Biomedical Engineering Reference
In-Depth Information
Mg
Al
Mg
1.20
1.50
Al
1.20
1.50
Fig. 8.6
SEM-EDS analysis of thixomolded AZ91: left the backscattered image, right the spectra
of (
a
) the islands (and hardly visible fine dendrites) of Mg
17
Al
12
and (
b
) the grey matrix of the
˛
-phase. Photo by P. Crabbe
Tabl e 8. 4
Commercial Mg grades for thixomolding. Composition in weight %; Mg: balance;
minor elements Si, Cu, Ni, Fe:
<
0.05%
Alloy
Al
Zn
Mn
Feature
AZ91D
8.5-9.5
0.45-0.90
0.17-0.40
High strength, good corrosion resistance
AM60B
5.6-6.4
0.20
0.25-0.50
Shock absorbing, high ductility
AM50A
4.5-5.3
0.20
0.28-0.50
Shock absorbing, high ductility
AS41B
3.7-4.8
0.10
0.35-0.60
8.4
Mg Foams
Tantalum has a high density and is too heavy for bulky implants. Therefore, we
paid much attention to its porous variant. Although Mg belongs to the lightest ele-
ments used for biomedical purposes, its porous variant also triggers our attention
and, when oxidation rate could be kept under control, a route is opened to intriguing
applications. An AZ91 foam with open cellular structure can be produced having a
density of 50 kg.m
3
. A cubic meter of pure solid Mg weighs 1,740 kg!
The fabrication starts with a polyurethane foam. Plaster is poured into that foam.
The polyurethane is removed by heating the plaster mold to 473
ı
C. The result-
ing plaster mold is an open porous fabric. Subsequently, the molten Mg alloy is
poured into the plaster mold either at atmospheric pressure or in vacuo and heated
to 573
ı
C. The plaster is removed by water jet. Pore sizes, estimated from a micro-
graph, range from less than a millimeter to a few millimeter and a wall thickness
around 0.3 mm [
266
].
An interesting property of foams, which was not discussed in Chap.
5
,isenergy
absorption during impact or loading. A look at the stress-strain behavior in com-
pression will clarify why. In a typical stress-strain curve of foams (Fig.
8.7
a), three
particular zones can be defined:
