Effect of melt convection on primary dendrite arm spacing in directionally solidified Pb-26%Bi hypo-peritectic alloys

Primary dendrite arm spacing（PDAS） of α phase in directionally solidified Pb-26%Bi（mass fraction） hypo-peritectic alloys was measured by considering the effect of melt convection in cylindrical samples with different diameters.The experimental results show the measured PDAS increases with increasing diameter of the sample.At the growth velocity of 5 μm/s,its value changes from 161.5 μm for the sample with 1.8 mm in diameter to 240.4 μm for the sample with 7 mm in diameter.The strong melt convection in large diameter samples causes a high bulk alloy composition and a high concentration gradient in peritectic β phase,resulting in a larger PDAS.Simultaneously,the high concentration gradient could effectively promote the peritectic transformation,enhancing the dissolution of the thin α dendrite.

Influence of directional solidification variables on primary dendrite arm spacing of Ni-based superalloy DZ125

The primary dendrite morphology and spacing of DZ125 superalloy have been observed during directional solidification under high thermal gradient about 500 K/cm. The results reveal that the primary dendrite arm spacing decreases from 94 μm to 35.8 μm with the increase of directional solidification cooling rate from 2.525 K/s to 36.4 K/s. The regression equation of the primary dendrite arm spacings A, versus cooling rate is λ1=0.013（GV）-0.32. The predictions of Kurz/Fisher model and Hunt/Lu model accord reasonably well with the experimental data. The influence of directional solidification rate under variable thermal gradient on the primary dendrite arm spacing has also been investigated.

Correlation Between Process Parameter and Primary Dendrite Arm Spacing in Laser Welding of Cu and Al

A combined numerical model of thermal field and the primary dendrite arm spacing （PDAS） was proposed to correlate the process parameters and PDAS in laser welding of Cu and A1. The solidification parameters simulated by the finite volume method with commercial software ANASYS FLUENT were applied in the PDAS model to predict the dendrite arm spacing of fusion zone. Dendrite was also examined by the metallographic method to validate the model. Results indicate that the calculated PDAS agrees with metallographic measurements reasonably, especially the Hunt model. PDAS increases apparently with increasing laser power while decreases slightly with increasing welding speed. Increasing laser power increases the secondary dendrite and increasing welding speed increases the microporosity in dendrite.

Kinetic nucleation of primary α(Al) dendrites in Al-7%Si-Mg cast alloys with Ce and Sr additions

Nucleation of dendritic primaryα（Al） phase with addition of element Ce and Sr in hypoeutectic Al-7%Si-Mg cast alloy was investigated by using differential scanning calorimetry （DSC） and scanning electron microscopy. DSC results were used to calculate the activation energy and nucleation work of primaryα（Al） phase. The results show that the values of activation energy and nucleation work are decreased and the nucleation frequency is increased with the additions of Ce and Sr to the alloys. Moreover, the grain size of dendriticα（Al） phase is well refined, and the nucleation temperatures of primaryα（Al） dendrites are decreased with the additions of Ce and Sr. The effects of elements Ce and Sr additions on kinetic nucleation of primary α（Al） phases were also discussed in hypoeutectic Al-7%Si-Mg cast alloy.

Prediction of Primary Dendrite Arm Spacing in Pulsed Laser Surface Melted Single Crystal Superalloy

Primary dendritic arm spacing(PDAS)is an important microstructure feature of the nickel-base single crystal superalloys.In this paper,a numerical model predicting the PDAS evolution with additive manufacturing parameters using pulsed laser is established,which combines the theoretical PDAS models with the temperature field calculation model during pulsed laser process.Based on this model,processing maps that related process parameters to the evolution of PDAS are generated.To obtain more accurate prediction model,the parameters of different solidification conditions,G^(-0.5)V^(-0.25) and G^(-0.5)V^(-0.25),are selected to calculate PDAS.The simulation results show that the PDAS increases as the arise of P and t.The processmgPDAS map can accurately predict the evolution of PDAS with pulsed laser process parameters,which is well in accordance with the experimental results.Additionally,the PDAS values calculated by the G^(-0.5)V^(-0.25) are more in line with the experimental results than those calculated by the G^(-0.5)V^(-0.25).

History-dependent Selection of Primary Dendritic Spacing in Directionally Solidified Alloy

Directional solidification experiments were carried out for succinonitrile-1.0 wt pct acetone alloy with the orientation of dendritic arrays being not parallel to the direction of the temperature gradient. Experimental results show that there exists an allowable range of primary dendritic spacing under a given growth condition.The average primary spacing depends not only on the current growth conditions but also on the way by which the conditions were achieved. The upper limit of the allowable range becomes smaller in comparison with that with 〈001〉 direction of dendrite arrays parallel to the direction of the temperature gradient, which means that the history-dependence of dendritic growth is weaker under this condition. The lower limit obtained is compared with a self-consistent model, which shows a good agreement with experimental results.

