｌｕｍｉｎｕｍａｌｌｏｙｉｎＬＦＣ ｐｒｏｃｅｓｓ,ｐｏｕｒｉｎｇｔｅｍｐｅｒａｔｕｒｅｏｆａｌｕｍｉｎｕｍｍｅｌｔｉｓｍｕｃｈｈｉｇｈｅｒｔｈａｎｔｈａｔｏｆｔｈｅｅｍｐｔｙ ｃａｖｉｔｙ…     

Permeability of coating in the lost foam casting process

This study measured the pressures developed during a filling experiment using molten grey iron in the lost foam casting (LFC) process. The measured pressures included back pressure and permeable pressure. The back pressure is created in-mould from the metal-foam interface. The permeable pressure was measured on the mould side of the coating, when gases introduced from the decomposing EPS were permeating through the coating. The permeable pressure reflected the permeability of the coating while filling molten iron. Variables, which included particle sizes, types of binders, weight per cent of binder in coatings, influenced the permeability of coating at room temperature and at casting temperature.

Coatings made from flaky alumina developed a greater total length of pore channels than those made from angular silica. Increasing the particle size of the refractory increased the permeability of the coating at room temperature. The highest permeable pressure was measured when the particle size was 74 μm and acrylic resin was used as a binder. The optimum binder content was less than 1% for flaky alumina and acrylic resin binder. The optimum coating thickness was less than 0.5 mm to develop a maximum permeable pressure associated with a minimum back pressure in the mould.     

Microstructure and dry sliding wear behavior of laser clad AlCrNiSiTi multi-principal element alloy coatings

The approximately equimolar ratio AlCrNiSiTi multi-principal element alloy(MPEA) coatings were fabricated by laser cladding on Ti-6A1-4V(Ti64) alloy.Scanning electron microscopy(SEM),equipped with an energy-dispersive spectroscopy(EDS),and X-ray diffraction(XRD) were used to characterize the microstructure and composition.Investigations show that the coatings consist of(Ti,Cr)_5Si_3 and NiAl phases,formed by in situ reaction.The phase composition is initially explicated according to obtainable binary and ternary phase diagrams,and the formation Gibbs energy of Ti_5Si_3,V_5Si_3 and Cr_5Si_3.Dry sliding reciprocating friction and wear tests of the AlCrNiSiTi coating and Ti64 alloy substrate without coating were evaluated.A surface mapping profiler was used to evaluate the wear volume.The worn surface was characterized by SEM-EDS.The hardness and wear resistance of the AlCrNiSiTi coating are well compared with that of the basal material(Ti64).The main wear mechanism of the AlCrNiSiTi coating is slightly adhesive transfer from GCr15 counterpart,and a mixed layer composed of transferred materials and oxide is formed.     

WC增强Fe基喷熔层组织及磨粒磨损性能

以Fe60自熔合金粉末和碳化钨粉末为原料,利用氧-乙炔火焰喷熔方法在45钢表面制备了碳化钨复合材料耐磨喷熔层;用扫描电子显微镜(SEM)、场发射扫描电镜(FEGSEM)和能量色散谱仪(EDS)等技术分析了喷熔层的显微组织结构;考察了复合喷熔层在磨粒磨损性能.结果表明:WC颗粒均匀分布在复合材料喷熔层组织中,其组织结构均匀并弥散分布着细小的硬质相;复合材料喷熔层在滑动干摩擦条件下表现出优异的耐磨粒磨损性能,碳化钨对喷熔层有显著的增强作用.     

Simulation of mould filling in lost foam casting process

In this investigation, an algorithm was developed to calculate the gas pressure at the melt/foam interface (gap) owing to degraded foam during mould filling in the lost foam casting process (LFC). The effect of back-pressure on mould filling was modelled using a new experimental function by the addition of a three-dimensional volume of fluid (3D-VOF) function. The molten flow and free surface were simulated using the solution algorithm-VOF (SOLA-VOF) numerical technique. To simulate the three-dimensional incompressible flow in the LFC, the pressure boundary conditions, heat transfer and foam gas pressure effect were modified. Finally, in order to verify the computational results of simulation, a thin plate of grey iron was poured into a transparent foam mould. The mould filling process was recorded using a 16 mm high-speed camera, then images were analysed frame-by-frame to obtain foam depolymerisation and gap volume during mould filling. The comparison of experimental and numerical results in the LFC filling sequence showed good agreement, which confirms the accuracy of the model. IJCMR/447     

Preparation and high temperature oxidation behavior of refractory disilicide coatings for γ-TiAl intermetallic compounds

