Last-stage solidification of alloys: Theoretical model of dendrite-arm and grain coalescence

Hot tearing in castings is closely related to the difficulty of bridging or coalescence of dendrite arms during the last stage of solidification. The details of the process determine the temperature at which a coherent solid forms; i.e., a solid that can sustain tensile stresses. Based on the disjoining-pressure concept used in fluid dynamics, a theoretical framework is established for the coalescence of primary-phase dendritic arms within a single grain or at grain boundaries. For pure substances, approaching planar liquid/solid interfaces coalesce to a grain boundary at an undercooling (ΔT _b ), given by

The mechanism of grain refinement during solidification of Zn-Ti base alloys

Through cooling curve determinations, metallography, and electron microprobe analysis the mechanism of grain refinement in Zn-Ti and Zn-Ti-Cu alloys was determined. It is shown that zinc-titanium-oxide particles in the melt act as nucleants. Through probe analysis and considerations of matching of crystallographic planes it appears that the nuclei are the spinel Zn₂TiO₄. Additions of lead to Zn-Ti melts drastically reduces the effectiveness of the nucleant.     

Thermodynamic calculations of the effective solidification range and its relation to hot cracking of aluminum-based ternary alloys

The correlation between experimental hot cracking (HC) and the calculated effective solidification range (ESR) (obtained as the difference between the formation temperature of a definite amount (65–90 wt %) of the solid phase and the nonequilibrium solidus temperature) is investigated by the example of Al-Cu, Al-Mg, and Al-Si binary systems. It is revealed that the location of the HB peak almost coincides with the calculated ESR peak. A good convergence between the calculated and experimental ESRs is established. The correlation between the HB and calculated ESR is, in general, considerably worse in the studied Al-Cu-Mg, Al-Cu-Si, and Al-Si-Mg ternary systems. It is shown that a correlation between the HB and calculated ESR that is similar to binary systems can be obtained for the radial joins of ternary systems if all alloys compared by the HB are crystallized by identical reactions with the participation of identical phases.     

Nonequilibrium solidification of Fe-P alloys

The specific features of solidification of Fe-P alloys in the concentration range 5?C25 at % P have been studied by differential thermal analysis and X-ray diffraction. The equilibrium ??-Fe + Fe₃P eutectic is shown to form immediately from melt only for compositions with 17?C19 at % P when a melt is superheated below temperatures of 1130?C1160°C, at which the type of composition short-range ordering changes (Fe₃P ?? Fe₂P) under insignificant (??50°C) supercooling conditions. A nonequilibrium ??-Fe + Fe₂P eutectic forms during cooling of the alloy at a large (??200°C) supercooling.     

Eutectic solidification of aluminum-silicon alloys

A mechanism that describes nucleation and growth as well as morphology modification by chemical additives of the eutectic phases in aluminum-silicon hypoeutectic alloys is presented. The mechanism is supported with results of nonequilibrium thermal analyses, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), selected area electron diffraction, and elemental X-ray mapping, as well as results of high-temperature rheological measurements that are performed on alloy samples of precisely controlled chemistry.     

Mixing enthalpies and calorimetric investigations of liquid iron-vanadium alloys

By means of a special levitation alloying calorimeter (LAC) the mixing enthalpies of liquid Fe-V alloys were determined in the iron rich range, up to 35 at.% V at a mean temperature of 1990 K; in the vanadium rich range, up to 32 at.% Fe at a mean temperature of 2320 K. The results were verified by a drop calorimetric experiment, where the enthalpy of an Fe-V alloy with 69 at.% Fe was measured. The total systematic error of the LAC was calculated to be within ± 10 %. Different thermodynamic models were applied to describe the results. By the quasi-chemical model the experimental results for the mixing enthalpies in kJ/mol are expressed by the equation ΔH^m=x_Fex_V(?28.61–80.58x_Fex_V).     

The nucleation and solidification of Al-Ti alloys

A series of hyperperitectic Al-Ti alloys at 0.35, 0.5, 0.7 and 0.8 wt pct Ti has been frozen at rates varying from less than 1°C/s to in excess of 100°C/s. Cooling-curve analyses, metallographic and microprobe examinations, taken altogether, allow identification both of the nucleants and the solidification modes acting in this important alloy system. Two sets of conclusions are drawn, one in general about low concentration peritectic systems like Al-Ti, and the other about particular interactions in Al-Ti. For example, it is revealed that Al_xTi compounds exist; Al₃Ti, Al_xTi and Al are nucleated by TiC; and Al_xTi and TiC are both nucleants for aluminum. Formerly of the Ford Scientific Staff, is now Senior Engineer, Société National d’Etude et de Construction de Moteurs d’Aviation, Gennevilliers, France.     

