Optimization of the Precision Casting Pouring System of Ti-3Al-2.5V Titanium Alloy

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(1. Beijing Engineering Research Center of Advanced Precise Forming of Titanium Alloys,Beijing Institute of Aeronautical Materials, Beijing 100095, China; 2. BAIMTEC Material Co.,Ltd., Beijing 100094, China)

1 Introduction

Due to low density, excellent mechanical properties at ambient temperature, good corrosion resistance and composite compatibility, titanium and its alloys have been widely used in the field of military and civil applications for aerospace structures, biomaterial substitutes[1—4]. However, the relatively high costs of metallic titanium (including the raw materials as well as fabrications) largely limit their application[4—9]. In the process of manufacturing a product, the cost can be directly or indirectly affected by many factors, such as material selection, fabrication process, fabrication equipment, metallurgical quality, fabrication properties and utilization rate of alloys, etc. Consequently, the comprehensive consideration of the performance and the cost of titanium alloys is a very important choice for utilization[10—11].

Among the various titanium alloys, Ti-3Al-2.5V alloy (ASTM Grade9[12]) is originally used for hydraulic and fuel structures in conventional airplanes, and now it has been widely used to fabricate medical and dental implants, sport equipment as well as other civil applicants. Ti-3Al-2.5V alloy has the intermediate properties(mechanical strength and corrosion resistance) between metallic titanium and the Ti-6Al-4V alloy[13]. It is classified as a near-α, α-β alloy or super-α titanium alloy,and is referred to as “half 6-4” or TA18[14—18]. The strength of Ti-3Al-2.5V alloy is 20%～50% higher than that of pure titanium, and the welding performance and cold forming performance are better than that of Ti-6Al-4V alloy under room temperature and high temperature conditions[19]. Therefore, the use of Ti-3Al-2.5V alloy is a good choice for non-bearing parts, such as a cover.

For the fabrication process of titanium alloy, investment (precision) casting is by far the most fully developed net-shape technology compared to Powder Metallurgy (PM), superplastic forming and precision forging[2,14,21], due to the utilization rate of precision casting alloy as high as 70%～90%[20]in the forming process. In contrast, the problem with the employment of PM is the presence of residual chlorides, which make the materials practically unwieldable. The precision casting of Ti-6Al-4V and other titanium alloys was reported in the literature, but little work has been performed on the castings of Ti-3Al-2.5V alloy. Previous studies showed that more than 95% of the Ti-3Al-2.5V alloys were forged into tubes[6,15,18,22—27]. Additionally,the traditional experiment and error methods usually lead to high cost and long reproduction cycle. In order to effectively solve this problem, numerical simulation has been used to study the mold filling and solidification in precision casting of titanium alloy[9,11,28—36], because it can predict the defect formation, optimize the casting pouring system, and reduce the cost by shortening the reproduction cycle.

In this paper, Ti-3Al-2.5V titanium alloy with four different pouring systems under gravity field and vertical centrifugal field are studied by numerical simulation,while the mechanical properties and shrinkage defects of the castings are experimentally measured.

2 Experimental procedures

The cover castings were chosen as the research object. Typical schematic illustration of the cover is shown in Fig.1.

Fig.1 Schematic illustration of the cover

2.1 Numerical simulations

The mold filling and solidification of Ti-3Al-2.5V alloy with four different pouring systems were simulated by a software ProCast. The detailed configurations and geometrical characteristics of four pouring systems are shown in Fig.2. Among the four pouring designs, (a) the top-bottom pouring is a combination of top-gating system and bottom-gating system; (b) the ingates are located at the side of the products in the spiral pouring system; (c) the ingates of the products are located directly in the pouring without any runner bars in the radial pouring system; (d) the tree pouring is composed of three branches, each branch contains four products. The first three pouring systems were numerically calculated in vertical centrifugal field and the last one in gravity field, respectively. The simulated pouring temperature was 1720 ℃, the mold preheating temperature was about 200 ℃ and the pouring time was 3 second.

Fig.2 Configurations and geometrical characteristics of four experimental pouring systems

2.2 Experimental Details

The experimental materials including commercial pure Ti and an Al-V intermediate alloy were melted in an Induction Skull Melting (ISM) furnace, which can melt 20 kg titanium alloy at one time. The experimental process was carried out in the vacuum environment.Details of the casting process are shown in Tab.1. The mechanical specimens annealed with the same lot castings were machined according to ASTM E8 standard.The hardness specimens cut from the castings and the chemical samples were also annealing treated. X-ray radiography method was used to examine the shrinkage defects for all the finished castings.

Tab.1 Parameters of the experimental and simulated pouring systems

3 Results and discussion

3.1 Numerical simulation

Among the four pouring systems of the cover, topbottom pouring system (a) and tree pouring system (d)represented more optimal designs according to the computer-predicted shrinkage, respectively, as shown in Fig.3.It is obvious that the shrinkage mainly appears in the four thick area of the large end face and four thick ribs of small ports for all four different pouring systems. The shrinkage in top-bottom pouring system shows the largest number and area than that in the other pouring systems.

