Semi-solid Region Research Articles

Defects detection of GMAW process based on convolutional neural network algorithm

It is significant to predict welding quality during gas metal arc welding process. The welding defect detection algorithm has been developed based on convolutional neural network (CNN). The sensing system and image processing algorithm for molten pools has been developed. It overcomes the interference caused by the arc light to obtain clear images of the molten pool's boundaries. The molten pools images are used to build up training set and test set for training and testing the CNN model. The model is designed to extract the visual features of molten pool images to predict the penetration state, the welding crater, and slags. Through optimizing the network parameters such as kernel-size, batch-size and learning rate, the prediction accuracy is higher than 95%. Moreover, the model enhances additional focus on the welding crater based on the welder experience. The mechanisms between molten pool characteristics and welding defects were analyzed based on the welder experience and the visual features of the model. It is found that the model judges the occurrence of burn-through with the black hole in the middle zone of the molten pool. When the surface pores are generated, the model exhibits a strong response to circular voids in the semi-solid region at the trailing end of the molten pool. The size and shape of fusion holes exhibit a strong correlation with the molten state. When the shape of the crater does not appear concave, it often signifies excessive penetration. It contributes to enhancing the algorithm's robustness during various welding scenarios.

Towards relieving center segregation in twin-roll cast Al-Mg-Si-Cu strips by controlling the thermal-mechanical process

Serious center segregation greatly limits the application of twin-roll casting (TRC) technology for producing 6xxx alloy strips. Herein, Al-0.9Mg-0.6Si-0.2Cu-0.1Fe (wt.%, 6061) strips with different thicknesses were fabricated by TRC, and we found that the center segregation was well relieved with the thickness increased from 3 mm to 4 mm. To reveal the mechanisms of mitigation of center segregation in the 4 mm strip, various techniques including solidification simulation, crystallographic calculation, electron backscatter diffraction (EBSD), and electron probe micro-analyzer (EPMA) were utilized. The results disclosed that the Fe-containing phase in the 3 mm strip was π-AlFeMgSi, while the counterpart in the 4 mm strip was α-AlFeSi. The α-AlFeSi could serve as nucleation substrates for Mg2Si and Q-AlCuMgSi phases, thus promoting the uniform distribution of elements and preventing the accumulation of phases in the center region. Three matching planes between the α-AlFeSi and Q/Mg2Si were examined as: (112¯0)α-AlFeSi // (0001)Q, (0001)α-AlFeSi // (110)Mg2Si, and (112¯0)α-AlFeSi // (11¯0)Mg2Si. Meanwhile, the smaller roll separating force during the TRC process in the 4 mm strip could weaken the force-induced liquid flow behavior in the semi-solid region, which is the other reason for the alleviation of center segregation. Owing to the elimination of the center segregation, a more excellent fracture elongation was achieved in the as-homogenized 4 mm strip (∼29%) compared with the counterpart of the 3 mm strip (∼20%). This work may provide a strategy to eliminate the center segregation, thus further promoting the application of TRC process and producing high-performance Al alloy strips efficiently.

Formation of banded intergranular segregation and control via micro-alloying in twin-roll casted Al–Zn–Mg–Cu alloy with high solidification interval

For twin-roll casting (TRC) aluminum alloys, research on segregation defects has mainly focused on center segregation. However, intergranular segregation has been rarely investigated. In this study, we found that the banded intergranular segregation was a distinctive feature in TRC aluminum alloys. The formation and coarsening mechanism of this banded intergranular segregation in TRC Al–Zn–Mg–Cu alloys were revealed for the first time. The results showed that the banded intergranular segregation was contributed to the shear-induced dilation as the cast-rolling force was gradually applied to the crystals in the semi-solid region. Macroscopic inverse segregation along the dendrite channels occurred under the strong shear-induced dilation, which resulted in significant coarsening of the banded intergranular segregation. Furthermore, a new path controlling the center segregation and the banded intergranular segregation simultaneously by micro-alloying was proposed, which provides a strategy to develop TRC aluminum alloys with a high solidification interval.

