Globular Grains Research Articles

Transmission Electron Microscopy Studies of X210CrW12 and 100CR6 Thixo-Cast Steels

Two kinds of deformed carbon steel were used as a feedstock for thixoforming: X210CrW12 tool steel cold worked, of composition 2,1 %C, 10,5 %Cr, 0,7 %W, 0,4 %Si (all in weight %), and bearing steel 100Cr6: (0,9% C, 1,4% Cr, 0,4% Mn, 0,3% Si, 0,2% Cu). The microstructures of X210CrW12 steel after thixoforming process at temperature of 1230°C, performed at 30 % of liquid show presence of globular grains (average size from 30 µm 50 µm) surrounded by the eutectic mixture. Measured chemical composition of the austenite in globules was: 2,5% C, 0,4% Si, 0,7% Mn, 12% Cr, 1,3% W, 0,1%V, 87% Fe and that of the eutectic mixture: 5%C, 0,4% Si, 0,6%, Mn, 1,1%, W, 17,5% Cr, 75,4% Fe in thixocast of X210CrW12 steel. Measured carbon content is too high, most probably due to contamination effect. TEM studies of this steel after thixoforming shows that the eutectic mixture consists of ferrite α-Fe and Cr7C3 carbides. Thixoforming of 100Cr6 bearing steel was conducted at temperature of 1420°C at about 25 % of liquid fraction. Thixocast microstructure consists of globular grains consisting of martensite needles and residual austenite (20 μm to 40 μm in size and average composition 0,9%C, 1,4 %Cr, 0,5%Mn, 0,3%Si). The chemical composition of the eutectic is following: 2,4% C, 2,6% Cr, 0,2%Si, 0,6%Mn.

Microstructure Formation Mechanism of AZ31 Magnesium Alloy Processed by Rheo-Forming

AZ31 magnesium alloy sheets with a cross section of 5mm×50mm were prepared by semisolid metallic rheo-rolling device self-designed, and microstructure formation mechanism of AZ31 magnesium alloy during rheo-rolling was investigated. The results reveal that during the preparation of semisolid slurry, melt firstly nucleates heterogeneously on the surface of sloping plate. Under the vibration and shear of sloping plate, a relatively homogenous temperature field and composition filed are formed around some grains and are favorable for the direct growths of globular grains; meantime, dendrites formed under the vibration and shear are broken up and rounded gradually and better semisolid slurry is formed. During rheo-rolling, under the action of roll interfacial friction and rolling force, laminar flow shear inside the alloy is enhanced, and grains are further broken up and elongated. When the pouring temperature is 670°C, excellent AZ31 magnesium alloy sheet with good microstructure can be obtained.

Composition segregation in semi-solid metal cast AA7075 aluminium alloy

The composition distribution and microstructural evolution associated with the characteristic globular grain structure in semi-solid metal cast AA7075 aluminium alloy has been investigated. The primary globular grains possess uniform composition over nearly 90% of their diameter with fluctuations only occurring in the near surface of the individual grains. The inter-globular regions are solute rich and contain non-equilibrium eutectic. The segregation pattern in semi-solid cast structures occurs over a much greater length-scale than the dendritic pattern prevalent in conventional castings and it is proposed that the distinction arises from predominant planar growth of the α-primary globular grains during the SSM conditioning stage. Examination of the in-grain structure using electron backscattered diffraction revealed a quasi-periodic fluctuation in misorientation that may have arisen from local perturbations on the planar solid interface as solidification progressed. Homogenisation prior to artificial ageing is severely compromised by the undesirable segregation pattern.

Microstructure Evolution of AS21 Magnesium Alloy via the SIMA Process

AS is a relatively new series of Magnesium alloys. The microstructure of this alloy can be improved for semisolid processing. The current research is concerned with the microstructure evolution of AS21 under the strain induced melt activated (SIMA) process. For this purpose, the AS21 alloy is cast and compressed 10-40% at 200 °C. The semisolid heat treatment is completed in a carbonate salt bath at different temperatures between 600-620 °C. The microstructure studies show that there is no favourable microstructure evolution between 600-610 °C. At 615 °C fine globular grains are obtained with the most desired mean grain size and sphericity of 67 µm and 81%, respectively. At 620 °C an undesirable coarsening phenomenon occurs that damages the microstructure globularity. SEM micrographs show that in a successful SIMA processing, the Mg2Si phases are broken into fine particles distributed within the grains and grain boundaries.

