lln-Situ Temperature !Measurements in Low-Pressure p,ermanent­Mold Casting

F. Paray McGill University Montreal, Quebec, CANADA

J. Clements Grenville Castings, Ltd. Merrickville, Ontario, CANADA

B. Kulunk Timminco Metals Haley, Ontario, CANADA

J.E. Gruz[eski Mc Gm University Montreal, Quebec, CANADA

ABSTRACT

The low-pressure casting process is widely used, as it allows a rapid production of components close to thefinalnear-net shape with a very good casting yield. Depending on the property requirements of the final product, it is often necessary to control casting soundness. Internal porosity can cause a loss of pressure tightness, a critical factor in parts, such as engine blocks and manifolds, which are required to keep separate various gases and fluids.

Fundamental to an understanding and control of porosity is a knowledge of the thermal conditions that prevail during the solidification of the casting. It is, therefore, necessary to acquire thermal data during casting solidification. A low-pressure cast­ing machine and die were instrumented to obtain in-situ thermal analysis curves during the solidification of flat plates of thick­ness varying from 118 to 314 in., in strontium-modified and unmodified 356 and 319 alloys.

This paper describes that effort and some of the obtained results. The soundness of the castings was investigated; some plates were x-rayed and porosity distributions were determined along the length.

INTRODUCTION

The properties ofa cast product depend critically on the quality of the casting, itself. Porosity causes costly scrap loss and can limit the use of castings in certain applications. The presence of porosity, inevi­table to a certain extent in any casting, can have a detrimental effect in terms of surface quality and a deleterious effect on the mechanical properties and corrosion resistance. Internal porosity can cause a loss of pressure tightness, a critical factor in parts, such as engine blocks and manifo]ds, which are i;equired to keep S·eparate various gases and fluids. Poros~ty occurs in cast aluminum aHoys due to shrinkage, resulting from the volume decrease accompanying solidification,

AFS Transactions 97-55

and due to the evolution of dissolved hydrogen, resulting from the decrease in solubility in the solid compared to the liquid metal.

Shrinkage and gas porosity can occur separately or together to produce undesirable features of castings. The formation of porosity in Al-Si alloys is also controlled by other factors, l-4 such as grain refinement, modification, inclusion content, cooling rate and alloy chemistry. Strontium is used as a modifier for eutectic silicon in the Al-Si alloy family. In the auto motive industry, there is an interest in the increased use of Sr to improve mechanical properties, to enhance machinability of castings and, in some cases, to control shrinkage. However, some foundries are still reluctant to adopt strontium because they associate it with an increase in porosity.

Casting industries are v1ery interested in the prediction of porosity without carrying out costly trial-and-error processes. Over the past decades, many efforts have been made in the development of models for the simulation of solidification phenomena in castings. A project is currently underway at McGill University, in collaboration with industrial partners, to study the feeding range of Sr-modified low­pressure permanent mold cast alloys for the automotive industry, and to develop criteria functions that should allow the prediction of the thermal conditions necessary to maintain porosity below some critical predetermined level. This will allow casting and die design­ers to better deal with the problem of porosity.

In the present study, experimental data required for the calibra­tion of the model used to simulate the solidification pattern were produced. In order to determine criteria functions, it was necessary to acquire thermal data during casting solidification. A low-pressure casting machine and die were instrumented to obtain in-situ thermal analysis curves during casting production. This paper describes that effort and some of the obtained results. The thermal data obtained will not only be used for modeling purposes, but will also provide useful information on the operation of low-pressure permanent mold casting machines ..

EXPERIMENTAL PROCEIDUIRE

Low-Pressure Permanent-Mold Machine

A low-pressure permanent-mold (LPPM) machine consists of two main parts: 1) the hydraulic casting unit witll tlle die and the ejection system, and 2) below it, a furnace that is a pressure-tight chamber containing a crucible.

The hydraulic casting unit is a simple four-bar hydraulic press. The bottom and top halves we mounted on the fixed lower platen and the moving middle platen, respectively. The lower platen has an opening through which the feed tube passes and makes a direct liquid metal contact with the bottom die half. A feed tube of cast iron, with an inside diameter of 5 in. (127 mm) was used. Protection must be provided or the molten aluminum will attack and dissolve the cast iron, contaminating the casting alloy.. In this case, a refractory material (Foseco Kornn) was brushed on the surface of the tube as a coating.