Novel refractory disilicide layers were applied to γ-TiAl to enhance oxidation resistance at 1050 °C. NbSi₂ and MoSi₂ layers were prepared by joining thin Nb and Mo foils to γ-TiAl surfaces, and siliconizing the combinations (Nb/γ-TiAl, and Mo/γ-TiAl) using molten salts. The coatings and their oxidation behavior were characterized using X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy techniques. Isothermal oxidation tests showed that the oxidation resistance of uncoated γ-TiAl at 1050 °C in air was insufficient, and scale spallation occurred. NbSi₂ coatings were formed and adhered firmly to the γ-TiAl substrate, whereas Mo film detached from the substrate surface causing failure of the MoSi₂ coatings. Oxidation of the NbSi₂-coated γ-TiAl (NbSi₂/Nb/γ-TiAl) at 1050 °C in air showed improved oxidation resistance at exposure times up to 100 h. Microstructural and compositional developments of the coating at prolonged time were discussed. The NbSi₂ coatings provided sufficient oxidation resistance for γ-TiAl at 1050 °C in air, and have potential use in high temperature applications.     

Numerical simulation and analysis of mould filling process in lost foam casting

In lost foam casting （LFC） the foam pattern is the key criterion, and the filling process is crucial to ensure the high quality of the foam pattern. Filling which lacks uniformity and denseness will cause various defects and affect the surface quality of the casting. The influential factors of the filling process are realized in this research. Optimization of the filling process, enhancement of efficiency, decrease of waste, etc., are obtained by the numerical simulation of the filling process using a computer. The equations governing the dense gas-solid two-phase flow are established, and the physical significance of each equation is discussed. The Euler/Lagrange numerical model is used to simulate the fluid dynamic characteristics of the dense two-phase flow during the mould filling process in lost foam casting. The experiments and numerical results showed that this method can be a very promising tool in the mould filling simulation of beads＇ movement.     

Microstructure and properties of thermal-sprayed NiCrWRE coatings

The powders of NiCrW and NiCrWRE alloys were flame sprayed on a medium-carbon steel substrate by thermal spray welding. The micro- structure and tribological behavior of coatings were studied experimentally by means of scanning electron microscopy （SEM）, field emission gun scanning electron microscope （FEGSEM）, and wear tests. The addition of CeO2 modifies the coating morphology from a needle-like structure to a roughly cubic morphology; the refining and purifying effect of rare earth elements makes the microstmcture more compact and finer. Analysis of the worn surfaces reveals that the coatings with CeO2 addition show improved abrasive wear resistance over those without CeO2. By adding CeO2, the hardness of the coatings is significantly increased, and the wear resistance of the coatings is enhanced.     

A study of thermal parameters and interdendritic feeding in lost foam casting

Thermal parameter-based criterion functions are of great practical importance to predict dispersed microporosity in castings. Using an experimental approach, the present study is a pioneering application of the thermal parameter-based criterion functions to predict the microporosity formation in lost foam casting (LFC) of A356 alloy. A series of plate castings with controlled hydrogen content were made with various thermal conditions by varying the dimensions of feeders and in gate design. Extensive analysis of the thermal parameters shows that changing the gate and feeder design modifies the distribution of the thermal parameters; furthermore, it influences the distribution of microporosity along the central line of the plate castings. The thermal parameters and criterion functions based on Darcy’s law show consistency, thus supporting the interdendritic feeding mechanism of microporosity formation. The criterion functions such as Niyama, LCC, and KCL can be used to predict microporosity formation during solidification of LFC, depending on the hydrogen content in the casting. The limitation of the interdendritic feeding mechanism and the effect of hydrogen on the gas pore formation are also discussed.     

反应等离子熔覆Fe-Cr-Ti-C涂层的组织与性能

采用等离子表面熔覆技术,以高能等离子束为热源在Q235基体钢板上熔覆无钛以及含钛(其它粉末成分基本不变)的铁基合金涂层.利用光学显微镜(OM)、扫描电镜(SEM)、X射线衍射仪(XRD)、电子探针(EPMA)、显微硬度计对涂层的组织、相组成和显微硬度等进行分析.结果表明,与无钛铁基粉末涂层相比,含钛铁基合金熔覆层晶粒组织明显细化,且含有较多带状晶,但随着合金粉末中钛含量的增多,熔覆层共晶组织中硬质相(Cr,Fe)7C3逐渐增多,抑制了硬质相的析出,熔覆涂层的平均和最高显微硬度值也相应降低.     

Heat transfer characteristics of lost foam casting process of magnesium alloy

1 Introduction The growing demand for mass reduction in aerospace and automotive industries has greatly increased the magnesium application. Currently, casting is the main industrial forming method for magnesium alloys, but the lag of research and develop…