High-temperature properties = better magnesium for automotive OEMs: automobile OEM's search for lightweight metal components has prompted the testing of magnesium's high-temperature properties to determine a viable option for lighter-weight vehicles.

As OEM automobile producers focus on the fuel economy of their end product by using magnesium alloys, several R&D labs are working to develop new such alloys with better high-temperature properties than those in the AZ and AM magnesium series. The AZ and AM alloys' have been used in many past applications, but they are unable to withstand increased-temperature requirements. Hence, research is currently in process to discover a diecast magnesium alloy that would perform well in applications under raised temperatures.

Today, magnesium is cast for a variety of components, including steering wheels, body panels, body frame sections, seat frames and valve covers. For weight reduction, however, applications must migrate to the drivetrain, and its higher operating temperatures of 150C-175C [302F-347F]. Many diecasters have searched for alloys to help reduce vehicle component weight without sacrificing material property. While magnesium alloy use in light vehicles has grown from 8 lb./vehicle in 2000 to 20 lb./vehicle today to a forecast of 30 lb. by 2008, the problem is whether existing alloys can meet the material required at the applications.

Researchers in Canada focused on comparing the high-temperature properties of two such alloys: AJ52x and AE42.

* AJ52x, with the key elements of aluminum and strontium (Table 1), is a fairly new alloy developed by Noranda Inc., and it showed good castability and high-temperature properties. In addition, this alloy is said to have mechanical properties equal or better than alloys that contain rare earth metals.

* AE42, a magnesium alloy consisting of aluminum and rare earths (Table 1). has existed for years and is proven to hold well at higher temperatures. Although fine alloy originally produced by Dow has been said to have poor castability and a high price, AE42's good creep and high-temperature properties make it a standard alloy upon which new alloys are often gauged.

While the limited success with the alloys and their advantageous properties provided a foundation for success, automotive OEMs must have mechanical property data on the alloys to embrace them. This article explains mechanical property data for AJ52x and AE42 that diecasters and automotive design engineers may consider for material used in high-temperature applications.

A Durable Investigation

The Materials Technology Laboratory (MTL)--Canada Centre for Minerals and Energy Technology (CANMET), Ottawa, Ontario, Canada, developed a program to examine and establish baseline tensile, impact, creep aim fatigue properties of AJ52x and AE42 magnesium diecast alloys for automotive applications. All the test samples were cast in three-, four- and six-cavity dies using a 600-ton cold chamber diecasting machine. The samples were X-rayed before testing, and with a few exceptions, all samples were test in the as-cast condition.

The effects of test bar thickness and test temperature on mechanical properties were investigated, and increases in tensile ductility and reduction in tensile properties were observed at higher test temperatures for both alloys.

Tensile

Both alloys' tensile properties decreased as the test temperature was increased from room temperature [22C, 72F] to 175C (Table 2). For the cylindrical test bars data, the ultimate tensile strength (UTS) and yield strength (YS) decreased, and the tensile ductility (percent elongation) increased during the temperature change.

For AE42, the average UTS and 0.2% YS from these test bars was 207 and 115 MPa at room temperature compared to 120 and 83 MPa at 175C. There was an improvement in tensile ductility--9.3% at room temperature and 21.5% at 175C.

For AJ52x, the UTS and 0.2% YS were 218 and 124 MPa at room temperature compared to 134 and 98 MPa at 175C. There also was an improvement in tensile ductility--7.9% at room temperature compared to 21.6% at 175C. The highest percent elongation was observed in the 3.25-mm thick plate of alloy AE42.

AJ52x exhibited higher UTS and 0.2% YS than AE42 at all test temperatures, which was noted during the flat bar testing. The tensile properties of the flat test bars were similar to those observed for the cylindrical samples. AJ52x exhibited higher UTS and YS than alloy AE42 at comparable temperatures. However, AE42 exhibited higher ductility than AJ52x, especially at room temperature and 150C.

Impact

The impact properties from the standard Charpy bars for both alloys revealed that at 150C, AJ52x (10.6 J) exhibited slightly better impact properties than alloy AE42 (9.3 J). On the contrary, the room temperature impact properties of AE42 (10.8 J) were better than those of AJ52x (7.4 J). However, for the sub-size test samples at room temperature and 150C, AE42 (8.5 J and 11.9 J) exhibited higher impact properties than AJ52x (5.6 J and 3.4 J).

