Aluminum Die Casting Alloy Studies L. Wang, D. Apelian and M. Makhlouf

Report No. 08-#2 Funded by US DLA – AMC CIR

________________________________________________ PROJECT TITLE High-Performance Die Casting Alloys: Castings for Improved Readiness Needs, Challenges, and Objective This project is part of the AMC’s Casting for Improved Readiness (CIR) Program, and sponsored by the Defense Supply Center, Philadelphia, PA, Defense Logistics Agency, Ft. Belvoir, VA. The Casting for Improved Readiness Program aims to provide tools to help the DLA to procure spare casting parts. Overall program needs are:

High quality parts; Capable sources; Reduced costs; and Reduced lead time

This project is focused on alloy selection. For producing spare parts, selecting the proper alloy is one of the major challenges. It affects part quality, cost, and lead time, and sometimes it can be problematic because for some parts the alloy specifications cannot be identified or the parts were made long time ago and the alloy used then is now obsolete. In Phase I of the project, we developed an electronic tool, i-Select-Al, for selecting alloys. However, for a specific application when an alloy is selected, the chemistry limits in the specification generally are wide resulting in wide variations in mechanical properties giving rise to non-compliance issues. In many cases, there is a need to reduce variation in properties in cast components in order to enable casting conversions to meet the demands for OEMs. In other cases, premium grade high performance alloys are needed. To meet these needs the current work, Phase II of the project, was initiated. Its objective is:

To select or develop a set of premium grade alloys that have properties that meet the high demands that are placed on OEM components with cost-effective cast products

STRATEGY (Solution Path–Technology Development Plan) Based on the requirements of the project set by DLA, the work is divided into the following tasks:

1. Determine target applications/alloy to be optimized; discuss with industry colleagues/partners.

2. Use i-select to optimize alloy(s) for the specified application. 3. Cast samples of the select grade (premium grade) alloy – laboratory

trials. 4. Validate the above select grade alloy(s) at industrial beta sites on

commercial die cast components. 5. Develop of the select grade die cast alloy(s). 6. Disseminate - Technology Transfer of the results.

Benefits The anticipated benefits are:

• Development of a set of new and premium grade alloys (within current specification limits)

• Reduced lead time and cost • Improved properties and ensured compliance • Reduction in scrap • Potential for fabricated parts to be converted to castings, thus reducing

production costs Industry Partners Contech (formerly SPX) Premier Tool & Die Cast ACHIEVEMENTS TO DATE Task 1: Target applications/alloy to be optimized; discussions with industry colleagues/partners. Task 2: Use i-select to optimize alloy(s) for the specified application. Task 3: Cast samples of the select grade (premium grade) alloy – laboratory trials

Tasks 1, 2, and 3 have been completed and reported in the last few program reports. This report will focus on the Task 4.

Task 4: Validate the select grade alloy(s) at industrial beta sites on commercial die cast components Two different samples are validated at beta sites. One is using standard samples (specimens), which was carried out at Premier. The other set was using the samples, which were machined from actual cast components. Contech die cast the components.

Validation on the Standard Specimens

Specimen Production

For casting the standard specimens NADCA funded, designed, and manufactured a special die for this project. The specimens were die cast at Premier. Premier contributed their facilities, resources and personals and made significant efforts in carrying out this project. They set aside a die cast machine exclusively for this project for a few weeks (see Figure 1 the die casting machine used at Premier). Besides the preparation in setting the machine and testing and adjusting the die just casting one alloy took minimum two days.

Figure 1 Die casting machine for producing the specimens at Premier.

Mr. Jeff Brennan is our liaison at Premier and is a wonderful collaborator. He organized and oversaw the whole production. So far, the specimens were

produced for 3 alloys, A380*, AMC380*, and the reference alloy, commercial A380. The compositions, target and measured, of these alloys are shown in Table 1. A run for alloy AMC1045* was also conducted but aborted because of alloy composition. Another run for the AMC1045* is being prepared and will be conducted soon.

