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Casting titanium in permanent mold: titanium alloys could be used in several automotive applications, but the current manufacturing processes are costly. Permanent mold casting could change that.

Increasingly stringent regulations on the fuel-efficiency and environmental impact of vehicles to achieve lower greenhouse gas emissions have focused interest on weight reduction and improved automobile performance. While the use of aluminum and magnesium parts has increased to help meet the new standards, another material could make its own mark on the automobile industry. Titanium and its alloys feature high strength, low density and excellent resistance to corrosion and oxidation, and they have the highest strength-to-density ratio of all metals from room temperature to 1,022F (550C). This translates to a high potential for weight savings.

[ILLUSTRATION OMITTED]

Titanium alloys currently are used in special racing cars and spring and exhaust components in motorcycles. Potential applications include connecting rods, pistons and valves.

Unfortunately, the cost of titanium and its alloys is high compared with aluminum and steel, both for raw materials and processing, which requires a controlled environment due to its high reactivity. The most common method of casting titanium is investment casting, but the reaction between the ceramic mold material and molten titanium results in the formation of a brittle surface layer that has to be removed by a special chemical milling process.

Gravity permanent mold casting would eliminate the brittle surface layer while still offering near-net-shape, fine microstructure, elimination of inclusions, enhanced mechanical properties and cost benefits. But before it can be used in production, the properties produced via permanent mold must be evaluated and compared to the properties of titanium processed with the traditional techniques.

A research project was initiated to cast a titanium alloy in permanent molds and evaluate the microstructure and mechanical properties with automotive applications in mind. The final results indicated that the mechanical properties in the as-cast condition were close to the values reported in the standard processing methods. Few casting defects were observed, and the loss of strength due to defects was not significant, proving that titanium castings with good mechanical properties can be cast in high-density graphite molds.

Encouraging Results

The project focused on the most common titanium casting alloy, Ti-6Al-4V, which contains 5.5-6.75% aluminum, 3.5-4.5% vanadium and trace amounts of carbon, iron, hydrogen, nitrogen and oxygen. It has a melting point of 3,020F (1,660C) and a solidification range of 77F (25C).

[FIGURE 1 OMITTED]

Two step plate castings with four steps measuring 3.9 x 2 in. (100 x 50 mm) each were cast vertically with the thin section at the bottom (Fig. 1). A superheat of only 86F (30C) could be maintained for the casting. Molten metal was poured at the stationary mold into the thickest step of the plate.

Both the castings exhibited good surface finish, but X-ray radiography indicated some shrinkage porosity in the thickest (1-in. [25-mm]) section, close to the pouring basin. Following X-ray examination, three flat tensile specimens were machined from the center and edge of each step and tested in the as-cast condition. Tensile properties for each specimen from the four section thicknesses of the two step-plate castings were recorded.

[FIGURE 2 OMITTED]

No noticeable difference in ultimate tensile and yield strength appeared between the two castings, except that the yield strength in the first casting was 8% lower than the second for the 0.2-in. (6.3-mm) section. The changes in elongation were not consistent for the two castings, but the 1-in. thick section of the second casting showed lower elongation values.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Ultimate tensile strength increased with decreasing section thickness, but a consistent trend did not appear for elongation percentage nor yield strength. In general, ultimate tensile and yield strength and elongation varied between 913 and 1,011 MPa, 773 and 850 MPa and 3.1-10.2%, with average values of 958 MPa, 826 MPa and 7.3% (Table 1). Values for the investment cast and hot-isostatic-pressed specimen were similar at 960 MPa ultimate tensile strength, 870 MPa yield strength and 10% elongation. Porosity and inclusions might have contributed to the relatively low yield strength, but this was not investigated in detail (Fig. 2).

Optical micrographs of the casting defects showed the typical Widmanstatten structure of the alpha and beta phases, similar to what is found with metal powder injection molding (MPIM) (Figs. 3-5). The size of the Widmanstatten pattern for the permanent mold castings varied with section thickness. The strength properties from MPIM are similar to those of the cast alloys, except for a higher elongation percentage.

The automotive industry, with its regulatory support, years of ongoing research on potential applications and the need to provide added value in a competitive environment, is best positioned to work toward development of cost-effective technologies for the production of titanium alloy components. As shown in this study, the permanent mold process is a viable method for producing titanium parts with the necessary physical and mechanical properties.

For More Information

"Cutting the Cost of Titanium," B.E. Hurles and F.H. Froes, Advanced Materials and Processing, Volume 160, 2002, p. 37.

Titanium on the Track

[ILLUSTRATION OMITTED]

Titanium already has proven itself a material of choice in several applications in racing cars. These applications include:

* brake calliper pistons

* bumper supports

* cam belt wheels

* clutch discs

* clutch springs and housings

* connecting rods

* drive shafts

* exhaust systems

* gear box housings

* gudgeon (wrist) pints

* high strength fasteners

* rocker arms

* torsion bars

* steering gear

* suspension linkages

* suspension springs

* valves

* valve retainers

* valve springs

* wheels

Kumar Sadayappan and Mahi Sahoo, CANMET-MTL, Ottawa, Ontario, Canada

Curt Lavender, Pacific Northwest National Lab, Richland, Washington

Paul Jablonski, Albany Research Centre, Albany, Oregon

Kumar Sadayappan is a research scientist and Mahi Sahoo is manager of casting technology for CANMET-MTL, Ottawa, Ontario, Canada. Curt Lavender is senior research engineer for Pacific Northwest National Lab, Richland, Wash., and Paul Jablonski is metallurEist for the Albany Research Centre, Albany, Ore.
Table 1. Summary of mechanical properties of titanium alloy Ti-6AI-4V
produced by different processes.

Process Ultimate tensile Yield strength *
 strength * (MPa) (0.2% offset, MPa)

Graphite mold 928 (913) 823 (815)
 (0.984-in. section)
Graphite mold 951 (940) 836 (815)
 (0.512-in. section)
Graphite mold 958 (950) 817 (773)
 (0.248-in. section)
Graphite mold 996 (984) 827 (801)
 (0.125-in. section)
Average 958 826
HIP 960 870
HIP, heat treated 1,103 1,055
Typical wrought 955 860
 (beta-annealed)
MPIM ** 953 839

Process Elongation * (%)

Graphite mold 7 (4.4)
 (0.984-in. section)
Graphite mold 7.2 (3.4)
 (0.512-in. section)
Graphite mold 8.6 (6.3)
 (0.248-in. section)
Graphite mold 6.5 (3.1)
 (0.125-in. section)
Average 7.3
HIP 10 (Ref. 5)
HIP, heat treated 8 (Ref. 5)
Typical wrought 9 (Ref. 5)
 (beta-annealed)
MPIM ** 11.8 (Ref. 6)

* Numbers in parentheses indicate minimum values.

** Produced by metal powder injection molding, 99.5% density.
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Author:Sadayappan, Kumar; Sahoo, Mahi; Lavender, Curt; Jablonski, Paul
Publication:Modern Casting
Date:May 1, 2008
Words:1125
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