The sealed chamber is resistance-heated and contains a crucible of about 500 pound capacity. Initially, the crucible was charged with molten metal by moving back the upper frame consisting of the hydraulic mechanism, the die and the ejection system, and by removing the furnace cover. Recharging is done without removing the entire unit. The molten metal is poured through a spout into the furnace of the LPPM machine, and this opening is sealed after transfer to allow the pressurization of the electric resistance furnace.

791

The casting cycle starts when the mold is closed. At this point, a 3-psi pressure was applied, causing the molten metal to rise steadily up through the feed tube into the die. This pressure is held for a time to provide liquid feed metal to feed solidification shrinkage. The furnace chamber is then released to the atmosphere and the casting is allowed to cool down :in the die, to ensure it has sufficient strength for ejection from the die. At the end of the cooling period, the middle platen moves up, thus extracting the casting from the bottom fixed portion of the mold. As the moving middle platen is moved further up, the ejector pins are activated and push the casting out of the mold.

The Die

An experimental die was designed for this study to produce castings with different thicknesses, in order to cover the usual range of wall sizes found in LPPM castings. The die produces four plates of different thicknesses: 118 in. (3.18 mm), 1/4 in. (6.35 mm), 112 in. (12.70 mm) and 3/4 in. (19.05 mm). This geometry was chosen because the flat plate is a basic fonn found in many castings and it is a simple design for thermal modeling. The choice of exact dimensions was limited by the maximum size of die that could be mounted on the LPPM casting machine. The length and the width of plates are 11 in. (270.40 mm) and 4 in. (101.60 mm), respectively, and! the thickness ratio between the gates and the plates is 2/3 for each plate.

A photograph of the casting is shown in Fig. l. The mold consists of two halves. Each plate cavity is inclined to avoid air entrapment, and! the parting lines correspond to the tops of the plates. During the advance of the molten metal into the die, the air escapes through the vents and parting lines of the die. The surfaces of the die in contact with the molten metal were coated by spraying a commercially available die coat product (Foseco Dycote 34ESS).

In the LPPM process, the die must be preheated before each run, and die temperature is an experimental variable to consider. A special gas heater having the same shape as the die was made to evenly preheat the die. A special tool was also designed to collect the casting after ejection from the die.

Rg. 1. Casting With its four plates of different thicknesses.

792

Thermal Analysis

It was necessary to be able to record the thermal history of a casting during solidification at different locations within the casting. The low~pressure casting machine and die were instrumented to obtain in-situ thermal analysis curves during the solidification of the flat plates with different thicknesses. This necessitates the insertion of thermocouples through the die wall and into the casting in such a way that the thermocouples were located in the area of interest, but they could be removed from the casting when it was ejected from the machine.

The thermocouples were inserted from the top half of the die, which was fixed on the moving platen .. For each plate, four holes were drilled along the longitudinal axis from the feeding to the free end at the mid-width. Theirdistancesfromthefeedingend were 1 in. (25.40 mm) for the thermocouple location #4, 4.75 in. (120.65 mm) for the location #3, 7.25 in. (184.15 mm) for the location #2 and 10 in. (254.00 mm) for the location #1.

The thermocouples, held in place by compression fittings, were aligned with the die ejection pins, so that they were pulled out of the casting when it was ejected from the d:ite. The tips of the thermo­couples were arranged at mid-height in each plate cavity, and a graphite die coat brushed onto the thermocouple tips prevented sticking in the casting. This application of graphite was found to be critical to ensure long thermocouple life. Without it, the wires would solder to the casting and pull out.

Pliable, type K, grounded thermocouples with a 1116 in. (1.59 mm) diameter were used. These were installed before mounting the die on the machine,. and they remained in pface during an entire testing campaign. Thus, thermal information could be recorded from the first casting to the last one produced, but the thermocouples were more exposed to damage and could not be replaced if they did fail. Some :lfailmres did occur, and, over a typica] two-day testing period, roughly four of the 16 thermocouples might be expected to have failed by the end of the tests. During the preheating of the die,. the tips of the thermocouples were protected by specially designed small cups, to avoid the heat of a direct flame. In addition, ceramic thermocouple connectors were preferred to plastic ones, as they were found to better support the heat.

