Dendrite Coherency Research Articles

Effect of Cooling Rate on the Dendrite Coherency Point During Solidification of Al2024 Alloy

Most research related to dendrite coherency point (DCP) has been done on cast aluminum alloys and at a low cooling rate condition. In this research, the DCP of a wrought aluminum alloy is calculated in the range of high cooling rates used in the direct-chill casting process. The two-thermocouple thermal analysis technique was used to determine the DCP of Al2024 alloy. The aim of this work is to investigate the effect of different cooling rates on the dendrite coherency characteristics of Al2024. The cooling rates used in the present study range from 0.4 to 17.5 °C s−1. Also, the effect of 1.2 wt pct Al-5Ti-1B grain refiner on the DCP was studied. To calculate the solid fraction at dendrite coherency, solid fraction versus time is plotted based on Newtonian technique. The results show that by increasing the cooling rate, both time and temperature of dendrite coherency are decreased. Also, by adding the Al-5Ti-1B master alloy, dendrite coherency temperature is reduced and dendrite impingement is postponed. To reduce casting defects occurring during equiaxed solidification, e.g., macrosegregation, porosities, and hot tearing, these two operations which lead to postpone the transition from mass to inter-dendritic feeding, or dendrite coherency, can be useful. By increasing the cooling rate, solid fraction at dendrite coherency increases initially and then decreases at higher cooling rates. Presence of grain refiner leads to increasing of solid fraction at DCP. Thus, by delaying the dendrite coherency and increasing the solid fraction at DCP, semi-solid forming can be performed on parts with higher solid fraction and less shrinkage. Microstructural evaluation was carried out to present the correlation between the cooling rate and solid fraction in 2024 aluminum alloy.

Interplay among solidification, microstructure, residual strain and hot tearing in B206 aluminum alloy

Hot tearing is a complex phenomenon attributed to alloy solidification, microstructure and stress/strain development within a casting. In this research, the conditions associated with the formation of hot tears in B206 aluminum alloy were investigated. Neutron diffraction strain mapping was carried out on three B206 castings with varying levels of titanium (i.e. unrefined, 0.02 and 0.05 wt%). Titanium additions effectively reduced grain size and transformed grain morphology from coarse dendrites to fine globular grains. Further, thermal analysis suggested that grain refinement delayed the onset of dendrite coherency in B206 and therefore enhanced the duration of bulk liquid metal feeding for the refined casting conditions. As a result, the interactive effects of such factors resulted in a more uniform distribution of strain, and subsequent higher resistance to hot tearing for the grain refined castings.

Effects of Sb Content on Solidification Pathways and Grain Size of AZ91 Magnesium Alloy

The phase constitution and solidification pathways of AZ91 + xSb (x = 0, 0.1, 0.5, 1, in wt%) alloys were investigated through ways of microstructure observation, thermal analysis technique, and thermodynamic calculation. It was found that the non-equilibrium solidification microstructure of AZ91 + xSb (x = 0.1, 0.5, 1) is composed of α-Mg matrix, β-Mg17Al12 phase, and intermetallic compound Mg3Sb2. The grain size of the alloys with different Sb contents was quantitatively determined by electron backscattered diffraction technique which shows no grain refinement in Sb-containing AZ91 alloy. Thermodynamic calculations are in reasonable agreement with thermal analysis results, showing that the Mg3Sb2 phase forms after α-Mg nucleation, thus impossible acts as heterogeneous nucleus for α-Mg dendrite. Besides, the solid fraction at dendrite coherency point (f DCPs ) determined from thermal analysis decreases slightly with increasing Sb content, which is consistent with the fact that Sb does not refine the grain size of AZ91 alloy.

The Mechanical Properties and Corrosion Behavior of Quaternary Mg-6Zn-0.8Mn-xCa Alloys

In present study, the influence of calcium content on the microstructure, mechanical properties and corrosion behavior of quaternary Mg-6Zn-0.8Mn-xCa alloys, where x = 1, 1.5, 3 or 4.5 wt.% Ca, was examined. The grain structure of this quaternary alloy system became more refined with increasing additions of Ca. In addition to α-Mg, the Ca2Mg6Zn3 phase was found to be present in Mg-6Zn-0.8Mn-1Ca and Mg-6Zn-0.8Mn-1.5Ca according to microstructural and thermal analysis (TA). In addition to the α-Mg and Ca2Mg6Zn3 phases, the Mg2Ca phase was found to be present in the Mg-6Zn-0.8Mn-3Ca and Mg-6Zn-0.8Mn-4.5Ca alloys. Alloys with 1 or 1.5 wt.% Ca led to increases in the tensile strength of Mg-6Zn-0.8Mn, although further Ca additions had a deleterious effect. The TA of Mg-6Zn-0.8Mn-xCa during its solidification indicates that the fraction of liquid phase increases with increasing Ca content at the dendrite coherency point, leading to an increase in secondary phases and increased corrosion rate of Mg-6Zn-0.8Mn-xCa alloys.

