Mech 473 Homework Assignment # 3
1. You graduate from UVic and are hired by a NASA contractor to place a three-foot
diameter microsatellite into orbit. The satellite will contain delicate electronic
equipment that will send and receive radio signals from the earth. Design the
shell within which the electronic equipment will be contained. What properties
will be required and what kind of materials might be considered. (1)
2. The hull of the space shuttle consists of ceramic tiles bonded to an aluminum
skin. Discuss the design requirements of the shuttle hull that led to the use of this
combination of materials. What problems in producing the hull might be
designers and manufacturers have faced. (1)
3. Aluminum has a density of 2,7 g/cm3. Suppose you would like to produce a
composite material based on aluminum having a density of 1.5 g/cm3. Design a
material that would have this density. Would introducing beads of polyethylene or
hollow glass spheres, which have a density of 0.95, into the aluminum be a likely
possibility. Explain. (1)
4. Explain why aluminum alloys containing more than about 17% Mg are not used.
Assume that the  phase is an intermetallic compound and a eutectic structure is
produced. (1)
When more than 17% Mg is added to Al, a eutectic microconstituent consisting of the
 phase and a brittle  phase is produced during solidification as seen in its phase
diagram. This eutectic contains
35.0  17.4
% 
 97.2%
35.5  17.4
Most of the eutectic is thus the brittle  intermetallic compound and it will likely
embrittle the eutectic. The brittle eutectic, which is the continuous microconstituent,
will make the entire alloy brittle.
5. Based on their phase diagrams, which of the following three alloys would be most
suited for thixocasting? Explain your answer using their respective phase
diagrams. (2)
a) Al—12%Si
b) Al-1%Cu
c) Al-10%Mg
Alloys best suited for thixocasting are those with a large freezing range. Of the
alloys listed, Al-10%Mg has a freezing range of 110 oC, which is the largest of
the three and is thus the most suitable. The Al-12%Si is a eutectic (approximately
0 oC freezing range) and Al-1%Cu has a freezing range of only 10 oC.
6. Suppose a 24-in long round bar is to support a load of 400 lbs without any
permanent deformation. Calculate the minimum diameter of the bar if it is made
of 6061-T6 aluminum alloy. Also calculate the weight of the bar and the
approximate cost. Pricing is typically based on the LME as per this link.
http://www.lme.co.uk/aluminiumalloy.asp (1)
Area 
Force
Yield Strength
A  400/40,000  0.01 in 2
diameter  4A/π  0.113 inches
Weight  24 in  0.01 in 2  0.097 lb/in 3
Weight  0.0233 lb
cost  $2.76 (USD)/lb  0.0233 lb  $0.06
The raw cost for the 6063 alloy (reference material) can be found at the above web site.
There is then an additional cost for 6061 (approx 10 cents) and an additional cost to convert
to billet (which is the raw mat'l at approx 10 cents per lb), on top of that you have the cost
to convert billet to extrusion.
As per the website, the current price for a cash buyer is 2340mt. (2340/2200 = 1.06/lb +
.10 for 6061 + .10 for conversion to billet = 1.26 US per pound for 6061 billet.
The conversion cost from billet to extrusion is typically between 1.35 & 1.65 US per pound
depending on the complexity.
So for 6061 aluminum extrusion, the pricing would be 1.26n + 1.50 (average conversion) =
2.76 US per pound.
Above is the response from Joe Jackman of ALMAG.
7. Compare the percentage increase in the yield strength of commercially pure
annealed aluminum and its 1100-H18 alloy and explain the difference. (1)
1100  H18 22,000

100%  440%
1100  O
5,000
Aluminum and its alloys have high strain hardening coefficients due to its FCC
structure and thus can be cold worked a large amount due to their good ductility.
8. Calculate the density of a cemented carbide, or cermet, based on a titanium matrix
if the composite contains 50 wt% WC, 22 wt% TaC and 14 wt% TiC. (See
Asklande & Phule, example problem 16-2 4th edition or 17-2 5th edition for
densities). (2)
9. (2)
10. A polyester matrix with a tensile strength of 13,000 psi is reinforced with Al2O3
fibers having a tensile strength of 300,000 psi. What vol% fibers must be added
to insure that the fibers carry 75% of the applied load? (1)
11. (2)
12. The company that you are working for welds aluminum parts together using the
GMAW (MIG) process. Currently, they are using CO2 gas to protect the
aluminum during the welding process. However, the CO2 gas is too cold and
leaves a rough finish on the welds. What gas would you choose to improve the
finish on the welds? Why would you choose this particular gas? (1)
Answer: Choose argon gas because it does not cool the weld as quickly. (This
allows the weld puddle to smooth out more before freezing.)
13. Stainless steels can suffer from “weld decay”. What is it and what two methods
can be used to eliminate it? (2)
Weld decay occurs in the stainless steels and is the precipitation of (CrFe)4C, which
contains 70 % Cr, at grain boundaries causes the concentration of Cr in the adjacent
grains to fall below 12%, which degrades the corrosion resistance properties of the
stainless steels. The optimum temperature for precipitation of (CrFe)4C is around
650 oC, which is attained in the heat affected zone adjacent to a fusion weld.
14. Submerged-arc welding is used for the high-output welding of large steel panels
(such as ship decks) in the flat position. Describe the Submerged-Arc Welding
process. (1)
Submerged-arc welding is both fast and automatic. Heat is supplied by an arc
between a bare metal electrode and the work piece while the shielding is provided by
a blanket of granular flux. The electrode and the flux are both fed continuously as the
arc is traversed along a pre-machined vee interface. The flux melts in the vicinity of
the arc and then solidifies to farm a protective cover over the cooling weld. The flux
is an insulator in the solid state but become highly conductive on melting and thus
maintains the submerged arc. High quality welds can be produced at high rates of
weld metal deposition. The high deposition rates are achieved with narrow electrodes
passing high currents, which results in a relatively coarse columnar structure and an
enlarged heat affected zone causing poorer low temperature ductility.