The thermocouples were numbered and were connected via chrome]-alumel wires extensions to two multichannel data acquisi­tion units to record the temperature profiles with time dming the solidification process. For each plate, the thermocouples were num­bered from 1 to 4; the thermocouple with the lowest number was near the free end, opposite to the mgate. The numbers correspond to the thermocouple locations described previously.

The Alloys

Two commercial alloys were investigated in this study: 356 repre­sentative of the Al-Si-Mg alloy system with 7.30 wt% Si and 0.34 wt% Mg; and 319 from the Al-Si-Cu family with 6.25 wt% Si and 3.62 wt% Cu. Castings containing 180 ppm of strontium were also produced.

Gas Level

A quantitative version of the reduced pressure test (Straube-Pfeiffer test) devdoped at McGill5,6 was used to determine the gas level. Constant volume samples with a riser were produced and the hydro­gen level of the melt was determined :from the density of these

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samples and calibration curves. Two gas levels were used: the normal gas level of the as-me[ted metal and a degassed level obtained by using a rotary impeller de gasser. For 356 aHoy, ilies,e levds were 0.31 mlH2/100 g Alarnd0.14mlH2/100 g, respectively; for 319alloy, they were 0.25 mlH2/100 g Al and 0.12 mm2/H>O g.

Pomsity Determination

The quantification of the porosity was done by density measure­ments. The plates were sectioned along the length to use the centerline slice. This slice was cut into 30x15 mm (1. l 8x0.59 in.) rectangular b]ocks having the plate thickness.

RESULTS .AND DISCUSSION

Castiing Cycle

Casting variables, such as melt temperature, pressure time and cooling time, were sdected., based on the experience of the machine operator. A casting cycle consists of a combination of three steps: a period when the pressure is applied, a time for cooling and a time when the mold is opened for the casting ejection. The cast time, or pressure time and the cool time, are selected by the operator and are changed several times during the transient period of the production run. They are then maintained at constant values when the operator fee]s that he has reached the steady state .. In the steady-state period, the pressure/cooUopen times used were 90sec/80sec/30sec and 90sec/ 90sec/31sec for 356 and 319 aHoys, respectively. On average, a casting was produced every three minutes and 25 sec, about 17 castings per hour.

The thermal behavior of the casting and the mold determines production parameters, such as the number of castings produced per hour and the "equilibrium'' mold temperature. The rate of cooling determines the time required for the aluminum alloy to completely solidify and r:each a temperature that will allow ejection without mechanical damage to the casting. Depending on the mold design, a certain number of casting cycles will be required to reach the steady­state period when the die temperature is more or [ess constant. and a constant casting cycle can be maintained.

Thermal1 Analysi:s Study

The temperature at different locations in the plates was recorded, as a function of time, during the production run, that is, during both transient and steady-state periods of the casting process. A very large number of thermal analysis curves was produced in this study and only a selection of thos,e will be discussed here.

T,emperature versus time curves are presented in Figs. 2 and 3 for the Sr-modified 356 alloy and unmodified 319 alloy. The graphs present the results obtained at location #4 (ilie closest to the feeding end) and at location# 1 (the furthest from the gate) in the 3/4-in. plate. These temperature-time plots are typical of the cooling curves obtained for 356 and 319 aUoys, and accurately r;eflect the time variation of temperature during the production run.

Significant differences were observed between the cooling curves obtained for the very first castings and those recorded at the end of the production run. This obs,ervation confirms the existence of a transient period in the LPPM casting process, in terms of thermal behavior, and it can be expected that castings produced during the transient will be very different in properties from those produced under steady-state conditions.

AIFS Tlransacti1ons

At the beginning, the casting cycle is not constant; the time interval between cycles varies, as do the highest and lowest tempera­tures measured for each casting cycle. Moreover, the solidification time at a specific location, defined as the time elapsed between the liquidus and solidus temperatures, increases during ilie beginning of the production run. For example, for the fifth casting in the 3/4-in. plate, the solidification times were 50 sec, 74 sec and 65 sec at locations #1, #2 and #4. For the sixteenth casting, they were 69 sec, 119 sec and 107 sec, respectively.

