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Zr and Mo macrosegregation in Ti6246 titanium alloy industrial-scale ingot by vacuum arc remelting

Zr and Mo macrosegregations were investigated in Ti6246 titanium alloy industrial-scale ingotby vacuum arc remelting. The formation mechanism of Zr and Mo macrosegregations was studied during the solidification process. Zr macrosegregation was characterized by low content in the equiaxed grain zone and high content in the hot top zone. Mo exhibits an opposite trend with Zr. The macrosegregations of Zr and Mo were the most pronounced, with a statistic segregation degree higher than Al and Sn. It could be concluded that temperature gradient and solidification rate dominated the macrosegregation formations during the solidification process. The thermal buoyancy made the negative segregation Zr be continuously discharged to the front of the solid-liquid interface. Mo was enriched in the solid phase at the solid-liquid interface as the positive segregation.

Titanium alloys exhibit many problems at the billet evaluation stage which are connected to the melting and casting processes used in their preparation [5][6][7]. Mitchell [5] concluded that β flecks are connected with the melting practice. Conventional melting by double or triple VAR gives a product whose reliability in these defects is uncomfortably close to the limits imposed by probabilistic design and fracture mechanics analysis. Dobatkin et al. [6] revealed that positive zonal segregation appears in titanium alloy ingots under the conditions of vacuum arc remelting. Positive segregation also occurs in aluminum alloy ingots during vacuum arc remelting or continuous casting accompanied by vigorous metal stirring in a molten pool. Various relationships of these parameters govern the development of a type of macrosegregation (positive or negative) or production of a rather homogeneous ingot. Liu et al. [7] reported a novel method for the analysis of composition distribution of titanium alloys over a large area (64 mm × 72 mm) was investigated by exploring the original position statistic distribution based on spark spectrum (OPA-SS) in Ti-6.5Al-1.0Cr-0.5Fe-6.0Mo-3.0Sn-4.0Zr titanium alloy. The macrosegregation of Sn was the most pronounced, with a statistic segregation degree higher than 18%; the macrosegregation of Mo followed with a statistic segregation degree of 10%; the macrosegregation of Al and Fe was relatively milder, lower than 8%. The main reason for the macrosegregation state of the as-cast Ti-6.5Al-1.0Cr-0.5Fe-6.0Mo-3.0Sn-4.0Zr alloy can be the solute redistribution during liquid solidification and the diff usion rate of each element in the solid phase. However, there are no introductions about the Zr segregation in that article. Although the details of the segregation in the solidification in remelting process have been discussed [8], many of the works were to analyze the smaller ingot less Φ380 mm [6,7], and focus on the significant quantities of β stabilizing elements such as Fe and Cr in particular and "hard-α" (LDI) and high density inclusions (HDI) in microsegregation. Some works just were to model the macrosegregation during the vacuum arc remelting [9]. The conclusions are still controversial and few experimental evidences have been published on the segregation behaviors of element macrosegregation in industry scale titanium alloy ingot. In Ti6246 alloy, the bulk composition contains 6%Al，2%Sn, 4%Zr and 6% Mo (wt.%, the same below). Lower melting-point and density elements (Al and Sn) maybe are not the origin of metallurgical quality problems, but higher melting-point and density elements (Zr and Mo) have not been investigated in detail as a source of segregation.
This article aims to illustrate the segregation behavior of alloying elements and explain the formation mechanisms of Zr and Mo macrosegregation in Ti6246 alloy industrial-scale ingot.

Material and experimental
The Ti6246 alloy ingot used in the experiment was melted by triple VAR with the actual production composition Ti-6.15Al-2.05Sn-4Zr-6Mo. Traditional technologies of VAR, i.e., the operations of electromagnetic stirring on melt pool and hot top set points at the end of the VAR process were used. While other parameters are listed in Table 1   The microstructure was examined on a scanning electron microscope (SEM, STEMI2000). The components in the different areas of the ingot were analyzed using inductive coupled plasma emission spectrometer (ICP) method. There is only a tiny equiaxed grain zone left in the surface of the ingot formed by the rapid solidification rate because the ingot of size Φ720 mm was machined into a smooth surface with a size Φ680 mm by milling. So the tiny equiaxed grain zone in the surface of the ingot can be considered to be the origin of the columnar grain zone. The columnar grain zone is about 40% of the ingot, and the columnar grain grows along the direction to two obvious shrinkage cavities in the hot top zone. There is a boundary between the equiaxed grain zone and the columnar grain zone. The equiaxed grain zone was in the shape of "circular truncated cone", the diameters of the circular truncated cone were about 460 mm on the top and 340 mm at the bottom. The equiaxed grain zone in the middle of the ingot is about 60% of the ingot including most of the hot top, which is similar as 60%-70% of the total ingot volume, which is still liquid at the time the process sequence enters into the hot-top cycle with the low solidification rate for equiaxed grain [8]. The hot top zone is on the top of the ingot, with two obvious shrinkage cavities in the center. There is almost equiaxed grain near the shrinkage cavities in the hot top center except a few columnar grains formed on the top of the ingot. There is no obvious change of grain shape zone with the ingot before heat-treating ( Fig. 3(a)).    Lots of micro-shrinkage cavities with the size of tens to hundreds of microns were found in the different positions of the ingot (Fig. 5). The shrinkage cavities are concave, which were formed in the solidification process because of the volume difference between the liquid and solidity. These zones can be recognized as the terminal of the solidification process, which also was the final solidification region of the interdendritic liquid phase [11,12].

