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Making A535, A206 viable in permanent mold.

Aluminum alloy A535 is well suited for automotive, marine and military applications because of its corrosion resistance in mild alkaline and salt spray exposure and combination of strength, ductility, shock resistance and dimensional stability in the as-cast condition. However, the alloy has poor hot-tearing resistance when poured in metal molds, partly due to its long solidification range of 144F (80C) with liquidus and solidus temperatures of 1,166F (630C) and 1,022F (550C).

Alloy A206 is subject to hot cracking when poured in metal molds for the same reason. As a higher purity version of aluminum 206, A206 is used in automotive, aerospace and other applications where high mechanical performance is needed.

With the increased interest in vehicle weight reduction and cost savings, development of viable permanent mold casting of A535 and A206 should be of interest to the automotive industry. But there is limited information on the gravity and low pressure permanent mold casting of these two alloys, so the selection of either for rigorous engineered components is uncommon.

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In order to study the issues encountered in the permanent mold casting of hot tear susceptible alloys, five engineered components were cast from the two alloys. The components included engine mount, rocker arm, swivel, elbow and swing arm castings.

Control of Hot Tearing

For alloys that solidify dendritically over a long freezing range, hot tearing is more prevalent close to 100% solidification because of a lack of liquid metal to feed the voids created by contraction and shrinkage. The growth of the primary interlocking dendrites commonly associated with long freezing range alloys is present with some residual liquid film close to the solidus temperature of the alloy. The trapped residual liquid freezes progressively until the process is complete at the solidus temperature.

Grain refinement improves the hot tearing resistance of A535 and A206, which could be due to the breakdown and refinement of the interdendritic structure, leading to improved feeding and a decrease in the amount of residual liquid during the last stages of solidification.

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A recent study showed that hot tearing of constrained rod castings from A206 and A535 in a metal mold was reduced when the mold temperature was more than 752F (400C). The hot mold provided the required thermal gradient during solidification that lead to improved feeding and reduced hot tearing. When there is a steep thermal gradient, the availability of liquid metal to fill the voids created during solidification compensates for any developed shrinkage stress.

With this understanding in mind, casting trials for the prototype components were performed.

Engine Mount Casting

The mold temperature for the engine mount castings ranged from 554-797F (290-425C) for the low pressure casting trials. The castings were poured in A535 and A206 at 1,242-1,346F (672-730C) and 1,285-1,389F (696-754C). An applied pressure of 350-400 mbar produced full castings for both alloys. Cracking during ejection occurred during preliminary casting trials because the part tended to drag due to an insufficient ejector system. The number of ejector pins was increased to prevent the twisting of the casting, which reduced and sometimes eliminated the physical cracking (Fig. 1). The majority of the hot tears in the castings occurred at sharp corners, and a generous radius there should eliminate the hot tear in future casting trials.

X-ray radiographic inspection revealed hot tear and sponge shrinkage that range from category 1 to 8, based on ASTM E155 (series 11) standards. Sponge shrinkage categories greater than 3 may not be acceptable for engineered components. The sponge shrinkage usually occurred on the vertical ribs of the casting furthest from the ingate and where a change in cross section occurred. The long freezing range in both alloys probably contributed to the poor feeding and formation of shrinkage porosity defects. The introduction of cooling channels in the mold would help to encourage directional solidification to improve soundness.

Rocker Arm Casting

The rocker arm mold and core temperatures ranged from 329-509F (165-265C) and 248-500F (120-260C). Alloys A535 and A206 were poured at 1,256-1,301F (680-705C) and 1,2831,301F (695-705C). The rocker arm molds generally were filled at no more than 122F (50C) superheat.

The use of a hollow metal core reduced rapid heat loss. Five of the 10 castings poured from A535 with reduced magnesium level were free of hot tear cracks. Lowering the copper level to 3.8% in A206 produced marginal improvement in hot tear resistance. Hot tear and surface shrinkage were present on some of the castings.

Extraction of the casting from the mold cavity was difficult due to the deep recesses of the mold, and the casting tended to stick to one side of the die (Fig. 2). A top coat of graphite wash on the insulating coating was not always effective as a mold release. Ejector pins were not used because of the limited capacity of the machine. The thermal expansion of the core could possibly have forced the casting toward the wall and increased the difficulty of ejection. The use of a composite mold and a core with lower thermal expansion than the mold material could alleviate this problem.

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Swivel and Elbow Castings

The swivel casting design used generous taper and no sharp corners and incorporated a generous gating system that provided molten metal to feed solidification shrinkage. In addition, the ejector mechanism worked well.

The average mold temperatures during the pouring of the swivel casting were 685F (363C) for A535 and 694F (368C) for A206, which is hotter than normally used for A356. The pouring temperatures ranged from 1,326-1,339F (719-726C) for A535 and 1,333-1,346F (723-730C) for A206. The success of the casting can be attributed to a combination of good casting design, fully grain-refined metal and good thermal management of the mold.

