Hot Isostatic Pressing (HIP)

The application of hot isostatic pressing (HIP) to aluminum alloys following casting is a key technology for improving prop-erties by reducing or eliminating the effects of porosity and in-clusions. This technology is sufficiently important that it is covered in a separate chapter (Chapter 6), following the discussion of the nature and cause of such imperfections.

REFERENCES

1. Principles of Purchasing Castings, American Foundry Society, 2002

2. Gating and Feeding for Light Metal Castings, American Found-rymen’s Society, 1946

3. Aluminum Casting Technology, American Foundrymen’s So-ciety, 1993

4. E.L. Rooy, Hydrogen in Aluminum, AFS Trans., 1993 5. Aluminum Now, The Aluminum Association Inc.

6. Engineered Casting Solutions, American Foundry Society SELECTED REFERENCES

• Basic Principles of Gating, American Foundrymen’s Society,

• Basic Principles of Risering, American Foundrymen’s Society,1967 1968

• G. Bouse and M. Behrendt, Metallurgical and Mechanical Prop-erty Characterization of Premium Quality Vacuum Investment Cast 200 and 300 Series Aluminum Alloys, Adv. Cast. Technol., Nov 1986

• Computer Gating Program, SDCE

• Core and Mold Process Control, American Foundrymen’s So-ciety, 1977

• A.K. Dahle, S.M. Nabulski, and D.H. St. John, Thermome-chanical Basis for Understanding Hot Tearing During Solidi-fication, AFS Trans., Vol 106, 1998

• Fundamental Molding Sand Technology, American Foundry-men’s Society, 1973

• J.C. Hebeisen, HIP Casting Densification, ASME, 1999

• E.A. Herman, Die Casting Handbook, Society of Die Casting Engineers, 1982

• W. Hunt, Jr., Metal Matrix Composites, Chapter 6.05, Com-prehensive Composite Materials, Pergamon Press, July 2000

• W.H. Hunt and D.R. Herling, Applications of Aluminum Metal Matrix Composites: Past, Present, and Future, Proc. Interna-tional Symposium of Aluminum Applications: Thrusts and Chal-lenges, Present and Future, Oct 2003 (Pittsburgh, PA), ASM International

• H. Koch and A.J. Franke, Ductile Pressure Die Castings for Automotive Applications, Automotive Alloys, TMS, 1997

• S.J. Mashl et al., “Hot Isostatic Pressing of A356 and 380/383 Aluminum Alloys: An Evaluation of Porosity, Fatigue Prop-erties and Processing Costs,” SAE, 2000

• Plaster Mold Handbook, American Foundrymen’s Society,

• H. Pokorny and P. Thukkaram, Gating Die Casting Dies, So-1984 ciety of Die Casting Engineers, 1984

• E. Rooy, Improved Casting Properties and Integrity with Hot Isostatic Processing, Mod. Cast., Dec 1983

• E.L. Rooy, Origins and Evolution of Premium Engineered Alu-minum Castings, MPI Symposium on Premium Engineered Castings, May 2002

• A.C. Street, The Die Casting Book, Portcullis Press Ltd., 1977

• M. Tiryakiog˘lu et al., Review of Reliable Processes for Alu-minum Aerospace Castings, AFS Trans., Vol 104, 1996

Chapter 3: Aluminum Casting Processes / 37

CHAPTER 4

The Effects of

Microstructure on Properties

Microstructural features are products of metal chemistry and solidification conditions. The microstructural features, excluding defects, that most strongly affect mechanical properties are:

• Size, form, and distribution of intermetallic phases

• Dendrite arm spacing

• Grain size and shape

• Eutectic modification and primary phase refinement

4.1 Intermetallic Phases

Controlling element concentrations and observing stoichiomet-ric ratios required for intermetallic phase formation results in pre-ferred microstructures for property development. Solidification rate and the rate of postsolidification cooling promote uniform size and distribution of intermetallics and influence their morphology.

Slower rates of solidification result in coarse intermetallics and second-phase concentrations at grain boundaries. Phase formation is diffusion controlled so that more rapid solidification and more rapid cooling to room temperature from solidification temperature

results in greater degrees of retained solid solution and finer dis-persions of smaller constituent particles.

4.2 Dendrite Arm Spacing

In all commercial processes, with the exception of semisolid forming, solidification takes place through the formation of den-drites from liquid solution. The cells contained within the dendrite structure correspond to the dimensions separating the arms of pri-mary and secondary dendrites and are exclusively controlled for a given composition by solidification rate (Fig. 4.1).

There are at least three measurements used to describe dendrite refinement:

• Dendrite arm spacing: The distance between developed sec-ondary dendrite arms

• Dendrite cell interval: The distance between centerlines of adjacent dendrite cells

• Dendrite cell size: The width of individual dendrite cells

Fig. 4.1 Dendrite arm spacing and dendrite cell size as a function of local solidification rate. Source: Ref 1 Aluminum Alloy Castings: Properties, Processes, and Applications

J.G. Kaufman, E.L. Rooy, p 39-46 DOI:10.1361/aacp2004p039

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All rights reserved.

www.asminternational.org

The larger the dendrite arm spacing, the coarser the microcon-stituents and the more pronounced their effects on properties. Finer dendrite arm spacing is desirable for improved mechanical prop-erty performance (Fig. 4.2, 4.3) (Ref 2).