Alternating grain orientation and weld solidification cracking

A new mechanism for reducing weld solidification cracking was proposed, based on the concept of the crack path and resistance to crack propagation, and its effectiveness was verified in magnetically oscillated GTA welds of a rather crack susceptible material 2014 aluminum alloy. This mechanism,i.e., alternating grain orientation, was most pronounced in welds made with transverse arc oscillation of low frequency and high amplitude, and solidification cracking was dramatically reduced in these welds. The effect of the arc oscillation pattern, amplitude, and frequency on the formation of alternating columnar grains and the reduction of solidification cracking in GTA welds of 2014 aluminum alloy was examined and explained. The present study demonstrated for the first time that columnar grains can, in fact, be very effective in reducing solidification cracking, provided that they are oriented favorably.     

Densities of Pb-Sn alloys during solidification

Data for the densities and expansion coefficients of solid and liquid alloys of the Pb-Sn system are consolidated in this paper. More importantly, the data are analyzed with the purpose of expressing either the density of the solid or of the liquid as a function of its composition and temperature. In particular, the densities of the solid, Eqs. [15] and [16], and of the liquid, Eqs. [24] and [25], during dendritic solidification are derived. Finally, the solutal and thermal coefficients of volume expansion for the liquid are given as functions of temperature and composition (Figure 9).     

On solidification of hypereutectic Al-Si alloys

Precipitation of primary silicon was studied in Al-Si hypereutectic alloys with 15, 18, and 25 wt. % silicon content. The alloys were solidified with different cooling rates from different super heat temperatures. The liquidus and eutectic temperature were evaluated from the cooling curves. The liquidus temperature was found to decrease with cooling rate. The evaluation of microstructure showed that the fraction of primary silicon decreased with increasing the cooling rate and super heat temperature. Furthermore, the morphology of the primary silicon changed as an effect of cooling rate and super heat temperature.     

The directional solidification of Pb-Sn-Cd alloys

The growing interest in composite structures for new material applications makes it necessary to determine just how generally we can apply existing solidification theory to controlled three-phase ternary solidification. The Pb-Sn-Cd ternary eutectic system was used as a suitable model system to completely map the phase morphology as a function of G/R and compositions. By carefully controlling the freezing rate and the thermal gradient in the liquid ahead of the solid-liquid interface (in the range 400 to 500 C/cm) the following areas of interest were investigated: 1) the effect of growth velocity and composition on coupled structures, 2) ternary impurities and their effect on the minimum G/R for coupled growth in a binary system, 3) the effect of growth velocity and composition on the nonplanar interface structures, and 4) the adaptability of present theories (the constitutional supercooling criterion and Cline’s binary analysis) in predicting the region of coupled growth in a three-component eutectic system growing at steady-state. It was found that much of the one and two-phase directional solidification theory and terminology can be directly extended to a ternary eutectic system. This suggests a further extension to n-phase, m-component systems (m ≥ n) with at least a qualitative understanding of the solidification process. The Authors wish to acknowledge the support of the National Science Foundation which made this study possible.     

Microstructure of Al-Ti-B-Er refiner and its grain refining performance

Al-Ti-B-Er refiner was successfully prepared by CR (contact reaction process), a process based on SHS (self propagating high-temperature synthesis). The microstructure of the alloy was studied by optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy-dispersive spectrometry. The results showed that Al-Ti-B-Er alloy was composed of a-Al, block-like TiAl₃ and flocked TiB₂. Compared with Al-Ti-B refiner, formation of TiAlEr compounds, Er modified the morphology of TiAl₃ phase, and dispersed the TiB₂ and TiAl₃. An excellent grain refining performance was obtained when adding 1 wt.% Al-Ti-B-Er in Al-10Zn-1.9Mg-1.6Cu-0.12Zr alloy, the average grain size was about 40 µm. The refinement mechanism of Al-Ti-B-Er was also discussed. Er changed the morphology of TiAl₃, TiB₂ phase, the refiner would be more efficient. The decomposition of TiAlEr compounds which released Er refrained the growth of TiAl₃ and made TiB₂ difficult to aggregate or deposit, therefore resulted in more particles being efficient nucleation substrate.     

Contraction of aluminum alloys during and after solidification

A technique for measuring the linear contraction during and after solidification of aluminum alloys was improved and used for examination of binary and commercial alloys. The effect of experimental parameters, e.g., the length of the mold and the melt level, on the contraction was studied. The correlation between the compositional dependences of the linear contraction in the solidification range and the hot tearing susceptibility was shown for binary Al-Cu and Al-Mg alloys and used for the estimation of hot tearing susceptibility of 6XXX series alloys with copper. The linear thermal contraction coefficients for binary and commercial alloys showed complex behavior at subsolidus temperatures. The technique allows estimation of the contraction coefficient of commercial alloys in a wide range of temperatures and could be helpful for computer simulations of geometrical distortions during directchill (DC) casting.