Effects of Y on hot tearing formation mechanism of Mg–Zn–Y–Zr alloys

The hot tearing behaviour of magnesium alloys is one of the very important parameters to estimate the alloys actual application. The hot tearing mechanism of different Y content of Mg–Zn–Y–Zr alloys was studied in the present paper. The several important parameters of the mushy zone of MgZn2·5YxZr0·5 (x = 0·5, 1, 2, 4, 6) alloys were collected by thermal analysis method. The solidification temperature and shrinkage stress during the solidification of MgZn2·5YxZr0·5 alloys were acquired by using the ‘T’ type permanent mould hot tearing testing instrument and the attached computer. The fracture morphology and the cross section of hot tearing regions were observed by scanning electron microscopy. The results shown that the first and the second characteristic temperature of primary crystal nucleation Tn1-cc and Tn2-cc decrease with increasing yttrium content except MgZn2·5Y2Zr0·5 alloy, and the dendritic coherency temperature Tcoh had the same trend, while the temperature difference of the first and the second characteristic temperature of primary crystal nucleation (Tn1-cc−Tn2-cc) increases slowly with yttrium content. The solid fraction of dendritic coherent, fscoh, of MgZn2·5YxZr0·5 alloys are from 0·38 to 0·74, and the fscoh of MgZn2·5Y2Zr0·5 and MgZn2·5Y6Zr0·5 alloys are relatively lower, while the MgZn2·5Y4Zr0·5 alloy has the highest fscoh. By analysing the effect factors of hot tearing susceptibility of MgZn2·5YxZr0·5 alloys, such as mushy zone properties, the morphology of hot tearing regions and the solidification shrinkage stress curve, the hot tearing mechanism of MgZn2·5YxZr0·5 alloys can be described as follows: with the lower Y content, the main mechanism of hot tearing of MgZn2·5YxZr0·5 alloys is dendritic bridging; with the higher Y content, the main mechanism of hot tearing of MgZn2·5YxZr0·5 alloys is liquid film combined with solidification shrinkage repairing.

Phase Evaluation of Pure Mg, Mg-1Ca and Mg-1Ca-3Zn ‎Alloys by Thermal Analysis Used in ‎Medical Applications

In this research paper phase identification was conducted based on the thermal and microstructural analysis of pure Mg, binary ‎Mg-Ca and ternary Mg-Ca-Zn alloys which have been received significant attention to be used as biodegradable implants. ‎Thermal analysis results show that nucleation temperature of primary Mg decreased and solidification range expanded with ‎adding Ca and Zn to the Mg melt. Moreover, characteristics temperatures of intermetallic phase assess for Mg-1Ca and Mg-‎‎1Ca-3Zn alloys. Microstructural evolutions of specimens were characterized by optical microscopy, scanning electron ‎microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). The result revealed that the identification of Mg2Ca and Ca2Mg6Zn3‎intermetallic phases by thermal analysis which were also detected by EDS. Furthermore with addition of Ca and Zn elements, the ‎coherency time increased while, the fraction of primary α-Mg at dendrite coherency point (fαDCP) decreased.‎

Effect of Al–5Ti–1B on grain refinement, dendrite coherency and porosity of AZ91E magnesium alloy

The dendrite coherency point is an important characteristic governing the castability of an alloy. In this study, the effect of Al–5Ti–1B additions on the grain size and dendrite coherency point of AZ91E magnesium alloy was investigated. It was observed that additions of 0·005 wt-%Ti led to a decrease in the grain size and the porosity level of AZ91E. This was accompanied by a significant increase in the coherency fraction of solid. However, with further additions of titanium up to 0·30 wt-%, the grain size increased and the coherency fraction of solid decreased. This was ascribed to increased growth rate, resulting from a change in the dendritic morphology from equiaxed to a lancet structure.