Effect of abruptly changing withdrawal rate on solidification microstructure in directionally solidified Al-4.5wt%Cu alloy

Al-4.5wt.%Cu alloy has been directionally solidified at constant and abruptly changing withdrawal rates, respectively. The effects of the withdrawal rate on solidification microstructure, primary dendrite arm spacing(PDAS) and liquid solute distribution in front of the solid-liquid interface were investigated. The experimental results for the PDAS at a constant withdrawal rate agree well with the values calculated by the Hunt, Trivedi and Hunt-Lu models. At an abrupt change in the withdrawal rate, the maximum to minimum ratio of the PDAS at a given solidification parameter, i.e. λ1max/λ1min, is more than 2, and the PDAS values are remarkably history-dependent. Further, the liquid-solute distribution curve based on theoretical calculation shows that the larger the initial withdrawal rate is, the smaller the minimum of liquid solute concentration in front of the solid-liquid interface is after the abrupt change in withdrawal rate.

Effect of Process Parameters on Defects,Melt Pool Shape,Microstructure,and Tensile Behavior of 316L Stainless Steel Produced by Selective Laser Melting

Previous studies have revealed that laser power and energy density are significant factors affecting the quality of parts manufactured by selective laser melting(SLM).The normalized equivalent density E_(0)^(*) and dimensionless laser power q^(*),which can be regarded as a progress on the understanding of the corresponding dimensional quantities,are adopted in this study to examine the defects,melt pool shape,and primary dendrite spacing of the SLM-manufactured 316 L stainless steel,because it reflects the combined effect of process parameters and material features.It is found that the number of large defects decreases with increasing E_(0)^(*) due to enough heat input during the SLM process,but it will show an increasing trend when excessive heat input(i.e.,utilizing a high E_(0)^(*))is imported into the powder bed.The q^(*) plays an important role in controlling maximum temperature rising in the SLM process,and in turn,it affects the number of large defects.A large q^(*) value results in a low value of absolute frequency of large defects,whereas a maximum value of absolute frequency of large defects is achieved at a low q^(*) even if E_(0)^(*) is very high.The density of the built parts is greater at a higher q^(*) when E_(0)^(*)remains constant.Increasing the melt pool depth at relatively low value of E_(0)^(*) enhances the relative density of the parts.A narrow,deep melt pool can be easily generated at a high q^(*) when E_(0)^(*) is sumciently high,but it may increase melt pool instability and cause keyhole defects.It is revealed that a low E_(0)^(*) can lead to a high cooling rate,which results in a refined primary dendrite spacing.Relatively low E_(0)^(*) is emphasized in selecting the process parameters for the tensile test sample fabrication.It shows that excellent tensile properties,namely ultimate tensile strength,yield strength,and elongation to failure of 773 MPa,584 MPa,and 46%,respectively,can be achieved at a relatively low E_(0)^(*) without heat treatment.

Microstructure and Stress-Rupture Property of Large-Scale Complex Nickel-Based Single Crystal Casting

The microstructure and stress-rupture property of the large-scale complex single crystal（SX） casting DD10 were investigated in high-rate solidification process. It is found that the primary dendrite arm spacing（PDAS） does not increase monotonically with the height increase. When across the platform, the temperature gradient increases due to the effect of platform, and the corresponding PDAS decreases. The distribution of eutectic volume fraction in large-scale complex SX casting is affected by PDAS, solid back diffusion, and the development of high order dendrites. The eutectic volume fraction contained in the sample taken below the platform decreases with the height increase. While the eutectic volume fraction contained in the sample taken upper the platform increases gradually with the height increase. After heat treatment,most of the γ/γ＇ eutectics are eliminated and the components are distributed uniformly. The similar stress rupture properties of the samples at different heights in the same direction are obtained.

Microstructure Evolution and Growth Orientation of Directionally Solidified Mg–4 wt% Zn Alloy with Different Growth Rates

The microstructure evolution and growth orientation of directionally solidified Mg-4 wt% Zn alloy in the growth rate range from 20 to 200μm/s were investigated. A typical cellular structure was observed with a growth rate of 20 μm/s, and the cellular spacing was 115 μm. When the growth rate increased to 60 μm/s, cellular structure with some developed perturbations was obtained and the cellular spacing was 145 μm, suggesting that the cell-to-dendrite transition happened at the growth rate lower than 60 μm/s. As the growth rate further increased, the microstructure was dendritic and the primary dendritic arm spacing decreased. The relationship between the primary dendritic arm spacings and the growth rates was in good agreement with Trivedi model during dendritic growth. Besides, X-ray diffraction and transmission electron microscopy analyses showed that the growth direction of directionally solidified Mg-4 wt% Zn alloy was （1120） lay in {0002} crystal plane, and the preferred orientation was explained with the lattice vibration model for one-dimensional monatomic chain.