Laminated Metal Composites by Infiltration

Abstract

We show that steel-magnesium alloy laminated metal composites (LMCs) can be produced by gas pressure infiltration of a liquid magnesium alloy between layers of stacked dimpled steel sheets. Resulting LMCs are amenable to subsequent warm rolling. The LMCs are free of pores or brittle intermetallics and feature, in the as-cast condition, metal layers of uniform thickness and spacing. The ultimate tensile strength of the as-cast LMCs, of 260 MPa, obeys the “rule-of-mixtures” (ROM). The uniform tensile elongation, of around 20 pct, makes the infiltrated LMC nearly as ductile as the bulk steel it contains, implying that the magnesium alloy in the as-cast LMCs has a substantially increased tensile ductility in comparison to its metallurgically equivalent bulk state. Rolling reduces the metal layer thicknesses, causes waviness in the interface, and makes the LMCs stronger but less ductile, by factors in the vicinity of 2 for both properties; the main cause for this is work hardening in the steel layers.

Appendices

Appendix A

1.1 Dimpling the steel

The steel sheets were dimpled using a small device including a punch and two aluminum plates (one upper and one lower, Figure A1). The upper plate contains cylindrical holes 1.02 mm in diameter, while the lower plate has matching conical indentations, 1 mm in diameter at the top and 0.5-mm deep. A drawing of the aluminum upper plate is given in Figure A2. Arrangements of dimples in dimpled and nondimpled specimens are given in Figures A3 and A4, for as-cast and rolled samples, respectively.

Fig. Al

Photograph of the punching device

Fig. A2

Aluminum upper plate of the punching device (not to scale). The bottom plate has conical indentations at exactly the same locations with the holes of the upper plate. All dimensions are in millimeters

Fig. A3

Arrangements of the dimples used in the production of tensile specimens of as-cast specimens: (a) dimpled specimens and (b) nondimpled specimens. The tensile samples were machined after infiltration and solidification

Fig. A4

Arrangement of dimples used in the production of tensile specimens of rolled specimens: with (a) 0.1-mm thickness in the nondimpled configuration, (b) 0.1-mm thickness in the dimpled configuration, (c) 0.2-mm thickness in the nondimpled configuration, and (d) 0.2-mm thickness in the dimpled configuration. All dimensions are in millimeters. The tensile specimens were machined after infiltration, solidification, and rolling

Appendix B

2.1 Assessment of steel sheet stability during infiltration

The resistance to flow of a channel varies as the square of its width: if only for this reason, the molten magnesium alloy is likely to infiltrate the various channels defined between individual steel sheets at different moments. As a result, one side of a steel sheet may be in contact with pressurized molten magnesium alloy while the other side is still under vacuum. This, in turn, will drive the steel sheet to bend.

In order to assess whether this situation will cause the steel sheets to undergo plastic deformation, bending stresses were computed based on elementary linear theory (considering small deflections and linear elasticity). A simple yet realistic one-dimensional geometrical model for the bending of the steel sheets in the present situation is a rectangular cross-sectional beam under a uniform load W with both ends clamped (Figure B1).

Fig. B1

Illustration of the deformation of a beam fixed at both ends under a uniform load W applied over its entire span

The moment of inertia I of a rectangular cross-sectional beam of width b and thickness t is given by[29]

$$ I = \frac{{b \cdot t^{3} }}{12} $$

(B1)

The maximum deflection of a beam of length l under a uniform load W (i.e., under uniform applied pressure) with both ends clamped is[29]

$$ \delta_{\max } = \frac{{ - W \cdot l^{4} }}{384 \cdot E \cdot I}{\text{at }}x = \frac{l}{2} $$

(B2)

where E is the Young’s modulus of the beam.

The maximum stress in the beam is[29]

$$ \sigma_{\max } = \frac{{M_{\max } \cdot t}}{2 \cdot I} $$

(B3)

where the maximum moment M _max is given as[29]

$$ M_{\max } = \frac{{ - W \cdot l^{2} }}{12}{\text{at }}x \, = 0{\text{ and }}x = {\text{l}} $$

(B4)

The temperature during infiltration was 983 K (710 °C). As a result, both the Young’s modulus and the proof stress of steel sheets are reduced compared to room temperature; these are estimated as 140 GPa[30] and 60 MPa,[31] respectively.

The maximum and minimum pressures applied during infiltration were 0.2 and 0.03 MPa, respectively. Table B1 summarizes maximum deflections and bending stresses for these two values, for sheets either 0.2- or 0.1-mm thick, with a free length between spacers equal to 30 mm, corresponding to the gage length of tensile samples. As seen, in either case, bending stresses far exceed the elastic limit of the steel. Predicted deflections generally exceed those admitted by linear theory and also exceed the distance between sheets. Therefore, it is likely that uncontrolled deflection of the steel sheets will occur during infiltration if no additional spacers are used. Preliminary experiments confirmed this; without dimples, composites contain stacked touching layers of steel pushed apart by molten magnesium.

Table B1 Computed Maximum Deflection δ _max of the Steel Sheets and the Maximum Stress σ _max Imposed on the Steel Sheets During Infiltration

Spacers are thus required in order to prevent uncontrolled bending. Table B2 examines the placement of additional spacers, created for example by dimpling the steel sheets. Here, a maximum allowable bending stress is set, at the elastic limit of the steel (60 MPa), and the maximum free distance between the dimples is computed. Computed deflection values remain well below the distance between the steel sheets. Depending on sheet thickness and loading conditions, distances between dimples have to be below 11 mm down to 2 mm if plastic deformation of the sheets is to be avoided. Dimple spacings used in the present experimental work are near 10 mm: the present computations suggest that the steel sheets bend at the onset of pressurization (i.e. at 0.03 MPa), this spacer separation distance being just short enough to avoid bending of 0.2 mm sheets, but not of the 0.1 mm sheets. Experiments show no sign of extensive bending with both steel thicknesses, Figure 1; assumptions used in the calculations are thus apparently conservative.

Table B2 Required Distance Between Spacers to Maintain the Stress on the Steel Sheets Below the Elastic Limit of Steel (60 MPa)