Interfacial reaction of aluminum borate whisker reinforced Mg-10Gd-3Y-1Zn-0.4Zr (wt%) alloy matrix composite

Magnesium matrix composites usually possess high strength and stiffness as well as low coefficient of thermal expansion, which makes them promising in lightweight and energy-saving strategies. Although the interfacial bonding state has a crucial influence on the properties of magnesium matrix composites, the interfacial reaction mechanism of Mg-RE alloy matrix composites is still unclear. Herein, the interfacial reaction and microstructure of Mg-10Gd-3Y-1Zn-0.4Zr (wt%) alloy matrix composite reinforced with aluminum borate whisker (Al18B4O33w) were reported by heating the matrix to a semisolid region and inducing the interfacial reaction to continue. After heat treatment at 600 °C for 6 h, the thickness of the interface layer increased from 50‐120 nm to about 335 nm, and the main interfacial reaction product was (Mg0.4Al0.6)Al1.8O4. With the increase of heating time to 12 h, a new double interface layer with (Mg0.4Al0.6)Al1.8O4 in the inner layer and MgO in the outer layer was formed. The difference of interfacial reaction products during isothermal heat treatment was related to the concentration gradient of Mg atoms along the radial direction of the whiskers and the hindrance of the migration of Mg atoms by reaction products formed in the early stage. These findings provide a further understanding of the interfacial reaction of Al18B4O33w reinforced Mg-RE alloy matrix composites.

Atomic-scale investigation of coarsening kinetics by the phase-field crystal model

Coarsening is a common physical process that occurs in polydisperse two-phase mixture systems, which had been widely studied for decades. However, accurate prediction of the volume fraction dependence of the diffusion-controlled coarsening kinetic process is still very difficult. In this work, by using the atomic-scale phase-field crystal model, we investigated the coarsening kinetics of crystalline nanoparticles in the semi-solid region. The results showed that the details of the atomic-scale nature of particles do not affect the kinetics of coarsening for low solid volume fractions and the coarsening process of the nanoparticles is in agreement with the classical coarsening theory. While, for high solid volume fractions, our simulation results show that the coarsening rates of crystalline particles decrease with the solid volume fraction, which runs counter to the theoretical models based on mean-field theories. By checking the competitive growth process of all the particles, we found the appearance of grain rotation and volume diffusion mechanisms leads to the failure of the classical coarsening models.

Effect of Microstructures and Mechanical Properties of Large Deformation High Entropy Alloy CoCrCuFeNi in Semi-solid Isothermal Heat Treatment

The semi-solid slurries of the CoCrCuFeNi high entropy alloy (HEA) were fabricated through the recrystallization and partial melting (RAP) process by cold-rolling and partial remelting. The temperature range of the semi-solid region and the relationship between the liquid fraction and the temperature were determined by the differential scanning calorimetry (DSC) curve. The effect of isothermal temperature and holding time on the evolution of the microstructure and mechanical properties of the rolled samples was analyzed. The results show that the microstructure was significantly deformed, and the tensile strength has been increased by 107% after 63% rolling deformation of the CoCrCuFeNi high entropy alloy (HEA). The high-entropy alloy after cold rolling was maintained at 1150 and 1300 ° C for 20, 30, 60, and 120 minutes respectively, the plasticity has been improved compared with the rolled high entropy alloy. The optimal plasticity was reached 13.7% and 7.9% at 1150 ℃ and 1300℃ for 30 minutes, respectively. After semi-solid isothermal heat treatment, the grain morphology changed from dendritic of as-cast or rolled to spherulite and the grain size increased significantly with time and the holding temperature increased.

Using the Phase Field Method to Investigate Microstructural Evolution of Semi-Solid 357.0 Slurries

Semi-solid aluminum slurries have globular grains, different from traditional dendritic solidification microstructures. The mechanism responsible for the formation of these globular grains is still in dispute. Some researchers suggest that the globular grains are formed by fractured dendrites, while others report it is due to copious nucleation. This study will model the growth of the α-Al phase during the production of semi-solid slurries using the swirled equilibrium enthalpy device (SEED) process, where liquid alloy with a low superheat is poured into a steel crucible and swirled within the semi-solid region. Experimental observations have shown that the final microstructure of the slurry produced by the SEED process is relatively non-uniform, with large dendrites at surface and fine spherical particles at the center. Open source code for the phase field method has been adopted to simulate the different microstructural evolution for semi-solid alloy 357.0. The effect of localized temperatures under isothermal condition on the morphology of the α-Al particles has been investigated according to the special conditions of the SEED process. In addition, suggestions for modifying the slurry production to achieve more uniform microstructure are discussed.