The Effect of TiB<sub>2</sub> Particles on the Microstructure of Semi-Solid 7075 Alloy Slurry

In situ reaction and low superheat degree pouring technologies have been successfully used to prepare semi-solid TiB2/7075 aluminum matrix composite slurry. The experimental results have shown that the TiB2 particulates become the stable nucleus of alpha-Al during the solidification of 7075 Al alloy. Moreover; high speed impact formed in the serpentine channel and stainless steel beaker is beneficial to get fine globular grains. The microstructure of 3 mass%TiB2/7075 composite is composed of uniform globular grains. The mean grain sizes and shape factors of the composite are 23 mu m, 0.90 and 26 mu m, 0.89 at 655 degrees C pouring temperature when one or two curved channels were used respectively. [doi: 10.2320/matertrans.M2011328]

Ultrasonic Processing and Microstructural Analysis of AZ91 Magnesium Alloy

In the present study ultrasonic vibrations are used for the grain refinement of AZ91 alloy during its solidification. The microstructures and mechanical properties of the treated AZ91 alloy are characterized. It is found that due to the acoustic cavitation and flows induced by ultrasonic vibration during the nucleation stage, the formation of the fine uniform globular grains are attributed to the combination of the enhanced heterogeneous nucleation and dendrite fragmentation. The average grain size decreases with increase in ultrasonic intensity but the largest reduction occurs mainly from 1.34KW/cm2 to 2.69K W/cm2; the benefits of further increase in the ultrasonic intensity from 2.69KW/cm2 to 4.031KW/cm2 are small.

Microstructure Evolution during Solution Heat Treatment of Semisolid Cast 2024 Aluminum Alloy

The aim of this study is to determine the appropriate solution treatment temperature and time of semi solid 2024 Al alloy. Solution heat treatment at 450°C and 480 °C for various times, from 4 hours to 16 hrs, were applied followed by artificial aging at 220 °C for 1 hr. Microstructure of the semi solid cast 2024 aluminum alloy mainly showed globular grain structure which consisted of matrix-α (Al) and grain boundary (GB) - eutectic phases (α+Al2CuMg/Al2Cu). Eutectic GB phases was found to completely dissolved after solution heat treatment at 480°C for 14 hrs while sample solution treated at 450°C for the same time showed the existence of remaining GB phases. Prolonging heat treatment after 14 hrs at both temperatures resulted in the formation of coarse black particles at the grain boundaries which were identified as Mg2Si phases. Therefore the suitable solution treatment of the alloy in this study was at 480°C for 14 hrs.

Microstructure and Wear Behavior of Ti-Matrix Functional Gradient Layer

To improve the wear-resistance of Ti600, and alleviate thermal stress as well under hyperthermal and excessive temperature difference condition, experiments of Ti-matrix FGM on Ti600 substrate by pulsed Nd:YAG had been carried out. The microstructures, microhardness and phase composition of FGM were investigated. And the friction wear properties of FGM, N-FGM and Ti600 substrate were examined in atmosphere. It was found that a rapidly solidified microstructure consisting of in situ synthesized TiC reinforced phase distributed on FGM substance evenly and dispersely in the form of globular grains had three main different shapes:bulky or imperfect arborescent crystal, fine or approximately equiaxial crystal and chopped fibriform crystal. With the increasing of original Cr3C2content, the number and size of arborescent crystal increased obviously. The distribution of main alloy components was gradient and continuous, presenting the same composition and regularity to the originally preset alloy powder. FGM and N-FGM had an average hardness of approximately 1450, 4.5-5 times of Ti600 substrate, attributed to TiC primary dendrites. With the reducing of weight percentage of TiC, microhardness decreased gradiently and continuously. Meanwhile, the wear resistance of FGM and N-FGM was improved obviously. Friction coefficient, wear loss and wear rate decreased significantly, 0.3-0.5 times of Ti600 substrate.

An investigation on the microstructural evolution and mechanical properties of A380 aluminum alloy during SIMA process

Microstructural evolution and tensile properties of thixoformed near net shape parts of A380 aluminum alloy have been studied. The effect of plastic deformation and holding time in the appropriate semi-solid temperature has been investigated in the strain-induced melt activating process (SIMA). The results of image analysis showed that by increasing the deformation value from 5% to 11%, the recrystallized α-Al grains gradually refined as a result of the increasing the nucleation rate. At the value of 11% plastic deformation a dominant globular structure of α-Al grains was obtained. However, further increasing in the plastic deformation (14%) reduced considerably the shape factor. The equivalent diameter and the shape factor of the globular grains were 70.3 μm and 79%, respectively, after about 11% of plastic deformation. Holding time of the specimens at the semi-solid temperature has been studied from zero to 30 min. It was found that the holding time of 25 min is the best to prevent grain growth. The mechanical properties of thixoformed alloys were also investigated. The results showed that the yield, tensile strength and elongation have been increased considerably using thixoforming, compared with the as-cast condition.