Cooling rates directly control dendrite arm spacing, which in-fluences property development and substantially improves ductil-ity:

Cooling rate Dendrite arm spacing

Casting processes °F/s °C/s mils μm

Plaster, investment 1.80 1 3.94–39.4 100–1000

Green sand, shell 18.0 10 1.97–19.7 50–500

Permanent mold 180.0 100 1.18–2.76 30–70

Die 1800 1000 0.20–0.59 5–15

4.3 Grain Refinement

Fine, equiaxed grains are desired for the best combination of strength and ductility by maximizing grain-boundary surface area and more finely distributing grain-boundary constituents (Ref 3).

Coarse grain structure and columnar and feather or twin-columnar grains that form with high thermal gradients in low-alloy-content

compositions are by comparison detrimental to mechanical prop-erties. The type and size of grains formed are functions of alloy composition, solidification rate, and the concentration of effective grain nucleation sites.

Increased solidification rate reduces grain size (Ref 4), but so-lidification rates in complex cast structures typically vary and the degree of grain refinement practically achievable in commercial gravity casting processes is lower than that obtained by effective heterogeneous nucleation through grain-refiner additions before casting (Fig. 4.4).

All aluminum alloys can be made to solidify with a fully equi-axed, fine-grain structure through the use of suitable grain-refining additions (Ref 5, 6). The most widely used are master alloys of titanium or of titanium and boron. Aluminum-titanium refiners generally contain from 3 to 10% Ti. The same range of titanium concentrations is used in Al-Ti-B refiners with boron contents from 0.2 to 1% and titanium-to-boron ratios ranging from 5 to 50. Se-lected carbides also serve grain-refining purposes in aluminum alloys (Ref 7).

Although grain refiners of these types can be considered con-ventional hardeners or master alloys, they differ from true master alloys added to the melt exclusively for alloying purposes. To be effective, grain refiners must introduce controlled, predictable, and operative quantities of aluminides and borides or carbides in the correct form, size, and distribution for grain nucleation. Refiners in rod form, developed for the continuous treatment of aluminum in primary operations and displaying clean, fine, unagglomerated microstructures, are available in sheared lengths for foundry use.

In addition to grain-refining master alloys in waffle or rolled rod form, salts, usually in compacted form that react with molten alu-minum to form combinations of TiAl₃and TiB₂, are also available.

Transduced ultrasonic energy has been shown to provide degrees of grain refinement under laboratory conditions (Ref 8, 9). No commercial use of this technology has been demonstrated. The application of this method to engineered castings is problematic.

4.4 Aluminum-Silicon Eutectic Modification

The properties of hypoeutectic aluminum-silicon alloys can be affected by modifying the form of the eutectic. A finer, more fibrous eutectic structure can be obtained by increased solidification rate and by the addition of chemical modifiers. Calcium, sodium, stron-tium, and antimony are known to influence the degree of eutectic modification that can be achieved during solidification. Figure 4.5 illustrates variations in degree of modification achieved by modi-fier additions.

Sodium is arguably the most potent modifier, but its effects are transient because of oxidation and vapor pressure losses. Strontium is less transient but may be less effective for modification under slow solidification rates (Fig. 4.6).

The combination of sodium and strontium offers advantages in initial effectiveness. Calcium is a weak modifier with little com-mercial value. Antimony provides a sustained effect, although the result is a finer lamellar rather than fibrous eutectic. The effects of sodium, strontium, and Na⫹ Sr on modification are shown in Fig.

4.6 and 4.7.

Fig. 4.2 Dendrite cell size effects on the strength and elongation of several aluminum casting alloys. Source: Ref 1

The addition of metallic sodium to molten aluminum creates turbulence that can result in increased hydrogen and entrained oxide levels. The use of hygroscopic salts including NaCl and NaF for modification also risks oxide formation and increased dissolved hydrogen content. Postaddition fluxing to restore melt quality in-creases the rate of sodium losses. The excessive use of sodium (>0.01 wt%) increases misrun tendencies through increases in sur-face tension and diminished fluidity.

Strontium additions are usually made through master alloys con-taining up to 10% of the modifier. While these additions are made

with minimum melt degradation, strontium is associated with an increased tendency for hydrogen porosity, either through increas-ing hydrogen solubility or decreased surface tension.

The greatest benefits of eutectic aluminum-silicon modification are achieved in alloys containing from 5% Si to the eutectic con-centration. The addition of modifying elements to these alloys results in a finer lamellar or fibrous eutectic structure. The modi-fying additions either suppress the growth of silicon crystals within the eutectic or equilibrate silicon-matrix growth rates, providing finer lamellae.

Fig. 4.3 Correlation between dendrite cell size and tensile properties of specimens machined from production castings in alloy A356.0-T62. The different data points indicate specimens from different heats. Source: Ref 1

Fig. 4.4 As-cast Al-7Si ingots showing the effects of grain refinement. (a) No grain refiner. (b) Grain refined. Both etched using Poulton’s etch; both 2⫻. Courtesy of W.G. Lidman, KB Alloys Inc.