Table 1 The compositions of test alloys

Composition (%) Alloy

Si Mg Cu Fe Mn Zn Ni Ti Sr

A380 Spec. 7.5- 9.5 <0.1 3-4 <1.3 <0.5 <3 <0.5 - -

Commercial A380 Measured 9.08 0.05 3.11 1.05 0.27 1.80 0.076 0.053 -

Target 7.5 0.1 4 0.7 0.5 3 <0.1 - -

Limits 7.0- 8.0

0.08- 0.12

3.8- 4.2

0.63- 0.73

0.47- 0.53

2.75- 3.25 <0.1 <0.2 <0.005 A380*

Measured 7.67 0.097 3.44 0.73 0.43 2.15 0.038 0.15 -

Target 9.5 0.3 3.0 0.7 0.5 3.0 <0.1 0.2 0.02

Limits 9-10 0.27- 0.33

2.8- 3.2

0.63- 0.73

0.47- 0.53

2.75- 3.25 -

0.18- 0.22

0.018- 0.022

AMC 380*

Measured 9.45 0.33 3.00 0.61 0.55 1.89 0.027 0.22 0.035

Target 11.0 2.5 2.0 0.3 0.4 <0.3 - 0 0.02 AMC

1045* Limits

10.5- 11.5

2.3- 2.7

1.8- 2.2

0.27- 0.33

0.37- 0.43 <0.3 <0.05 <0.01

0.018- 0.022

For each run a minimum of 1500 lbs of the alloy was prepared. The initial 10 shots were discarded to ensure steady state conditions and the subsequently 100 to 120 shots were made for specimen production. Process details have been recorded. Five specimens were tested for tensile properties in the same day after casting at Premier.

Property tests

Considering that the properties of these alloys can be affected by natural aging due to the presence of Mg and Cu and fast cooling of die casting the property tests were conducted for several times in different intervals after casting. Including the test in the same day after casting, 4 batches of specimens were tested, which were at within 24 hours, 1.5 months, 3 and 4 months after the specimens were cast. For each alloy totally over 40 specimens were tested. The first test was conducted at Premier and the rest of them were at WPI. It showed that the properties varied with time. The changes became slower after 1.5 months and nearly stable after 3 months. The test results are shown below.

5 specimens were tested for each alloy within 24 hrs after casting. The results are shown in Table 2 and the comparisons of the tensile properties of alloys A380* and AMC380* with the commercial A380 alloy are shown in Table 3.

Table 2 Tensile properties of the alloys measured within 24 hours after casting.

TS Ksi (MPa)

YS Ksi (MPa)

Elongation % QI*

Commercial A380 41.2±1.8

(284.1±12.0) 21.1±0.2

(145.5±1.4) 2.55±0.39 195.0±6.1

A380* 39.7±1.4

(273.7±9.5) 21.8±0.2

(150.3±1.1) 2.58±0.33 200.3±4.2

AMC380* 43.3±0.9

(298.6±6.5) 25.5±0.3

(175.8±1.7) 2.33±0.23 221.9±3.3

* QI – Quality Index (QI = YS (MPa) + 90•log E + 13)

Table 3 Comparison of the properties measured within 24 hours after casting.

Changes (%) of tensile properties comparing with commercial A380 alloy

TS YS Elongation QI A380* vs Commercial A380 -3.6 3.3 1.2 2.7

AMC380* vs Commercial A380 5.1 20.9 -8.5 13.8

In the second batch, one and half months after casting, 15 specimens were tested for each alloy, a few data from severely defected specimens were discarded, and results from 11 to 13 specimens (11 for commercial A380, 12 for A380*, and 13 for AMC380*) are shown in Table 4. The comparisons of the properties are shown in Table 5.

Table 4 Tensile properties of the alloys measured at one and half months after casting.

TS Ksi (MPa)

YS Ksi (MPa)

Elongation %

QI*

Commercial A380 40.9±0.6

(282.2±4.4) 22.6±0.5

(155.9±3.5) 2.27±0.09 200.9±4.0

A380* 41.1±0.9

(283.5±6.2) 23.4±0.5

(161.5±3.5) 2.58±0.21 211.5±3.6

AMC380* 45.3±1.3

(312.3±9.1) 27.6±0.4

(190.5±2.9) 2.38±0.27 237.5±4.1

Table 5 Comparison of the properties measured at one and half months after casting.