English

The effects of different cooling rates on thermal profiles and microstructures of aluminium 7075 are presented in this paper. The 7075 alloy was heated in a graphite crucible to 750 °C. In the experimental work two thermocouples were used to record the temperatures at the centre and 30 mm from the centre of the graphite crucible. A slow cooling rate condition was achieved by placing the crucible into a chamber with Kaowool insulation. A higher cooling rate was achieved by placing the crucible in open atmosphere with controlled air flow over the crucible. The slow and high cooling rates were 0.03 °C/s and 0.4 °C/s respectively. The Data Acquisition (DAQ) system implemented using LabVIEW software to record the temperature-time profiles. Using Differential Thermal Analysis (DTA), the enthalpy of phase change at each temperature was estimated from the cooling curves. The changes of cooling rate were directly related to phase transformation including at liquidus, eutectic and solidus temperatures. The dendritic coherency point (DCP) was determined from analysis of the temperature difference between two thermocouples. The formation of DCP was found to be delayed with use of the slow cooling rate. DCP occurred at 610.2 °C (0.75 fraction solid) for the slow cooling rate and at 633.1 °C (0.3 fraction solid) for the higher cooling rate. The microstructure features were also found to alter significantly with the different cooling rates used. The microstructure was more spheroidal for the higher cooling rate compared with the slow cooling rate.

Synchrotron Radiography Studies of Shear-Induced Dilation in Semisolid Al Alloys and Steels

An improved understanding of the response of solidifying microstructures to load is required to further minimize casting defects and optimize casting processes. This article overviews synchrotron radiography studies that directly measure the micromechanics of semisolid alloy deformation in a thin-sample direct-shear cell. It is shown that shear-induced dilation (also known as Reynolds’ dilatancy) occurs in semisolid alloys with morphologies ranging from equiaxed-dendritic to globular, at solid fractions from the dendrite coherency point to ~90% solid, and it occurs in both Al alloys and carbon steels. Discrete-element method simulations that treat solidifying microstructures as granular materials are then used to explore the origins of dilatancy in semisolid alloys.

Front tracking approach to modeling binary alloy solidification

Purpose – The purpose of this paper is to endorse the idea of using a special post-calculating front tracking (FT) procedure, along with the enthalpy-porosity front tracking (EP-FT) single continuum model, in order to identify zones of different dendritic microstructures developing in the mushy zone during cooling and solidification of a binary alloy. Design/methodology/approach – The 2D and 3D algorithms of the FT approach along with different crystal growth laws were implemented in macroscopic calculations of binary alloy solidification with the identification of different dendrite zones developing during the process. Findings – Direct comparison of results predicted by the FT model with that based on the concept of the critical value of the solid volume fraction shows the sensitivity of the latter on an arbitrary assumed value of the dendrite coherency point (DCP). Moreover, for a carefully chosen DCP value the second model provides results that are close to those given by the FT-based approach. It is also observed that the macro-segregation pattern obtained by the proposed method is hardly influenced by chosen dendrite tip kinetics. Originality/value – To the best authors’ knowledge, for the first time the 3D FT model has been used along with the enthalpy porosity approach to simulate the development of zones of different dendrite morphology during binary alloy solidification. And, a weak influence of assumed different dendrite tip kinetics on the macro-segregation pattern has been proved, what justifies this underlying assumption of the EP-FT method.

Effects of Y on hot tearing susceptibility of Mg–Zn–Y–Zr alloys

The hot tearing susceptibility of MgZn2.5YxZr0.5 (x=0.5, 1, 2, 4, 6) alloys was evaluated by thermodynamic calculations based on Clyne–Davies model. The microstructure and morphology of hot tearing regions of the alloys were observed by X-ray diffraction and scanning electron microscopy. The solidification temperature and shrinkage stress during the solidification of MgZn2.5YxZr0.5 alloys in the “T” type hot tearing permanent-mold were acquired with the attached computer. The effect factors of hot tearing susceptibility of MgZn2.5YxZr0.5 alloys, such as the solidification temperature interval, the variation of solid fraction in vulnerable region, the residual liquid fraction in the final stage, the type of the second phase of the alloys were discussed based on the above calculation and observation. The results demonstrated that the hot tearing susceptibility in the investigated alloys was found as follows–MgZn2.5Y2Zr0.5>MgZn2.5Y0.5Zr0.5>MgZn2.5Y4Zr0.5>MgZn2.5Y6Zr0.5>MgZn2.5Y1Zr0.5. The highest hot tearing susceptibility of MgZn2.5Y2Zr0.5 alloy related to the following reasons: the largest freezing range, the biggest changing of the variation of solid fraction in vulnerable region, the least liquid film in the final stage of solidification, the formation of the second phase which worsens the liquid flow and interdendritic feeding after dendrite coherency.