These results show that both alloys reveal good impact properties at 150C for the standard Charpy bars, and that alloy AJ52x could be preferred over alloy AE42. It is not clear why the impact energy was higher for the sub-size sample of AE42 at 150C. It is probable that better casting soundness observed in the X-ray radiography of the sub-size bars and the observed porosity in the Charpy samples must have contributed to the scatter in the results.

Fatigue

Two or more samples were tested at the same stress level for each alloy. There was no apparent fatigue limit of both alloys based on the limited data, and in fact, several test samples didn't fail before the test was stopped. For example, none of the three samples tested at room temperature at 74 MPa broke after 30 million cycles. A similar observation was made for two other samples tested at 84 MPa. Two of the three samples tested at 94 MPa broke after 240,000 cycles, but the third one didn't break after 30 million cycles.

For the samples tested at 150C, one of the three samples tested at 50 MPa broke after 5 million cycles. The other two didn't break even at 6 million cycles, and the test was stopped. None of the three other samples tested at 60 MPa failed after almost 8 million cycles. All the other samples tested at 70 MPa failed after fewer than 1 million cycles, except for one of the samples tested at 70 MPa.

AE42 had a slightly better fatigue resistance at 74 MPa at room temperature, but all the samples tested at about 85 MPa for both alloys broke at relatively low cycles. At room temperature, AE42 seemed to exhibit better fatigue properties. However, alloy AJ52x tended to have better fatigue properties at 150C.

Due to the fact that all samples were tested in the diecast condition, any subsurface defects (porosity) as well as external surface discontinuities, such as parting line flash and ejector pin marks, could lead to the scatter observed in this data. Despite the scatter, it is apparent that the applied stress during testing and the test temperature are key process parameters in determining the fatigue properties of the alloy.

Creep

Creep elongation for 1.8- and 6.35 mm flat samples was similar for both alloys at 35 MPa and 100-hr. rupture time. At 50 MPa, AE42 showed a significantly higher creep elongation than AJ52x. For the round test bars, AE42 again had a higher creep elongation than AJ52x at 200-hr. rupture time. The creep rate for the 1.8, 3.25 and 6.35 mm plates of AE42 were similar at 35 MPa. At 50 MPa, the 1-mm sample showed relatively high creep elongation (0.275%) compared to about 0.025% for the 1.8-mm thick sample. AJ52x exhibited significantly better creep properties than alloy AE42, especially at 50 MPa and 200-hr. rupture time.

A Better Performance By Which Alloy?

Before analyzing the results of each test, it must be recognized that the condition of the diecast test samples acted as a significant parameter in the research. The revealed mechanical properties of the alloys are dependent on the quality of each bar, and one bar might not be as optimized as another.

Although the properties of both alloys can be further examined, it can be said that based on these results, AJ52x performed slightly better overall than AE42. AJ52x exhibited better tensile properties at all temperatures, better fatigue properties at higher temperatures and better creep properties at higher stress levels. The latter two reasons can be more attractive to OEMs who appreciate an alloy that works well under high temperatures, especially inside a running vehicle.

The results of these tests show there has been some improvement of diecast magnesium alloy properties. But because an alloy performs well at one temperature does not mean it will be satisfactory at another temperature. On the contrary, the usage of a new alloy tends to depend on the manufacturer. European OEMs are more liberal with new alloy usage titan those in North America, and a number of OEMs look for other properties, such as pressure and stress values, as well as tests performed with different product forms at different temperatures.

Despite the positive results from these tests, magnesium, when compared to aluminum and zinc diecasting alloys, has been said to be relatively low-strength and have low impact strength. Many manufacturers, however, would welcome the use of durable, light-weight magnesium alloys. The investigation of these properties is intended to assist OEMs with such data so that large magnesium castings with good structural components can be made.