Changes (%) of tensile properties comparing with commercial A380 alloy TS YS Elongation QI

A380* vs Commercial A380 0.5 3.6 13.5 5.3 AMC380* vs Commercial A380 10.8 22.2 5.0 18.2

In the third batch, 3 months after casting, the results from 15 specimens are shown in Table 6 and the comparisons of the properties are shown in Table 7.

Table 6 Tensile properties of the alloys measured at 3 months after casting.

TS Ksi (MPa)

YS Ksi (MPa)

Elongation %

QI

Commercial A380 41.6±1.5

(286.8±10.4) 23.5±0.6

(162.0±4.4) 2.26±0.26 207.0±5.5

A380* 41.3±1.2

(284.5±8.6) 24.1±0.5

(166.0±3.4) 2.49±0.25 214.6±5.4

AMC380* 45.6±1.0

(314.3±7.1) 28.5±0.5

(196.6±3.1) 2.27±0.17 241.6±4.4

Table 7 Comparison of the properties measured at 3 months after casting.

Changes (%) of tensile properties comparing with commercial A380 alloy TS YS Elongation QI

A380* vs Commercial A380 -0.8 2.4 9.9 3.7 AMC380* vs Commercial A380 9.6 21.3 0.3 17.0

In the fourth batch, 4 months after casting, 5 specimens were tested and the results are shown in Table 8. The comparisons of alloy properties are shown in Table 9.

Table 8 Tensile properties of the alloys measured at 4 months after casting.

TS Ksi (MPa)

YS Ksi (MPa)

Elongation % QI

Commercial A380 42.7±0.5

(294.2±3.6) 23.6±0.1

(163.0±0.9) 2.40±0.11 210.2±2.4

A380* 43.1±0.4

(297.0±2.8) 24.2±0.2

(167.1±1.3) 2.81±0.19 220.5±2.8

AMC380* 45.7±1.7

(314.8±12.0) 28.5±0.3

(196.4±1.9) 2.40±0.13 243.6±3.0

Table 9 Comparison of the properties measured at 4 months after casting.

Changes (%) of tensile properties comparing with commercial A380 alloy TS YS Elongation QI

A380* vs Commercial A380 0.9 2.5 17.1 4.9 AMC380* vs Commercial A380 7.0 20.1 0.2 15.9

The summary of the test results measured at different time is shown in Table 10.

Table 10 Summary of the properties measured at different time after casting.

Alloy Test Time

TS Ksi

YS Ksi % QI

1 Day 41.2 21.1 2.55 195.0 1.5 Mo 40.9 22.6 2.27 200.9 3 Mo 41.6 23.5 2.26 207.0

Commercial A380

4 Mo 42.7 23.6 2.40 210.2 1 Day 39.7 21.8 2.58 200.3 1.5 Mo 41.1 23.4 2.58 211.5 3 Mo 41.3 24.1 2.49 214.6 A380*

4 Mo 43.1 24.2 2.81 220.5 1 Day 43.3 25.5 2.33 221.9 1.5 Mo 45.3 27.6 2.38 237.5 3 Mo 45.6 28.5 2.27 241.6 AMC380*

4 Mo 45.7 28.5 2.40 243.6

It can be seen that:

• A380* and AMC380* both have better properties than commercial A380. Compared with commercial A380, when the properties are stabilized, e.g. in about 4 months after casting, the A380* has a substantial increase in %, about 10%, and a slight increase in YS (~3%); the AMC380* alloy has significant increase in YS (~22%) and similar %. Both of A380* and AMC380* have higher QI than commercial A380 with the increases of about 4% and 17%, respectively.

• The project goals are to increase QI for 12-15%, YS for 8-10%, and % for 20-30% (3-4% to 4.5%). The results from the standard 2” gage length specimens show that the goals for the YS and QI are achieved but % increase did not meet the goal.

• For the same alloy, generally TS and YS increase with time but % has little change.