Computer Aided Cooling Curve Analysis and Microstructure of Cerium Added Hypereutectic Al–Si (LM29) Alloy

Thermal analysis of LM29 alloy and Ce added LM29 alloys was carried out. The effect of cerium addition on solidification parameters and microstructural features of hypereutectic Al-Si (LM29) alloy was studied using Newtonian analysis technique. Thermal analysis parameters such as primary and eutectic phase nucleation and solidus temperatures were determined. The addition of Ce to LM29 alloy decreased the nucleation temperature of primary silicon and eutectic silicon. The microstructural examination of Ce added LM29 alloys revealed the presence of a polyhedral shaped Al–Si–Ce compound that might have caused the refinement of primary and eutectic silicon. The dendrite coherency point temperature of LM29 alloy was found to be suppressed on addition of Ce.

Determination of the Columnar to Equiaxed Transition in Hypoeutectic Lamellar Cast Iron

Shrinkage porosity as a volume change related casting defect in lamellar cast iron was reported in theliterature to form during solidification in connection to the dendrite coherency. The present w ...

Determination of Rigidity point/temperature using thermal analysis method and mechanical technique

The aim of this paper is to introduce a new method for determining Rigidity temperature applying cooling curve analysis. The determined values of Rigidity temperatures for three AlSi8Cu3 alloys with different contents of Strontium using thermal analysis technique have been compared with Rigidity temperatures obtained using mechanical technique (viscosity measurement). It was established that there is good correlation between measured values of Rigidity temperatures determined using both above mentioned techniques. Characteristic solidification temperatures such as liquidus, dendrite coherency point, Rigidity and solidus temperature have been recognized as important parameters of solidify aluminum alloys because they mark transitions between several types of feeding mechanisms. So far the Rigidity temperature can be only determined using mechanical technique. Potential application of Rigidity temperature as an input parameter for simulation software has been also discussed.

Effects of Zn and Ca Additions on the Solidification Behavior and Microstructure Analysis of Mg and Mg–6Zn Alloys

In this paper the influence of Zn and Ca additions on solidification behavior and microstructure analysis of Mg–6Zn and Mg–6Zn–4.5Ca alloys were investigated. Solidification behavior was evaluated by two thermocouple thermal analysis method. Microstructure and phase identification were performed via OM, SEM and EDX analysis. First derivative curves of Mg–6Zn alloys reveals two peaks related to α-Mg and MgZn at 520 and 370 °C respectively, While in Mg–6Zn–4.5Ca three peaks were detected. The microstructure result shows that the Cacontaining phase formation related to Ca content and Zn/Ca ratio in Mg–6Zn–4.5Ca. Thus the microstructure of Mg–6Zn–4.5Ca consists of primary Mg and eutectic (α-Mg+Mg2Ca+Ca2Mg6Zn3) phases. However the microstructure of Mg–6Zn composed of primary Mg and the second phase γ-MgZn which precipitate within the grain boundary. Thermal analysis result also exhibited that the addition of Zn elements to Mg alloy decreased the fraction of dendrite coherency point (f DCP α ) whilst the f DCP α increased by adding Ca to Mg–6Zn. Moreover coherency time increased with the addition of Zn and combination of Zn and Ca elements to Mg alloy.

Thermal characteristics and corrosion behaviour of Mg–xZn alloys for biomedical applications

The thermal parameters of Mg–xZn cast alloys with 0·5–9 wt% Zn were evaluated by using computer aided cooling curve thermal analysis (CA–CCTA), whereas the corrosion behaviour was investigated by potentiodynamic polarization and immersion tests. Thermal analysis results revealed that the dendrite coherency temperature (T DCP ) decreased from 642·2 to 600 °C with the addition of Zn from 0·5 to 9 wt%. The liquid fraction at coherency point (\({f}_{ L}^{ DCP}\)) increased by 72% when Zn was increased up to 9 wt%. MgZn intermetallic phase was observed in samples with <3 wt% Zn. At higher percentages of Zn, the Mg 51Zn 20 intermetallic phase was also detected in addition to α-Mg and MgZn by first derivative cooling curves under non-equilibrium solidification. All these phases were observed along the grain boundary when Zn was rejected from the solid/liquid interface and enriched in the triple conjunction of grain boundary. The grain size decreased from 185·2 to 71·5 μm when Zn content was increased. The addition of Zn content had a significant effect on the corrosion rate and the corresponding mechanisms. The corrosion rate decreased from 2·1 to 1·81 mmpy as Zn content increased from 0·5 to 3 wt%; afterwards, however, this value increased with further increase of Zn. Mg–3Zn also had the lowest degradation rate and highest corrosion resistance which can be fully utilized for biodegradable orthopedic applications.