Solidified mechanism of marco-structures and micro-shrinkage cavities
The grain structure of VAR ingots may vary from almost completely columnar for small diameter ingots, to a mixture of columnar grains near the crucible walls and equiaxed grains in the central part of the ingot. The size of the central equiaxed region largely depends on the diameter of the ingot, the alloy properties, and the arc current, which is related to the intensity of the fluid flow in the sump [9]. As can be seen from Fig. 2, the columnar grain formed along the opposite direction of the temperature gradient in the industrial ingot. According to the VAR process, there is a bigger water flu in the bottom than in the round, the temperature gradient is also bigger so the columnar grain was in the shapes of "long stick"; the columnar grain was in the shapes of "short stick" because of the smaller temperature gradient in the round. There is an obvious phenomenon that almost all the columnar grain in the round grows toward two shrinkage cavities in the hot top. It means that temperature gradient is not to the center of the ingot horizontally, but the last solidified zone on the top of the ingot at an angle.
The micro-shrinkage cavities were found in the metal casting and melting process. They are usually considered to be caused by the density difference between the solid phase and the liquid phase formed during metal solidification. Some small volumes of liquid space are usually formed at the end of solidification, and the space is relatively closed. The fluidity of the liquid phase is poor, and density difference results in insufficient liquid feeding at the end of solidification. Finally, the solid volume shrinks to form pores [11][12][13]. No clear microstructure differences were found near the micro-shrinkage cavities, so it means that there are no obvious elements of micro-segregation such as high melting point Mo at the end of solidification.

Segregation mechanism of Zr and Mo
The macrosegregation in the columnar grain zone is slighter than that in the equiaxed grain zone and hot top zone because of the higher temperature gradient. The electromagnetic stirring accelerated the melt flow in the molten pool, the strong melt flow broke the correspondence between the grain structures and promoted the transition from the the columnar grain to equiaxed grain during the VAR process. It means that the electromagnetic stirring decreased the temperature gradient in the center. The center segregation in the equiaxed grain zone (Fig. 7) had not been improved by electromagnetic stirring. The result is different from Ref. [14] which shows that adding magnetic stirring and adoptting small melting current decreases the macrosegregation rate of Fe element in Ti-10V-2Fe-3Al. The macrosegregation in the center of the Ti-1023 alloy ingot was more serious when the melting current increases [9,14], so small melting current must be the main factor on the decrease of macrosegregation rate of Fe element in the Ti-1023 titanium alloy but the electromagnetic stirring. Furthermore, the results of Ref. [15] also show that the center segregation was effected by electromagnetic stirring parameters such as current intensity and stirring pool width.
Large ingot size will increase the degree of macrosegregation [16], electromagnetic stirring can effect the Lorentz force distribution inside the metal on the liquid pool in the VAR process and deformation of the free surface of the bath [17]. It could be concluded that the temperature gradient and solidification rate in the industrial-scale ingot dominated the macrosegregation formations during the solidification process.
In consideration of the interaction between elements, the distribution of solute elements in the solid-liquid phase was calculated by Jmatpro software (Fig. 8).   The  (Fig. 2).
The β-flacks formed in the center of the ingot are due to the falling of equiaxed grains, which is called "crystal rain" [19]. The volume of the liquid metal poll is large when the melting rate is higher of VAR, which is up to 60%-70% of the total ingot in some titanium alloy ingots [8]. At this time, the solidification process is closer to the common large volume casting process. During solidification, the accumulated crystals capture the segregated liquid phase in the molten pool and fix the liquid phase in the crystal space, resulting in the formation of β-flacks in this region. The equiaxed crystal is nucleated in the molten pool when the molten pool is characterized by low circulating flow and low temperature gradient [20]. In Ti alloys, the solidified crystal sinks to the bottom of the molten pool in the form of a raindrop because the density is higher than that of liquid.
This kind of equiaxed solidification segregation often occurs in alloy solidification with a large melting pool, slow cooling rate and low temperature gradient [21,22]. In this work, the Ti-6246 alloy industrial-scale ingot has the same big volume of the liquid metal poll, low circulating flow and low temperature gradient. It means that the solidified equiaxed crystal sinks to the bottom of the molten pool in the form of a raindrop. However, the accumulated crystals did not capture the segregated liquid phase depleting Zr in the molten pool and did not fix the liquid phase in the crystal space during solidification.To verify the above viewpoint, the density change of Ti6246 alloy during solid-liquid phase transformation was calculated by Jmatpro software (Fig. 9). With the solidification process, the liquid density of Ti6246 alloy increases gradually. The density of the liquid phase is about 0.213 g/cm 3 lower than that of the solid phase. This density difference is enough to drive the solid phase to sink in the equiaxed region of the ingot center. 2. The macrosegregation in the columnar grain zone is slighter than that in the equiaxed grain zone because of the higher cooling rate, but the center segregation could not be improved more obviously by electromagnetic stirring because of center liquid solute enrichment and liquid phase accumulation in the stirring zone.
3. As the positive segregation, Mo is enriched in the solid phase at the solid-liquid interface.
The volume fraction of the solid phase is higher than that of the liquid phase at the end of the solidification so that Mo is fully diluted in the liquid. Furthermore, the diffusion and fluidity of the solid phase are much lower than that of the liquid phase so that it is hard to form high concentration enrichment of Mo in the micro-area. Zr is the negative segregation in the solidification and enriched in the last solidification zones. The thermal buoyancy made the negative segregation Zr be continuously discharged to the front of the solid-liquid interface.