Fig.3 Simulation results of shrinkage in four pouring systems

The mold filling process of top-bottom pouring system in the gravity field is shown in Fig.4, and the red liquid represents metal flowing through the runner from the pouring into the mold cavity, while the green liquid on behalf of the cooled metal. Due to the gravity, fluid flows along the wall from the gate into the cavity (Fig.4a). With the pouring continues, the metal liquid continues to fill the mold in the fan-shaped form, and the last filling area is in the runner on both sides. Apparently, there is some surface turbulence in the process of mold filling. Splash and folding-over of the melt are observed when the liquid metal enters the mold cavity (Fig.4c and d).

Fig.4 The four moments to show the mold filling process of the top-bottom pouring

In contrast to the top-bottom pouring system, the spiral and radial static pouring systems (Fig.5a and 5b)have a common weakness of metal liquid turbulence.Due to the limited space in the radial pouring system, it is difficult for the subsequent coating and cleaning shell,and the temperature field of the products is affected by neighboring heat radiation effect besides the mold filling process[37]. The metal liquid turbulence increases the proportion of scab and sweat in the deep groove on the products.

Fig.5 Mold filling process of the electric power tool with

A satisfied filling result was obtained in the tree pouring system under the centrifugal force field (Fig.6).Because of the centrifugal effects, mold filling is different from static mold filling, free surface of liquid is circular arc during filling process and the melt of titanium alloy sticks to one side of the wall to fill the cavity in the centrifugal field. The centrifugal force can enhance the mold filling ability of liquid melt and help the metal liquid remove entrapped gas pores. Generally,the influence of gravity field upon the fluid flow is negligible in the centrifugal casting[3,33,38]. The fluid fills along the runner channels and stuffs the cavity layer by layer, and the final stuffing place is the small port of the casting. It avoids more shrinkage defects appearing in the big end surface where subsequently machined four holes, because the final fusion area (i.e. independent liquid zone) is prone to shrinkage.

3.2 Mechanical properties

After pouring, the chemical composition of the casting parts in top-bottom pouring system and tree pouring system were tested, shown in Tab.2. It can be seen that the chemical composition of static casting(top-bottom pouring) fluctuates obviously (the content scope of oxygen). The composition segregation leads to inhomogeneity of microstructure, and even unstable mechanical properties.

Fig.7 shows the mechanical properties (a)σbandσ0.2(b)δ5andΨof the parts in top-bottom pouring static system and tree pouring centrifugal system.Compared to the historical production data (tensile strength: 700～830 MPa; elongation: 13%～16%), it is obvious that the values of the tensile strength in tree pouring are higher than that of top-bottom pouring system.

Fig.8 shows the values of Vickers hardness in top-bottom pouring static system and tree pouring centrifugal system. The hardness test specimen and the testing surface of the parts are shown in

Fig.8a. Compared with the historical production data (about 230～270) of Ti-3Al-2.5V parts after annealed in different batches, the hardness values of the two systems within the scope of the historical production data. The value of hardness in bottom pouring is similar to that of tree pouring, but the value of hardness in top pouring is relatively low. In the mold filling process of top pouring system, the area of hardness test specimen finally filled is prone to shrinkage, leading to a relatively low hardness value.

Fig.6 Filling sequence of tree pouring system under the centrifugal force field

Tab.2 Chemical composition of Ti-3Al-2.5V parts (wt.%).

Fig.7 Mechanical properties of top-bottom and tree pouring systems

Fig.8 Vickers hardness of top-bottom and tree pouring system

3.3 Shrinkage

The shrinkage distribution of the top-bottom pouring and the tree pouring system were compared with that of computer-predicted, shown in Fig.9 and Fig.10. The predicted shrinkage level coincided wellwith the experimental observations. The shrinkage defects are near to the four thicker areas in the big end surface and are easily exposed to the surface after machining. The shrinkage defects in the top-bottom pouring system are larger than that in the tree pouring system.

Fig.9 Experimental castings and X-ray radio photographs of top-bottom pouring system

Fig.10 Experimental castings and X-ray radio photographs of tree pouring

In short, it indicates that the numerical simulation can be used as an efficient tool to aid in casting design and shrinkage control. Based on the comprehensive evaluation of various performances, the tree pouring(centrifugal casting) system seems more ideal. But the shrinkage in the big end surface has the risk of exposure after machining, and it needs to be get rid of by further process optimization.

4 Conclusions

In summary, the mold filling and solidification of Ti-3Al-2.5V alloy with four different pouring systems were studied by means of numerical simulation. The mechanical properties and shrinkage defects of the castings were examined by actual experiments. According to the experimental results, we can conclude that.

1) The chemical composition of static (top-bottom)pouring system is not stable, and oxygen content range is very large, which can result in uneven alloy microstructures, and even unstable mechanical properties.

2) The different casting systems have little influence on the hardness or the mechanical properties of Ti-3Al-2.5V casting alloy.

3) The predicted shrinkage level coincided well with the results of X-ray radio photographs. The defects of static (top-bottom) pouring system are bigger than that of centrifugal (tree) pouring system. So the numerical simulation is a good method to predict the shrinkage defect distribution effectively.

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