• Property variation rates decrease with time, however the rate of decrease also decreases with time. Initially – first 45 days, the observed decrease is in range of 6-7%, whereas 90 days later it is almost stabilized.

Analyses of Reasons for Low % and TS

The properties for these compositions can be predicted using i-Select-Al based the alloy compositions shown in Table 2. The predicted and measured values are shown in Tables 11.

Table 11 Predicted properties using i-Select-Al and the achieved properties for different alloy compositions.

Predicted Tensile Property [Actually Achieved] Alloy

UTS (Ksi/MPa) YS (Ksi/MPa) % Quality Index

A380 - Specified 47 (324) 23 (159) 3-4 215-216

Commercial A380

Measured Composition

41.4 (285) [42.7 (294)]

23.3 (161) [23.6 (163)]

3.61 [2.40]

224.2 [210.2]

Target Composition

44.3 (306) 24.4 (168) 4.72 231.7 A380*

Measured Composition

43.9 (302) [43.1 (297)]

23.6 (163) [24.2 (167)]

4.96 [2.81]

238.6 [220.5]

Target Composition 46.7 (322) 26.7 (184) 4.52 256.0

AMC380* Measured

Composition 44.6 (307)

[45.7 (315)] 26.7 (184)

[28.5 (196)] 4.09

[2.40] 252.1

[243.6]

We can see that the YS achieved in this work is approximate the same to or a little higher than the predicted values and the TS is about the same to the predicted. However, the % is much lower than predicted. In addition, the % and TS are both lower than the specified values for A380. The typical TS and % elongation for commercial A380 should be 45 to 47 Ksi and 3 to 4%, respectively; however, the measured values were only about 43 Ksi and 2.5%.

The reasons for low % and TS were analyzed. When the specimens were first tested at Premier Mr. Jeff Brennan thought that the reasons could be related to the die design used in producing the specimens. It is probably right and the low % and TS may stem from the following reasons.

In general, for the same alloy the die casting has better tensile properties than the other castings, for example, permanent mold casting, especially in TS and %. It is mainly because that die casting generally has thin wall and cooled faster under high pressure, resulting in finer and denser microstructure. The differences can be clearly seen when we compare the properties of the castings produced by die casting and permanent mold for the same alloy. In this project, we did cast the A380 and AMC380* alloys in permanent mold and their TS and % (shown in Table 12, tested 30 days after casting) are much lower than the die castings.

Table 12 Permanent mold casting properties of A380 and AMC380* (Specimen diameter of 0.5”, tested 30 days after casting.

Alloy UTS (Ksi/MPa) YS (Ksi/MPa) % Quality Index

A380 31.0±1.1 21.1±0.8 1.00±0.11 158.5

AMC380* 38.6±1.3 26.1±0.5 1.49±0.16 208.5

In the same die casting the properties also vary with locations and are affected by casting design. Look at the die used in this work (see Figure 2 below). The tensile bar is located at the upper side. It is far from the main gate with a long narrow ingate leading the metal in. The long narrow ingate incorporated with fast cooling by the die can cause significant temperature drop of the metal entering the tensile bar cavity. The temperature drop makes the melt to solidify before the bar cavity is completely filled. The resistance formed by the partially solidified metal in the long narrow channel will reduce the pressure exerted on the metal filled in the bar cavity during the final solidification.

Figure 2 Casting of the standard specimens produced at Premier.

The melt enters the bar cavity at the end of one grip section. The connection between the ingate and the bar cavity is a “neck” area with a smaller cross section than both the bar grip and the ingate, which is designed for easily breaking the bar from the gate. So, the pattern of bar cavity filling is as follows. The melt flow along the long narrow ingate, through an even narrower “neck” enters a bigger cross section (grip section of the bar), then leads to a small cross section (the gage section of the bar), and finally enters the bigger cross section (another grip section). Generally, in the die casting when the melt flows from a small to a bigger cross section, the area with bigger cross section may solidify under a little lower pressure than the area with small cross section.

