Table 2. Effect of AlCuMnFe Phase on Yield Strength in B206 Castings Alloy
Sample B2-2
B3-2 B4-2 B5-2 C2-2 C3-2 C4-2
Yield T7 Yield T4 Amount of Al7 (MPa)
325 287 319 258 326 294 349
(MPa) 210
187 180 207 220 220 210
mold castings. This result was encour- aging but may not be generalized, as other metalcasting facilities may use a different binder system that results in harder molds producing more cracks. Wedge castings made in sand molds
with a metal chill in the bottom also were poured to determine the effect of solidi- fication time on mechanical properties. The solidification was rapid in the metal located next to the chill and became progressively slower away from it. During the casting trials, tensile
properties were found to be a strong function of solidification rate. The reasons for this may be seen in the mi-
Cu2 /Al20
0.5 1.4 1.3 1.1
1.05 1.9 1.6
* Indicates the element whose composition increased from the prior heat.
crostructure of the material. As freezing becomes slower, porosity, the amount of undissolved copper-bearing phases and grain size increase. The amount of porosity and undis-
solved copper-bearing phases increases regularly until the directional solidifica- tion is lost at 800-1,000 seconds. Reason- ably rapid solidification is necessary for the best elongation and strength results.
Optimal B206 Composition The best combination of mechanical
properties was obtained when B206 alloy castings were heat treated to the T4 temper. In this condition, the most
rapidly solidified sections of the wedge casting had 15-21% elongation, a yield strength of 220-280 MPa and an ulti- mate tensile strength of 370-450 MPa. Increasing the magnesium content
increased the strength of alloys heat treated to the T4 temper without reducing elongation. Yield strength also increased. Therefore, increas- ing the maximum limit for magne- sium in B206 from 0.35% to 0.55% would allow for an additional 30 MPa of yield strength and ultimate tensile strength. Zinc additions to B206 were found
to have no beneficial effect, but a small loss of strength was noted when sig- nificant amounts of zinc were present. The recommended maximum limit of zinc in B206 alloy is 0.05%. Iron and manganese additions
had an unexpected effect on yield strength (Table 2). The yield strength of the alloy after T7 heat treatment decreased by 30-40 MPa when iron was added. It appeared that iron ad- ditions tied up some of the copper in the alloy so it no longer acted to strengthen the alloy during subsequent aging and precipitation hardening. Castings heat treated to the T4 temper,
where no artificial aging occurred, had a reasonable tolerance for the impurity ele- ments iron and silicon. Depending on the desired elongation and freezing rates in a casting, the maximum allowable limits for these elements can be increased, which may reduce casting costs and make internal returns easier to recycle within the facility.
METAL About the Authors
Andres Rodriguez and Ruben Chavez iare research development engineers for Nemak, S.A., Nuevo Leon, Mexico. Jacobo Hernan- dez is a PhD student at the Univ. of Quebec, Chicoutimi, Quebec, Canada. J. Fred Major is internal consultant, metalcasting alloys and casting, and Yves Raymond is metallographer for Rio Tinto Alcan, Kingston, Ontario, Canada.
40 Metal casting Design anD purchasing March/april 2010 Mn3
(Fe, Mn) Cu2
% Cu % Mg 4.03
0.2
3.93 3.92 4.02 4.52 4.35 4.48
0.18 0.17
0.161 0.24 0.24 0.24
% Mn % Fe 0
0 0
0.34* 0.217 0.23 0.24
0.068
0.222* 0.215 0.215 0.098
0.221* 0.235
% Si 0.06
0.06
0.215* 0.18 0.09 0.05
0.145*
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