Thermal characteristics, such as solidus and liquidus tempera­tures, eutectic temperature and solidification time can be determined from the curves, for all ilie plates at different locations. These values represent valuable thermal data required for the calibration of any eventual model to simulate the solidification pattern. Additional information can also be obtained from the cooling curves, such as times when the pressure was applied, when the cooling starts, and when the mold is opened. These were recorded for each casting. Tables 1 and 2 present some examples of temperatures all: different steps in the process for 356 and 319 alloys, respectively. In the steady-state period, the pressure time should be selected, in order to hav,e a temperature lower than the eutectic temperature at the end of the pressure time, in order to take advantage of the pressure in terms of soundness.

Table 1. Temperatures ("C) at Different Steps of Casting Cycle

for 356 Alloy With Sr

casting melt step* 314" plate 112" plate 114" plate mnperablre

location 4 location 1 location 4 location 4

p 302 270 340

#1 747 c 562 567 S44

0 442 467 411

p 276 279 349

#10 757 c 584 S72 S47

0 S2S 493 464

p 321 306 383

#20 7Sl c 565 S62 S48

0 520 482 470 ": wnen pressn11e ts appne 1, auratton 4U sec, :m sec, ~u sec tor castmg l, W,Z( C : when cooling starts, duration 60 sec, 80 sec, 80 sec for casting 1, 10, 20 0 : when the mold opens

Table2. Temperatures ("C) at Differ:ent Steps of Casting Cycle

for 319 Alloy With No Sr

-401

403

377

S16

414

382

444

410

casting melt step* 314" plare 112" plate 114" plate temperature

location 4 location 1 loc.ation4 location 3

p 22[ 195 260 247

#1 743 c 561 561 614 461

0 403 460 350 284

p 242 221 291 287

#10 744 c 561 561 507 436

0 461 449 403 314

p 264 I

247 325 )04

#20 745 c 560 I

556 491 385

0 471 459 ·420 325

p 305 267 3S7 322

#30 747 c 564 11 560 522 405

0 494 480 44S 348 : wnen p.resswe is appuea, ouration 'IQ sec, w sec,'~ sec, 9U sec tor casting 1, II, zo, ::Kl

C : when cooling starts, duration SO sec, 60 sec, 90 sec , 90 sec for casting L, rn, 20, 30 0 : when the mold opens

793

750..-------~--~-~~-~~~--~~~

700 650

_ 600 1·-·

:"55o,···' -; 500 I

i 4SO

l400 j 350

300

250

200 1.50 .___....._ __ ...._ __ __._ ___ _.... _ __......__._ __ ...._......___.___....__._ _ __.

2700 2800 2900 3000 3100 3200 3300 3400 3500 3600 3700 time (sec)

(2a) transient period, location #1

750r----~--~--~---~--~~-~--

700

650

,....,.600 ~550 ·--· ···

::: 500

i 450 l4001 J 350

300

250 200 .... .

............. •···-···· ·····--·-· ··-·-· ··--· · · . ..... . .......... 1 · ·-· ·· -·-· · ·-··-· ·---·-·-·· · -·--

150 .___....._~_,_ _ _.__ ........ _ __._......._....._ ........ ..._ _ _,_ __ __._ _ __,

5300 5400 5500 5600 5700 5800 5900 6000 6100 6200 6300 time (sec)

(2b) steadycstate period, location #1

750..--~------~~--~-~--~--~--

700 ··-·-·-···········--·········-·····-·--······-···--- -·-·-·······--·--·--····-··-·····--······---- --.. · ···-·······--···-·· ··--

650 I -

600 I ••••

Es5o : ..... "500 .

i 450 I l400 ....... . J ~= I~-· ~:. :·==:~=- ::::~ =:~~= .. :: . ...... 1 :~:~=-~=~= . -~=~-=--=: :

250 200 150 .__...__......_ _ _,_ __ __._ _ _.... _ __..___..__ ........ ...__-'-__ __,_ _ __,

2700 2800 2900 3000 3100 3.200 3300 3400 3500 3600 3700 time(sec)

(2c) transient period, location #4

150..----~----~--~~--~---~~~

700'

650

...... 600 ~ 550 t5oo' i 450 ........ .

l400 j 350

300

2SO

200 150"'-........ ..._ __ _._ __ __._ __ _.... _ __..___...__ __ _,_ __ _,_ __ __,_ _ _,

5300 5400 ssoo 5600 5700 5800 5900 6000 6100 6200 6300 time(sec)

(2d} steady-state period, location #4

Fig~ 2. Cooling curves for several castings produced in sequence; Sr-modifiied 356 alloy (314-in.. plate).