Microstructure and Fabrication of Cu-Pb-Sn/Q235 Laminated Composite by Semi-Solid Rolling

In the present work, Cu-Pb-Sn and Q235 laminated composite were fabricated by a horizontal semisolid rolling procedure. The interfacial structure, elemental distribution, and properties of the composite were investigated. Finite-element simulation was conducted to analyze the temperature field and solidification process during the semisolid rolling. An appropriate semi-solid region was observed at a pouring temperature of 1598 K in the simulation, which would effectively kept fluidity and avoided casting defects. The experimental results showed that good interface between Cu-Pb-Sn alloy and Q235 steel was achieved by the proposed process at 1598 K, without casting defects or excessive deformation. The Cu and Fe alloys were bonded mainly by the diffusion of Fe into Cu matrix, and a handful of microscopic Pb-rich layer. Fine Pb-rich precipitates were uniformly distributed in the Cu-Pb-Sn alloy, and were considered to be advantageous to the self-lubrication property. The average tensile-shear strength of the interface was higher than 57.68 MPa at a pouring temperature of 1598 K, which fulfilled the requirements for a further extrusion process.

Semi-Solid Remelting of Magnesium Chips

Compact and loose magnesium chips were processed by means of remelting. The remelting was successfully performed using the new method of semi-solid melting, without the addition of flux, at temperatures between 580°C and 600°C. In this temperature range, the exothermic reaction between magnesium and the oxygen present in the surrounding atmosphere is avoided; in addition, the oxygen layer of the chips is stripped off by the particles of the semi-solid melt. Results show that more than 95% of the magnesium chips can be recovered as metal. Experiments were performed on different scales to obtain production parameters for the recycling process. Larger particle sizes of magnesium chips can be remelted faster than smaller ones. The ability to remelt at temperatures in the semi-solid region of alloys demonstrates the possibility of recovering virtually all of the metal from the chips.

Microstructural Features and Wear Characteristics of Semi-Solid Processed A356 Aluminum Alloy

In the present study the microstructural features and tribological characteristics of hot forged A356 Al alloy subjected to SIMA (Strain induced melt activation) and T6 heat treatment (semi-solid) processes have been investigated. The SIMA process consists of hot forging of alloy at 325˚C followed by cold forging at room temperature, isothermal holding at 580˚C for 10 min and quenching. In case of T6 heat treatment, the hot forged alloy was solution treated at 540˚C for 4 hours followed by quenching in cold water and artificial aging at 155˚C for 3 hours. The microstructure of the alloy exhibited a spherical or globular morphology of the primary α-phase with uniform distribution of solutes in the interdendritic region. A detailed analysis of the solidification behaviour of the melt from semi-solid region of the alloy is reported. The reasons for the consequential changes in tribological properties of A356 Al alloy processed by SIMA andT6 heat treatment have been clearly brought out.

Microstructure and Tribological Characteristics of Strain-Induced Melt Activation (SIMA)-Processed Al–10Cu–Fe alloy

In the present study, the influence of synthesis methods, namely the conventional metal mould casting (MMC) and strain-induced melt activation (SIMA) processes, on the microstructure and tribological characteristics of Al–10Cu–Fe alloy has been studied. The alloy was prepared by melting the commercial purity aluminium and copper. The effect of holding time in semi-solid region on the microstructure of the alloy subjected to SIMA was studied. Microstructural characterization was performed using optical and scanning electron microscopy, while chemical segregation is investigated using energy-dispersive spectroscopy. Hardness and strength of the resulting alloy are measured using macroindentation and tensile testing, respectively. Near-spherical grains were achieved in the SIMA process with an average grain diameter varying from 44 to 67 µm for holding times ranging from 30 to 55 min. The wear properties of stirred MMC alloy which was subjected to SIMA process are considerably better than those of either conventional MMCs or unstirred MMC subjected to SIMA process. The 50% pre-deformation followed by intercritical annealing at 580 °C and 30 min holding time is the optimum parameters to obtain the wear properties of Al–10Cu–Fe alloy. The improved wear and mechanical properties of the alloy are discussed in the light of the microstructural features of SIMA-processed alloy and the nature of the worn surfaces.

Compound forming of 7075 aluminum alloy based on functional integration of plastic deformation and thixoformation

A compound forming technology of functional integration of plastic deformation and thixoformation was proposed. The feasibility of the proposed forming technology was verified by near net shaping of empennage-shape component with complex geometric shape including a handle and 6 blades with a small thickness of 1.5mm. The handle and the blades were formed by plastic deformation and thixotropic deformation, respectively. 7075 alloy slurries with gradient microstructure containing thermoplastic, transition, and semisolid regions were obtained by gradient induction heating. The effect of heating rate during gradient induction heating on recrystallization of 7075 alloy was investigated. When the temperature of semisolid region was controlled in a suitable processing window between 590 and 630°C with a heating rate of 4°C/s, and empennage-shape component with homogenous distribution of hardness could be formed successfully by the compound forming technology.