[GRAPHIC OMITTED]

Table 1. Chemical Analysis of AE42 and AJ52x During
Diecasting Operation

                       Composition (wt. %)

 Sample     Mg      Al         Zn          Mn       Sr
 Number
                           Alloy AE42

ASTM spec   bal   3.6-4.4   0.20 max.   0.10 min    --
    1       bal    4.644      0.146      0.285      --
   2&3      bal    3.861      0.148      0.333      --

                          Alloy AJ52x

    1       bal    4.92      0.0235      0.3598    1.91
    2       bal    4.89      0.0093       0.23     1.63

                        Composition (wt. %)

 Sample       Si       Fe       Cu       Ca       R.E
 Number
                            Alloy AE42

ASTM spec   <0.010   0.003    <0.010     --     2.0-3.0
    1       0.043    0.0036   0.0055   0.011     2.498
   2&3      0.031    0.0031   0.0046   0.0053    2.268

                           Alloy AJ52x

    1       0.0114   0.0045   0.0004   0.0025     --
    2       0.0016   0.0023   0.0003   0.0022     --

Table 2. Average Mechanical Properties of AE42 and AJ52x

                 UTS (MPa)      0.2% Offset
                                  YS (MPa)

Test Temp (C)   AJ52x   AE42    AJ52x   AE42

6.35 mm round test bar

     23         128.2   207.0   123.7   115.0
     100        195.1    --     109.8    --
     150        154.0   140.0    99.5    89.0
     175        134.4   120.0    97.6    83.0
     200        118.8    --      47.4    --

6.35 mm fl at test bar

     23         187.2   194.0   124.8   113.0
     100        171.0    --     107.3    --
     150        146.4   125.0    92.5    80.0
     175        128.2   113.0    89.1    74.0
     200        112.5    --      47.8    --

3.25 mm fl at test bar

     23         201.6   199.0   133.2   116.0
     100        193.0    --     115.2    --
     150        155.4   139.0   110.1    90.0
     175        136.0   113.0   100.7    81.0
     200        123.1    --      44.5    --

1.8 mm fl at test bar

     23         206.8   200.0   142.1   124.0
     100        174.5    --     112.0    --
     150        145.2   133.0   107.9    91.0
     175        133.5   114.0   103.8    84.0
     200        119.7    --      51.2    --

1 mm fl at test bar

     23         127.7   138.0   120.9   112.0
     100        158.1    --     115.4    --
     150        128.6   103.0   108.5    84.0
     175        130.7    89.0   105.1    75.0
     200        118.8    --      73.5    --

                  Elongation      Modulus
                      (%)          (MPa)

Test Temp (C)   AJ52x   AE42   AJ52x   AE42

6.35 mm round test bar

     23          7.9     9.3   43636   41115
     100        13.5     --    36824    --
     150        16.6    19.2   39142   34145
     175        21.6    21.5   37816   37144
     200        17.0     --    13655    --

6.35 mm fl at test bar

     23          4.0     7.3   48132   42724
     100         6.0     --    41386    --
     150        11.9    11.7   39348   37508
     175        12.6    17.2   35623   38328
     200        12.3     --     4516    --

3.25 mm fl at test bar

     23          4.6     6.7   44788   42181
     100         9.0     --    41803    --
     150        15.8    24.8   41206   36790
     175        13.2    27.7   38828   38671
     200        22.9     --     9657    --

1.8 mm fl at test bar

     23           3.9    6.2   39903   42372
     100          7.2    --    40254    --
     150          7.5   19.6   36706   35024
     175          8.5   23.7   40624   37542
     200         17.8    --    12073    --

1 mm fl at test bar

     23           0.3    2.0   40277   38946
     100          4.3    --    42357    --
     150          4.7    3.3   30923   35882
     175         13.3    2.4   35726   30135
     200          9.6    --    10605    --

This article was adapted from a paper presented at the 2003 North American Die Casting Association Congress.

For More Information

Visit www.moderncastings.com for the entire report, "Mechanical Properties of High Pressure Diecast Magnesium Alloys AE42 and Noranda New Alloy (AJ52x) for Hight-Temperature Applications."

Yemi Fasoyinu, Terri Castles, Real Bouchard and Mahi Sahoo, Materials Technology Laboratory--CANMET, Ottawa, Ontario, Canada, Pierre Labelle, Noranda Inc. Technology Centre, Montreal.

Yemi Fasoyinu, Real Bouchard, Terri Castles and Mahi Sahoo are all part of the Materials Technology Laboratory (MTL)/CANMET, Ottawa, Ontario, Canada, which is part of Natural Resources Canada. In collaboration with industry; MTL conducts applied research, and develops and deploys technologies to improve all aspects of producing and using value-added products from minerals and metals. Pierre Labelle is a material and applications engineer with Noranda Inc. Technology Centre, Montreal. and supports customers in prototyping computer modeling/simulation and applications.