Table 1. Permanent Mold Typical Aluminum Alloy Mechanical Properties Alloy 319
Ultimate tensile strength (ksi)
333 355
C355(T6) 356
A356 443
34 34 27 48 26 27 23
Ultimate tensile strength (ksi)
383 384
B390 A413
46 45 48 46 42
Yield strength (ksi)
19 19 15
28 18 13 9
Yield strength (ksi)
25 22 24 36 19
Elongation in 2 in. (%)
3% 2 4 8 5 8
10
Table 2. Diecasting Typical Aluminum Alloy Mechanical Properties Alloy A360
Elongation in 2 in. (%)
3 4 3
<1 4
Hardness (HB)
85 90 -
90 - -
45
Hardness (HB)
75 75 85
120 80
Fatigue ultimate limit (ksi)
–
15 - - - -
8
Fatigue ultimate limit (ksi)
20 21
20 20 19
Modulus of elasticity (106
psi)
10.7 –
10.2 10.2 10.5 10.5 10.3
Modulus of elasticity (106
psi)
10.3 10.3 -
11.8 -
Diecast parts trend toward the less
complex, partly because the metal cores must be designed to be pulled straight out of the casting. T is limits the shapes of the cores and passage- ways of the casting. Diecast parts also have strong
dimensional accuracy and excellent surface fi nishes. Aluminum alloys can be diecast to tolerances of +/-0.004 sq. in. and feature fi nishes as fi ne as 50 RMS. Walls can be cast as thin as 0.04 in. In the diecasting process, also
called high pressure diecast- ing, metal molds, or dies, are preheated and coated with a die release agent prior to each shot of metal. Premeasured amounts of molten metal then are metered into a shot sleeve and forced into the die under extreme pressure (usually from 10,000 to 15,000 psi). Rapid fi lling of the mold and solidifi cation under pressure can produce a dense, fi ne-grained and refi ned surface structure with excellent properties, including fatigue strength. But the typical injection speeds of the metal into the mold do not allow enough time for air to escape
the die cavity. If turbulence occurs as the metal fl ows through the shape of the casting, porosity results. T e use of a vacuum during die fi lling (vacuum diecasting), larger ingates with slower shot velocities (squeeze casting) or semi-solid metalcasting (in
which metal somewhere between the liquid and solid phase is injected into the die) can overcome these problems and produce parts that can be heat treated and welded. In designing for a die casting,
thick sections may be less strong than thinner areas, because they can breed shrink porosity as the outer layer so- lidifi es before the interior metal. Dies have a relatively long wear
life and can be used for up to 100,000 shots, depending on the application, so when large quanti- ties are required, diecast parts cost less in the end, despite the high start-up costs. However, because the molds used in diecasting must be stronger than those used in perma- nent molding, they can be more costly, and the number of castings required to justify the use of diecasting is higher than permanent mold. For high volume jobs, the
diecasting process, which is highly automated, often produces parts
Fig. 2. The permanent mold process gave this suspension support casting the mechanical properties needed to meet the minimum strength and stiffness requirement of 180 MPa ultimate tensile strength and 5% elongation.
with the lowest per-unit price. Production runs above 10,000 pieces are connected
Nov/Dec 2014 | METAL CASTING DESIGN & PURCHASING | 41
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60