Optimising the porous structure of heterogeneous reforming catalysts with a globular model of the grain

The theory of a globular grain model was used to construct an algorithm for analysing the problem of how the porous structure of heterogeneous catalysts affects their efficiency in model reforming reactions. Our analysis has substantiated the undeniable advantage of applying the bidispersive grain structure since its effectiveness is many times as high as the effectiveness of the monodispersive grain structure. Apparently, the method described in the present paper will be equally efficient when applied to heterogeneous catalysts made use of in other technological processes.

Effects of Sr Addition on as Cast Microstructure of AS31 Alloy

The effect of Sr addition on the as-cast microstructure of AS31 alloy was studied. The experimental results show that the grain size decreased obviously from 1116μm to 221μm for AS31 alloy with addition of 0.05%Sr, and promote the formation of globular grains. The morphology of eutectic Mg2Si phase is changed from Chinese script to polyhedral shape or fine fibers shape. The grain refinement mechanism is attributed to the enrichment of Sr element at solidification interface front, which restricts the growth of α-Mg primary dendrite. The modification of the eutectic Mg2Si phases results from the dissociative Sr changing the preferred growth manner of them.

Characteristics of Eutectic Precipitations in Selected Magnesium Cast Alloys

. The paper focuses on characterization of eutectic precipitations in three magnesium alloys (MSR-B, ZRE1 and WE43) containing rare earth elements. It was shown, that the structure of the tested alloys in a cast state consists of globular grains of the α-Mg solid solution and eutectic areas of different chemical composition and morphology. An untypical influence of the pouring temperature on the eutectic precipitations area fraction was observed. This phenomenon can be explained by impoverishment of the eutectic-forming elements in the areas of the slowest solidification. This results from the fact, that at the highest pouring temperature a considerable amount of eutectic forms in places solidified as the first.

Microstructure evolution and mechanical properties of an intermetallic Ti-43.5Al-4Nb-1Mo-0.1B alloy after ageing below the eutectoid temperature

Intermetallic γ-TiAl based alloys with a chemical composition of Ti-(42–45)Al-(3–5)Nb-(0.1–2)Mo-(0.1–0.2)B (in atom percent) are termed TNMTM alloys. They exhibit several distinct characteristics, including excellent hot-workability and balanced mechanical properties. In this study, the relationship between microstructure and mechanical behavior in a Ti-43.5Al-4Nb-1Mo-0.1B alloy after two different heat treatments was investigated. One of the analyzed microstructures consisted of lamellar γ-TiAl/a2-Ti3Al colonies with a small volume fraction of globular γ-TiAl and β0-TiAl grains at their grain boundaries, whereas the second microstructure basically exhibited the same arrangement of the microstructural constituents, but a fraction of the lamellar colonies was altered by a cellular reaction. The prevailing microstructures have been analyzed by means of scanning electron microscopy and transmission electron microscopy. Macro- and micro-hardness measurements as well as room temperature tensile tests have revealed that the sample with both cellular and lamellar features show lower yield stress and hardness than the ones exhibiting undisturbed lamellar microstructures. The strength and hardness properties are primarily connected to the lamellar spacing within the colonies, where strength increases with decreasing lamellar spacing. The appearance of a cellular reaction leads to a refinement of the lamellar colonies which in turn influences positively the plastic fracture strain at room temperature.

Influence of ultrasonic melt treatment on microstructure and mechanical properties of AlSi9Cu3 alloy

A novel MMM (Multi-frequency, Multimode, Modulated) ultrasonic (US) technology was used to refine the as cast microstructure and improve the mechanical properties of a AlSi9Cu3 alloy. Ultrasonic vibration was isothermally applied to the melt for 120 s at different temperatures slightly above the liquidus temperature of the alloy, using different electric power values, before pouring into a metallic mould. The microstructure of the cast samples was characterized by optical and scanning electron microscopy and energy dispersive spectrometry. Ultrasonic vibration promoted the formation of small α-Al globular grains, changed the size and morphology of intermetallic compounds and distributed them uniformly throughout the castings. Ultimate tensile strength and strain were increased to 332 MPa and 2.9%, respectively, which are 50% and 480% higher than the values obtained for castings produced without vibration. The microstructure morphology and the alloy mechanical properties were found to depend on the electric power and the melt temperature, and by using a suitable combination of these parameters it is possible to achieve high refinement efficiency by treating the melts in the liquid state.

PREPARATION OF SEMISOLID 5052 ALUMINUM ALLOY SLURRY BY INDIRECT ULTRASONIC VIBRATION

A new indirect ultrasonic vibration (IUV) apparatus was employed to prepare semisolid 5052 aluminum slurry. The effects of IUV temperature and time on the morphology of the semisolid slurry were studied. The results indicate that semisolid slurry with fine and globular primary grains can be obtained by applying IUV from the liquidus temperature (643 ) of this alloy or a higher temperature for about 50 s. When IUV is applied through the bottom of the metal cup at 643 ,a lot of nuclei form near the bottom of metal cup and these nuclei grow into globular grains. In this stage, the average diameter of primary grains decreases as IUV time increased up to 30 s and then it increases with further increasing of IUV time from 30 s to 60 s, and the average shape coefficient of primary grains increases as IUV time increased. When the IUV is applied at 640 , dendrites can be broke up and a mixture structure consisted of rosette and globular grains with a average diameter of 105 μm is obtained.