Effect of Silicon Concentration on the Dendrite Coherency Point in Al–Si Binary Alloys

Dendrite coherency is important to the formation of the solidification structure. The coherency point is a temperature at which the microstructure starts to bridge and develop some mechanical resistance. It is still too early in the solidification process for hot tearing to develop. Dendrite coherency point (DCP) characteristics in Al–Si binary alloys have been studied by double thermocouples method during solidification process. The results indicate that the DCP and solid fraction at DCP are decreased with an increase in silicon concentration. As for the unrefined Al–xSi (x = 1, 3, 5, 7 and 9 wt%) system alloys, the solid fraction at DCP varies from 0.14 to 0.38 and the corresponding dendrite coherency temperature varies from 598.6 to 653.8 °C. In addition, there is an approximate nonlinear relationship between DCP and silicon concentration. For the binary Al–Si hypoeutectic alloys, the change of DCP is not obvious by the grain refinement and modification treatment for the melt.

Thermal Profiles and Fraction Solid of Aluminium 7075 at Different Cooling Rate Conditions

In order to determine suitable processing conditions for semi-solid aluminium 7075 thermal analysis (TA) was performed in order to obtain the relationship between fraction solid and temperature. During experimental work, the alloy was heated to 750°C by induction furnace and solidified at various cooling rates. Cooling curves for the metal were recorded with two thermocouples, one at the centre of the melt volume and one beside the containing crucible wall. A specially designed chamber with kaowool blanket was used to achieve the slowest cooling rate. The faster cooling rate was achieved with the crucible in open atmosphere with a set air flow rate over the crucible. A Data Acquisition (DAQ) system controlled by LabVIEW software was used to record the temperature-time profiles. From these cooling curves, the phase change at any corresponding time and temperature was estimated. The temperature difference between centre and wall of crucible was used to determine dendritic coherency point (DCP). Results show that, the slowest cooling rate with the kaowool blanket was at 0.03°C/s. An intermediate cooling rate of 0.21°C/s was achieved by leaving the melt to cool without kaowool blanket or forced air flow, and the fastest cooling rate was 0.43°C/s. The change in cooling rate altered the temperatures at which phase changes occurred, including those for eutectic and solidus. It was found that for lower the cooling rates that the DCP occurred at lower temperatures. The DCP for the cooling rate of 0.03 °C/s was found to be 574°C (corresponding to 0.85 fraction solid) whereas the DCP for 0.43 °C/s was found to be 623°C (corresponding to 0.55 fraction solid).

Computer-aided cooling curve thermal analysis of near eutectic Al–Si–Cu–Fe alloy

The effects of bismuth (Bi), antimony (Sb) and strontium (Sr) additions on the characteristic parameters of the evolution of aluminium dendrites in a near eutectic Al–11.3Si–2Cu–0.4Fe alloy during solidification at different cooling rates (0.6–2 °C) were investigated by computer-aided cooling curve thermal analysis (CA-CCTA). Nucleation temperature (\( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \)) is defined with a new approach based on second derivative cooling curve. The results showed that \( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \) increased with increasing cooling rate but both the growth temperature (\( T_{\text{G}}^{{\alpha {\text{ - Al}}}} \)) and the coherency temperature (TDCP) decreased. Increase in the temperature difference for dendrite coherency (\( T_{\text{N}}^{{\alpha {\text{ - Al}}}} - T_{\text{DCP}} \)) with increasing cooling rate indicate a wider range of temperature before the dendrite can impinge on each other and higher fraction solid (\( f_{\text{S}}^{\text{DCP}} \)). Additions of Bi, Sb and Sr to the base alloy produced only a minor effect on \( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \). Additions of Bi and Sb resulted in an increase in fraction solid and an increase of 30 % in the value of \( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \, - \,T_{\text{G}}^{{\alpha {\text{ - Al}}}} \) to almost 13 °C.

Numerical study of dendrite coherency during equiaxed solidification by the Discrete Element Method

Equiaxed solidification of Al alloys has been simulated by a continuum model in 2D, producing morphology variations from near-globular to highly-branched dendritic. The resulting microstructures were taken as initial samples to perform direct-shear simulations using the Discrete Element Method (DEM) and study the dendrite coherency point. Crystal rearrangement in response to direct-shear is analysed in different grain morphologies with a focus on force transmission and crystal translations and rotations. The simulations show that the coherency point decreases significantly as the morphology becomes more dendritic. Significant rotation was observed around the shear plane, leading to local dilation. The modelling results reproduce the key trends reported in prior experiments on the effect of grain size and morphology on dendrite coherency, and suggest that the coherency point depends on both the internal fraction of liquid within the crystal envelopes and also on the shape of envelopes.