Metal Processing Institute
Advanced Casting Research Center

Research Programs

SSM Alloy Development


Research Team:

Qingyue Pan
Patrick Hogan
Diran Apelian

Introduction

Often newly developed manufacturing processes are evaluated with existing alloys rather than optimizing a special alloy that can take advantage of the attributes of the new process. Currently, conventional cast aluminum alloys, such as 356 and 357 are widely used for SSM processing. SSM alloy development/optimization remains a significant issue in SSM processing.

Historically, the trial-and-error method has been employed for alloy development. This approach has been proven to be cost-intensive and time-consuming. With the development of robust aluminum alloy databases, a new approach based on thermodynamic simulations has emerged. This approach provides a powerful tool for alloy design. In this approach, the Gibbs free energy of individual phases is calculated as a function of alloy composition, temperature and pressure, and then collected in a thermodynamic database that enables calculation of multi-component phase diagrams. The calculation results provide critical information for alloy design such as the phase formation and transformation temperatures, and the solidification characteristics of the alloy.

Objectives

The aim of this project is to optimize/develop alloys that are better suited for SSM processing. In order to achieve the goal, the following strategies are being pursued:

Methodology

Several important factors that need to be considered for SSM alloy development/optimization are outlined as follows:

  1. Solidification range (ΔT): is defined as the temperature range between the solidus and the liquidus lines of the alloy. Pure metals and eutectic alloys are not suitable for SSM processing, whereas, alloys with too wide a solidification range experience poor resistance to hot tearing. It is therefore suggested that the solidification range of an SSM alloy be between 40-130K.
  2. Temperature sensitivity of fraction solid: For a given alloy composition, temperature sensitivity of the fraction solid (fs) is defined as the slope of the fs vs.T curve, i.e., it is dfs/dT. In order to obtain stable and repeatable processing conditions, the temperature sensitivity of the fraction solid should be as small as possible in the fraction solid range of commercial operations (ideally fs should be 0.3-0.5 for rheocasting, and 0.5-0.7 for thixocasting/thixoforging).
  3. Temperature process window (ΔT): Depending on the application, for rheocasting, ΔT is defined as the temperature difference between 0.3-0.5 fraction solid, whereas, for thixoforging, ΔT is defined as the temperature difference between 0.5-0.7 fraction solid. Considering temperature variations during commercial forming operations, a relatively large temperature window is expected.
  4. Potential for age hardening: In order to achieve high strength, the alloys designed for SSM processing need to have high potential for age hardening. During a T5 temper, SSM parts ejected from the die are quenched immediately in water and then artificially aged at a relatively low temperature. Therefore, the potential for age hardening of a phase can be gauged by the concentration difference (ΔC) of the major alloying elements in the α-phase between the quenching and ageing temperatures

Salient Results:

In this study, extensive thermodynamic calculations are being conducted to evaluate the SSM processability of commercial alloys. These include 356/357, 380/383, 319, 206, and wrought alloys. Subsequently, the effects of various alloying elements on the SSM processability of these alloys are characterized and recommendations are made to allow the optimization of the alloys for semi-solid processing. Some salient results are highlighted below:

Figure 1 compares the fraction solid (fs) vs temperature (T) curves of A356/380/319/206 alloys with nominal composition. Table 1 gives important simulation results. From Figure 1 and Table 1, one can see that:

Figure 1: Fraction solid (fs) vs. temperature (T) curves of A356, 206, 380 and 319 alloys with nominal composition.

Table 1: Simulation Results of 319/380/206/A356 Alloys with Nominal Composition

Figures 2 and 3 illustrate the effects of Si, and Ni content on the fs vs T curves of 380 alloy. Simulation results point out that:

Table 2: Recommended Composition Window of 380 for Semi-solid Processing

Figure 2: Effect of Si content on fraction solid vs. temperature curves of 380 alloy.

Figure 3: Effect of Ni content on fraction solid vs. temperature curves of 380 alloy.

SSM Related Publications

Related Publications

Maintained by webmaster@wpi.edu
Last modified: September 12, 2007 16:39:05