A Comparative Study on the Microstructure Evaluation of Thixoforged A356 and A380 Aluminium Alloys

Abstract Semi-solid processes of aluminium alloys have been a focus of interest for many years. Thixoforging is a kind of forging process which is carried out in the semi-solid region of the alloy. Ingots used in thixoforging process cannot be produced by conventional ways; they must be prepared with different processing routes. In this study, low superheat casting (LSC) technique was used for ingot fabrication in thixoforging process. LSC can be summarized as the pouring of molten metal at a temperature very close to liquidus followed by rapid solidification. Low heat input combined with sudden cooling suppresses dendritic growth and the conventional casting structure converts into a more spherical structure. A356 and A380 are well-known casting aluminium alloys which are commonly used in sand and die casting processes. These alloys are appropriate for semi-solid process and there are many applications that researches have been carried out in time. In view of that, A356 and A380 alloys were selected as subject of this comparative study and final structures were evaluated together. A LSC combined thixoforging process was successfully applied to both alloys and a spherical microstructure was obtained. High sphericity with a simple microstructure is the characteristic of the A356 alloy. A380 shows relatively lower sphericity with more complex microstructure which has numerous different intermetallic compounds.

Different Techniques of Determination of the Cracking Criterion for Solidification in Casting

Cracking which occurs during the solidification process is a serious defect in castings and welds. Alloys are susceptible to cracking during solidification. Al alloys, stainless steels and Ni-base alloys are notable examples. Cracking takes place in the semisolid region called the mushy zone. During solidification, shrinkage and thermal contraction of the mushy zone and its surrounding solid are hindered, tensile deformation can induce cracking (in the semisolid region) along grain boundaries that are not fed with sufficient liquid phase. The cracking criterion is focused on events occurring at the grain boundary, including separation of grains from each other, lateral growth of grains toward each other, and liquid feeding between grains. The susceptibility factor of a binary aluminum alloy to cracking during solidification was also described. The criterion has an effect upon: (a) the lateral growth rate of two neighboring grains toward each other to bond together to resist cracking, and (b) the length of the grain-boundary liquid channel through which feeding has to occur to resist cracking. In future work the criterion will be compared with experimental data and computer simulation during solidification of binary aluminum alloys in welding.

Constitutive behavior of aluminum alloy in a wide temperature range from warm to semi-solid regions

The constitutive behavior of aluminum alloy 7075 was characterized using compression tests performed over a wide range of temperatures (350–600 °C, from warm deformation temperature to semi-solid region) and strain rates (10−3–1 s−1). This work is of significance for the development of compound forming which combines plastic forming and semi-solid processing. The constitutive behavior in the wide temperature range was divided into two types: plastic deformation when the alloy is in solid state and containing a small fraction of liquid (350–490 °C); thixotropic deformation when the alloy is in the semi-solid state (above 490 °C). An adjusting factor which denotes the effect of liquid on the constitutive behavior was proposed. The constitutive model of peak stress as a function of temperature and strain rate was obtained for the wide range of temperatures (350–600 °C). The average relative errors between the calculated and measured values are 3.88% (350–490 °C) and 4.189% (above 490 °C), respectively.

Liquid rolling of woven carbon fibers reinforced Al5083-matrix composites

A liquid rolling method was successfully developed for the fabrication of woven carbon fibers reinforced Al5083-matrix composites. The woven fibers were coated with Ni/P to facilitate wetting of the fibers by the molten matrix. It was found that the composite fabricated in this way had a good microstructure with few defects, which was most likely due to the occurrences of pressure infiltration, rapid solidification, and hot-deformation strengthening on the materials. Finite-element simulation was conducted to analyze the temperature field and solidification process during the liquid rolling. A wide semi-solid region was observed in the simulation, and it would effectively avoid the fiber damages. The results of three-point bending measurements implied that the application of liquid rolling and the addition of reinforcements could both offer obvious improvements to the materials. The liquid rolling method could increase the matrix bending strength by 43% than the casting method. Furthermore, when the liquid rolling was used, the addition of woven fibers could offer an improvement of 22% to the matrix.