In addition, the melt overflow and gas release channels of different specimens in the die are interconnected. Thus, it is possible the melt flows out from the other specimens first and blocks the gas-escaping path from tensile bar.

In summary, three factors mentioned above are:

1. The long narrow ingate leading the bar to be filled with higher portion of solid and solidified under a lower pressure,

2. The cross section variations in the melt flow path of filling the bar cavity resulting in the area with bigger cross section solidifying under lower pressure, and

3. The possible blockage of the gas escape channel.

All of them can result in severer porosity in the tensile bar, especially in the lastly filled grip section, produced in the currently used die. The microstructure analyses confirmed the speculation. Figures 3 and 4 show the microstructures of the cross sections at the connecting area between the specimen gage and grip sections for alloys A380 and A380*. Figures 5 and 6 show the fracture surfaces of alloys A380* and AMC380*. All of then contain large amounts of porosity with big pores.

The porosity will make the specimen to have lower TS and % but does not affect the YS much. YS is mainly determined by the alloy composition, phases, phase size and distribution. This can be seen from the testing results. We tested 15 specimens for each alloy in the second batch of tests and many bars were broken at the lastly filled grip section (left grip section in the photo) though this part had a larger cross-area than the gage section. The number of bars, which were broken at the left grip section, the gage section, and the right grip section, were 10/5/0, 6/4/5, and 6/7/2, for alloys commercial A380, A380*, and AMC380*, respectively. The test results show that the TS and % have larger variation than YS.

The reasons for the differences in % and TS between the predicted and our obtained values are also attributed to database used for the prediction. The data, which the predictions were based on, were generated using die, where the tensile bar was close the main gate with short ingates. The alloys were provided by Wabash Alloys and were degassed. The specimens were die cast using a vacuum die casting machine.

Figure 3 Microstructure of alloy A380 specimen at the connecting area between the gage and grip sections.

Figure 4 Microstructure of alloy A380* specimen at the connecting area between the gage and grip sections.

Figure 5 Fracture surface of alloy A380 specimen.

Figure 6 Fracture surface of alloy AMC380* specimen.

Property Tests using 1” gage Bars

All the results discussed above were generated using the standard test bar with 2” gage length. The same die also produces a 1” gage tensile test bar. This 1” gage bar has a little shorter ingate and bigger grip sections, which can be seen in Figure 2, where the 1” gage bar is located at the bottom. So, the 1” gage bar should have a better filling and solidification condition than the 2’ gage bar and we believe that the 1” gage bars should have better properties. Accordingly, 8 specimens for each alloy were tested. The data and comparisons are shown in Tables 13 and 14. For all the alloys the 1” gage bars show much higher TS and % elongation. The data also show that the YS and QI of AMC380* are higher than A380 by about 28% and 19% (both meet the targets), and the % elongation and QI of A380* are higher than A380 by about 24% (meets the target) and 10%, respectively.

Table 13 Tensile properties of the alloys measured using 1” gage specimens.

TS Ksi (MPa)

YS Ksi (MPa)

Elongation % QI

Commercial A380 45.4±1.0

(313.0±7.1) 21.8±0.8

(150.1±5.7) 3.81±0.32 215.4±8.0

A380* 46.4±0.6

(319.7±4.4) 23.7±0.5

(163.1±3.5) 4.72±0.43 236.7 ±4.0

AMC380* 50.0±0.7

(345.0±4.6) 27.9±0.9

(192.5±6.0) 3.71±0.29 256.7±5.6

Table 4 Comparison of tensile properties of alloys A380* and AMC380* with commercial A380 alloy (measured from die cast specimen, 1” gage length).

Comparison of tensile properties with commercial A380 alloy

TS YS Elongation QI

A380* vs Commercial A380 +2.1% +8.7% +23.7% +9.9%

AMC380* vs Commercial A380 +10.2% +28.3% -2.6% +19.2%

Work in Progress

Microstructural and fracture surface analyses of specimens of 1” gage bars and cut from cast components and comparisons of them with those of 2” test bars are in progress.

The work in progress and to be done

• More 1” gage specimens are being tested. • Testing the specimens cut from the casting components. • Microstructure analyses