Study of Semisolid and ECAP Processes on Al-Fe-Si Alloy - Microstructure and Kinetic Grain Growth

ECAP (Equal Channel Angular Pressing) is one of the useful methods of Severe Plastic Deformation (SPD) to reach ultrafine grain size. ECAP was carried out on Al-Fe-Si alloy. All grains stretched along the ECAP direction. Samples were ECAPed for one pass into an ECAP mold with two equal channels (1 cm × 1 cm) with 90o between them. After ECAP, specimens were heat treated (semisolided) for different times and temperatures for achieving globular grains. Optical microscopy has been used to evaluate the microstructure. By ECAP and semisolid processes, the structure becomes fine and globular. The kinetic grain growth has been studied for this alloy by the calculation of the D = Ktn equation and log(D)-log(t) curves.

Influence of conditions of consolidation on the structure of mechanically doped Ti-Al-Nb composition

The article discusses the capability of producing materials of intermetallic type of the Ti-Al-Nb system by mechanical alloying of elemental powders with subsequent sintering or hot pressing. Temperature intervals of active shrinkage of the material at sintering and variation of the phase composition at hot pressing are established. The mechanically doped alloy based on titanium aluminides has globular grain structure irrespective of conditions of consolidation.

Study of effect of boric acid on Zn–Co alloy electrodeposition from acid baths and on composition, morphology and structure of deposit

The effect of boric acid as additive in electrodeposition of Zn–Co alloy coatings from an acidic sulphate electrolyte containing NH4Cl, Na2SO4 and citric acid Na3C6H5O7, at ambient temperature, without agitation has been investigated. The deposition process was investigated by cyclic voltammetry. It was found that the current density first decreased and then increased, due to adsorption of a boric acid complex and/or changes in the morphology, but the initial deposition potential was not affected. The addition of boric acid to the bath led to the formation of improved Zn–Co deposits, composed of coalesced globular fine grains smaller than ∼2 μm in diameter. Also, all of the Zn–Co deposits studied consisted of η phases. The Co content in the Zn–Co deposits produced in the presence of boric acid increased from 2·5 to 4·3 wt-%. It is suggested that Zn–Co deposits produced in the bath containing 60 g L−1 boric acid probably offer sacrificial protection to the steel substrate.

Preparation of Semisolid A356 Alloy by a Cooling Slope Processing

In order to obtain the non-dendritic feedstock for the semisolid forming process, a cooling slope processing was used. In this work, the effects of the angle, length of cooling slope and pouring temperature on the microstructure of A356 aluminum alloy were investigated. It showed that these parameters affect the size and morphology of α-Al phase to some extent. The results indicate that a pouring temperature of 650°C and a cooling slope with 45° in angle and 50 cm in length are the optimum parameters for preparing fine and globular grain structures. To eliminate the solidification shell formed in the surface of cooling slope, a nitride dope was coated on the surface of the cooling slope.

Microstructural evolution and mechanical properties of back-extruded Al 7075 alloy in the semi-solid state

The thixoforming process is a new method for manufacturing complicated and net shape components through which high strength materials can be formed more easily. In this study 7075 Al alloy which has low extrudability has been thixoformed by backward extrusion process. The recrystallisation and partial melting (RAP) route was used to obtain the semi-solid feedstocks for thixoforming. Microstructural evolution during partial remelting was studied at temperatures for times. Results showed that a fine and globular microstructure can be obtained by the RAP route. The results showed that high semisolid isothermal temperature would increase the liquid volume fraction and accelerate the spherical processing of the solid particles. Furthermore at long holding time, the globular grains coarsened slightly and the average grains size are increased. The experimental results showed that when the semisolid billet is hold at 580°C with the holding time, less than 30 min, the microstructure of the billet is composed of spherical grains and remnant liquids, the average grain size are smaller than 100 μm. So the remelted billet is suitable for thixoforming. In this paper, a back-extruding of 7075 Al alloy with a high solid fraction in the semi-solid state at 580°C for 10 min was performed. Mechanical properties of thixoformed components at room temperature were examined. Tempering treatment T6 has been applied after thixoforming to investigate the effects of heat treatment on mechanical properties of thixoformed parts. The tensile properties and low hardness values in the as-thixoformed 7075 Al alloy were improved by subsequent heat treatment. Post-forming heat treatment is one of the key parameters for improving the mechanical properties of thixoformed parts.