The mold temperature of the elbow component, which used a sand core, was kept at 675 [+ or -] 5F (357 [+ or -] 3C) for A535 and 650 [+ or -] 5F (343 [+ or -] 3C) for A206, and the pouring temperatures ranged from 1,420-1,447F (771-786C) and 1,420-1,438F (771-781C).

X-ray radiography of the swivel castings in both alloys showed that they were free of hot tears. The castings met Grade A for AMS-STD-2175, which is equivalent to less than a frame on the ASTM E155 standard. Radiography of the elbow castings returned Grades B and C quality in alloys A206 and A535 (Figs. 3-4).

Swing Arm Casting

The swing arm part was designed with an open elastic concept from inside out with a collapsible sand core and a goal of 0.2-0.24-in. (5-6-mm) walls throughout the casting. In areas where it was not possible to adhere to these criteria, recoverable steel chills were inserted in the sand core, and aggressive water cooling provided the needed cooling rate to preclude the formation of hot tears. In addition, cross hatching was added to all stress concentration locations, and the parts were ejected as early as possible to limit stress.

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The low pressure machine used to produce the parts was rebuilt with rack and pinion hydraulic ejection so they could eject as square and soft as possible. Every effort was made to ensure the lowest oxide content through the use of multiple filters in the feed tube and casting ingate.

Intensive solidification modeling was used to aid mold design by predicting the areas on the part that could produce cracks where the die hit certain maximum temperatures based on the wall thickness and casting geometry. The feeding distance of the alloy and gating were limited, and the model suggested that it be distributed across much of the part for feeding. This also helped limit the hot spots that could contribute to tearing.

The first eight swing arm castings poured before grain refinement showed hot tears. Addition of the grain refinement and thermal management of the permanent mold die during casting eliminated those tears. Grain refinement shrank the grain size from 105 to 65 [micro]m.

Production Mode

Once it was demonstrated that prototype components could be produced successfully from the hot tear-susceptible aluminum alloys A535 and A206, production parts and industrial scale casting machines were used to implement some of the ideas that made this possible. The casting trials at two production metalcasting facilities showed that engineered components could be cast successfully in permanent mold without hot tears from both alloys, as long as special attention was paid to the following:

* Casting and mold design (utilization of generous tapers and draft angles and avoidance of sharp corners and abrupt changes in section thickness);

* Mold coating and mold temperature;

* Effective ejector mechanism (placement and generous ejector pin allowances);

* Molten metal processing (effective degassing and grain refinement techniques);

* Processing temperatures (melting and pouring);

* Establishment of appropriate thermal gradient (effective cooling rate control) during casting solidification.

For More Information

"Grain Refinement of Aluminum Casting Alloys," G. Sigworth and T. Kuhn, International Journal of Metalcasting, Fall 2007, p. 31.

"Gravity Permanent Mold Casting of AI-Mg Alloy 535,"F. Fasoyinu, J. Thomson, D. Cousineaou, T. Castles and M. Sahoo, 2002 AFS Transactions 02-145.

Paul Burke, Grenville Castings Ltd., Perth, Ontario, Canada

David Weiss, Eck Industries Inc., Manitowoc, Wisconsin

Festus Fasoyinu, Jim Thomson and Mahi Sahoo, CANMET Materials Technology Laboratory, Ottawa, Ontario, Canada

RELATED ARTICLE: Making permanent mold casting possible.

A few of the specifics that made it possible to use permanent mold casting technology to produce castings in A535 and A206 without hot tears are given below.

MOLTEN METAL TREATMENT

* The metal must be fully grain refined to eliminate hot tearing for both alloys. This was achieved when the titanium level ranged from 0.02 to 0.26% with 5 to 20 ppm boron content. However, a combination of grain refinement and strict thermal management of the mold temperature provided the best change for producing hot-tear-free castings.

* Effective degassing and removal of oxide films and other nonmetallic inclusions is recommended. The use of the reduced pressure test technique was effective in ensuring gas-free castings.

* The engine mount, rocker arm, swivel, elbow and swing arm castings were poured from fully degassed and fully grain refined metal. The swing arm component incorporated filters in the feed tube and the casting ingate to ensure that only clean metal was used during the low pressure casting process.

MOLD COATING, MOLD TEMPERATURE, CASTING EJECTION

* Although the effect of mold coating was not specifically studied, the use of commercially available insulating and refractory coatings, sometimes followed by a lubricating graphite top coating to promote casting release, were effective for all the castings poured.

* Hot tear susceptible alloys must be poured in hotter metal molds than those used for A356 in order to preclude the formation of a hot tear, even from fully grain-refined metal. Based on casting design and cooling conditions, hot tearing appears to be less prevalent at a minimum mold temperature. For these casting trials, the minimum ranged from 662-752F (350-400C).

* Casting ejection time must be carefully controlled to allow the casting to cool with minimum restraint in the mold before it is ejected.

MOLD AND CASTING DESIGN

Swing Arm

* Solidification modeling helped predict hot spots so appropriate thermal management of the mold could be implemented.