Cross-Permeability of the Semisolid Region in Directional Solidification: A Combined Phase-Field and Lattice-Boltzmann Simulation Approach

Based on the results of microstructure simulations, fluid flow through the semisolid region during directional solidification of the technical Ni-base alloy 718 has been studied. Three-dimensional microstructures at different positions in the semisolid region were obtained by using a multicomponent multiphase-field model that was online coupled to a commercial thermodynamic database. For the range of five different primary dendrite distances λ 1 between 50 µm and 250 µm, the flow velocity and the permeability perpendicular to the dendrite growth direction was evaluated by using a proprietary Lattice-Boltzmann model. The commercial CFD software ANSYS FLUENT was alternatively applied for reference. Consistent values of the average flow velocity along the dendrites were obtained for both methods. From the results of the fluid flow simulations, the cross-permeability was evaluated as a function of temperature and fraction liquid for each of the five different primary dendrite distances λ 1. The obtained permeability values can be approximated by a single analytical function of the fraction liquid and λ 1 and are discussed and compared with known relations from the literature.

An Investigation on the Effects of Pre-deformation and Temperature on Thixotropic Microstructure of AA2014 Aluminum Alloy

With copper as the main alloying addition, alloy from the 2XXX family offers high strength at low specific weight and are widely employed for aircraft, wheels and major structural components. However, they suffer from low forming speeds and require high forming pressures. Hence, it is very attractive to form these alloys in semisolid state for technical and economical reasons. In this study, one of the most important wrought alloys for aerospace applications, AA2014 was used. In order to form the best semi-solid forming products, thixotropic character must be known. For this purpose, AA2014 alloy as-cast and warm extruded samples were heated to 600, 610 and 620 °C in semi-solid region and held for 0, 5 and 10 min in the induction coil. All samples were cooled with water at room temperature. Microstructure analysis was conducted with metallographic methods and shape factor values were calculated. With increasing predeformation rate and heating temperature, the shape factor increases. Experimental results show that spherical shaped thixotropic microstructures which is required for semi-solid forming can be created in AA2014 alloy.

Characterization of Newly Developed Semisolid Stir Joining Method for Cast Cu Base Alloy (Cu-Al-Si-Fe) and Effect of Stirrer Type on Uniformity of Microstructure

In this research, the semisolid stir joining method was used to overcome the problem of hot cracking in welding aluminum and silicon bronzes. Moreover, the effects of grooved and cylindrical tools on the microstructure and mechanical properties of samples were examined. After welding specimens, mechanical tests were carried out to find differences between the cast and welded samples. Optical microscopy and scanning electron microscopy were used to study microstructure. X-ray diffraction was used to investigate compounds formed during casting and welding. The solidus and liquidus temperatures of the alloy were measured by differential scanning calorimetry. In this study, the temperature of the work pieces was raised to 1203 K (930 °C) that is in the semisolid region, and the weld seams were stirred by two different types of tools at the speed of 1600 rpm. Macro and micro-structural analyses show uniformity in the phase distribution for specimens welded by cylindrical tool. Desirable and uniform mechanical properties obtained when the cylindrical tool was used.

Unconventional Microstructures in Tool Steel Obtained by Semi-Solid Processing and Subsequent Heat Treatment

Mechanical properties of all metals depend predominantly on the type and morphology of their microstructure. Microstructure attributes can be altered by various heat treating and thermomechanical treatment procedures. One of the advanced techniques profoundly affecting the microstructure evolution is semi-solid processing. It can produce unconventional microstructures even in conventional steel types. Moreover, subsequent heat treatment can also deliver a wide range of microstructures and correspondingly varied mechanical properties. In the present experimental programme, the X210Cr12 ledeburitic tool steel was studied. Its initial annealed microstructure consisted of ferritic matrix, chromium carbides and globular cementite particles. The semi-solid processed structure, on the other hand, contained polyhedral austenite grains embedded in carbide-austenite network. The austenite volume fraction exceeded 95 %. This microstructure was then altered by subsequent heat treatment or thermomechanical treatment. The experimental programme comprised three stages. At the first stage, the effects of the rate of cooling from the semi-solid region to the ambient temperature on the nature and morphology of the ledeburitic network and the austenitic grain size were explored. The second stage was aimed at the impact of tensile and compressive deformation applied after transition through semi-solid state on the microstructure evolution and, in particular, on grain size. Once suitable processing conditions and parameters were identified, the treatment led to a recrystallized austenitic microstructure with an average grain size of less than 3 μm. As high volume fractions of austenite were obtained, the third stage involved exploring the effects of thermal exposure. The stability of austenite and the decomposition of austenite into other microstructure constituents were mapped. Metallographic observation revealed a resulting wide range of microstructures from fine pearlite to martensite, depending on the heat treating schedule.