* Water-cooling of the perimeter of the part opposite the gates facilitated the creation of a strong thermal gradient and reduced solidification time.

* A shell core with a hollow center provided the required core collapsibility after solidification. Reusable steel chills within the core provided aggressive chilling in the inside radii.

* Cross hatching of stress concentration locations and ejection of the casting as early as possible limited stress on the parts.

Swivel and Elbow

* Both castings were designed with rounded corners, generous taper and an effective ejection mechanism.

Engine Mount and Rocker Arm

* Challenges of producing these two components were associated with the mold design, which was not optimized for A535 and A206.

Paul Burke is metallurgical engineer at Grenville Castings, Perth, Ontario, Canada, and David Weiss is vice president of sales and engineering at Eck industries, Manitowoc, Wis. Festus Fasoyinu is a research scientist, Jim Thomson is a quality assurance manager and Mahi Sahoo is manager of casting technology at CANMET Materials Technology Lab, Ottowa, Ontario, Canada.
Table 1. Chemical Analysis of A535

Melt # Mg Mn Si
AA spec 6.2-7.5 0.1-0.25 0.15 max

Engine Mount and Rocker Arm Castings

N5115 6.9 0.18 0.040
N5193 7.2 0.19 0.050
N5136 7.0 0.18 0.070
N5171 7.0 0.18 0.047
N5188 7.1 0.29 0.035
N5235 5.0 0.20 0.066
N6125 7.6 0.20 0.051

Swivel and Elbow Castings

535-1 7.1 0.18 0.083
535-2 7.1 0.18 0.076

Melt # Fe Cu Ti
AA spec 0.15 max 0.05 max 0.1-0.25

Engine Mount and Rocker Arm Castings

N5115 0.060 0.010 0.130
N5193 0.070 0.010 0.120
N5136 0.059 0.004 0.150
N5171 0.060 0.010 0.035
N5188 0.050 0.010 0.035
N5235 0.068 0.005 0.033
N6125 0.08 0.012 0.150

Swivel and Elbow Castings

535-1 0.10 0.01 0.071
535-2 0.097 0.01 0.12

Table 2. Chemical Analysis of A/B206

Melt # Cu Si Fe
AA spec 4.2-5.0 0.1 0.15

Engine Mount and Rocker Arm Castings

N5111 4.5 0.022 0.090
N5192 4.8 0.040 0.070
N5137 5.0 0.040 0.086
N5201 3.8 0.040 0.070
N6124 5.0 0.041 0.059

Swivel and Elbow Castings

A206-1 4.9 0.061 0.065
A206-2 4.9 0.061 0.068

Swing Arm Casting

Gren-1 4.7 0.223 0.048
Gren-2 4.4 0.228 0.045
Gren-3 4.5 0.218 0.049

Melt # Mn Mg Ti
AA spec 0.2-0.5 0.15-0.35 0.15-0.30

Engine Mount and Rocker Arm Castings

N5111 0.28 0.320 0.224
N5192 0.34 0.214 0.065
N5137 0.37 0.230 0.053
N5201 0.25 0.200 0.080
N6124 0.38 0.29 0.075

Swivel and Elbow Castings

A206-1 0.26 0.31 0.23
A206-2 0.26 0.30 0.26

Swing Arm Casting

Gren-1 0.362 0.269 0.0085
Gren-2 0.356 0.257 0.0156
Gren-3 0.355 0.263 0.0289

Table 3. Processing Parameters for Engine Bracket Castings

 No. of Processing temp
Melt castings Range (F)
No. poured Mold Pouring

A206

N5111 23 572-734 1,292-1,317
N5192 8 608-707 1,285-1,301
N6124 10 554-797 1,368-1,389

A535

N5115 18 662-680 1,242-1,278
N5193 14 526-752 1,261-1,301
N6125 12 671-761 1,308-1,346

Table 4 Processing Parameters of the Rocker Arm Castings

Melt Castings Processing temp range (F)
No. poured Mold Core Pouring

A535

N5136 11 424-509 302-446 1,256-1,263
N5171 13 383-464 284-446 1,256-1,301
N5188 5 329-401 248-437 1,252-1,299
N5235 10 374-478 293-446 1,250-1,260
N5147 10 374-518 293-428 1,261-1,292

A206

N5137 11 356-451 320-446 1,283-1,301
N5201 7 378-500 293-500 1,283-1,301

Table 5. Processing Temperatures for Swivel Castings

 A535.0 A206.0
 Temperature (F) Temperature (F)

Cast No. Mold Pouring Cast No. Mold Pouring

 1 700 1,339 1 710 1,342
 2 690 1,332 2 700 1,334
 3 680 1,337 3 690 1,337
 4 680 1,332 4 690 1,339
 5 675 1,326 5 680 1,346
 AVG 685 1,333 AVG 694 1,340
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Author:Burke, Paul; Weiss, David; Fasoyinu, Festus; Thomson, Jim; Sahoo, Mahi
Publication:Modern Casting
Date:Feb 1, 2008
Words:2615
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