794

700 ....................... ·--·-··-·-·-·-·--·--.. ·-··-·-·---·-··-·-··· .... ..... ·-·-·--······--··-·-·····-··-·"·-·······-···· ···-6SO ·-·-····-········· -·-··-----··-····- ·-······-- ····-· ·············-···· ···· ·---·-·-···-·-·· ·············-· ---·-

600 ~550

1: J 350

300 2SO 200 -·· 150"---'---_.__._......L._._-..t.__..--1......,1...._i.,_..__..1-....... _._ __ -'-__ _.

0 100 200 300 400 500 (j()() 700 800 900 lOOJ time (sec)

(3a) transient period, locaUon #1

750..---~~~--------~-----~~--.

700

650 600 .....

E5so ::: 500

i 450

l400 j 3SO

300

250

200 lS0'---..._ ....... ...._ __ __.__...__.___.__.__....._....._ ........ -'-..._-'---......1...--11

4400 4500 4600 4700 4800 4!H)() 5000 5100 5200 5300 5400 time(sec)

(3b) steady-state period, location #1

750,...-~~-~~-~~--~~~-~--~----

100 ··-·-·······-·--·-··-·-·-··-- ·-·-·-·--···-----·····--·---·-- ·-·-··--····-·--···-··-

6SO _600 ts5o f! 500

t: J 350

300 250

200

100 200 300 400 500 600 700 800 900 1000 time (see)

(3c) transient period, location #4

7SOr----------~~-~~--~~~~-.

700

_600

~550 f 500 a 4so i.400 j 3SO

300

250

200 lS0'-----'---~ ........ -'---.....1.---...1......_. ...... ....._ ........ ..i.....---'---.....__..._,

4400 4500 4600 4700 4800 4900 5000 5100 5200 5300 5400 time(sec)

(3d) steady-state period, location #4

Fig. 3. Cooling curves for several castings produced in sequence; unmodified 319 alloy (314-in. plate).

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Meta~I Temperature

In the LPPM process. the melt temperature is controlled via the temperature of the sealed chamber, which is monitored during a production run. In these experiments, the metal temperature was directly measured by plunging a temperature probe into the melt For each production run, measurements were t~en just before the first casting and after each ten castings produced. In all cases, the metal temperature was approximately constant, fluctuating within a few degrees. An exception was observed for the unmodified 356 alloy (with normal gas level) where the temperature increased from 746C to 763C and then to 789C. This variation in temperature was significant and was reflected in the thermal analysis, as discussed further.

The fluid flow phenomena and temperature distribution/ variation of the molten metal during mold filling have great effects on the quality of the casting obtained. Thermal information on the molten metal during filling is important to determine operating conditions, such as pouring temperature or cooling system design, which may be required in the metallic die to avoid problems, such as misruns or localized hot spots. The highest temperature on each cooling curv,e obtained indicates, at each thermocouple location, the drop in metal temperature while filling the mold. Examining ilie highest tempera­tures recorded by an thermocouples, as a function of time, will give some indication as to when the transient period is completed for each plate or thermocouple location.

Because the molten metal does not contact all the thermocouples at the same time, and for eas,e of comparison, the results were plotted as a function of casting number. Figure 4 presents some results for the 356 alloy. For each plate, the highest temperature recorded at locations #2 and #4 were plotted. In general, during a production run, the highest temperature recorded at any location first increased during a transient period and then reached a plateau. However, for the unmodified 356 alloy, this plateau was more difficult to reach, due to the fact that ilie melt ternperatme increased with time.

As can also be seen in Fig. 4, a significant difference exists between the highest temperature measured at location #4 and the one measured at location #2 in a plate. Along the distance of 6.25 in. (158.75 mm), which separates these two Bocations, a metal tempera­ture loss of several degrees occurs. It should be noted that, in both cases, without and with Sr, the highest temperature recorded at location #2 in the l/4-in. plate is very low, even lower than what was measured by the thermocouple furthest from the in-gate, near the free end (location #1). where the plateau was around 635C.

The thinner the pfate, the lower was the metal temperature at the feeding end. Similar results were observed for the 319 alloy. as shown in Fig .. 5a. where the highest temperature at the same location (location #3) in the 3/4 in. and 1/4-in. plates are compared. Again, on average, the loss in temperature for the metal during mold filling is smaller during the steady-statte period, compared to the beginning of the production run: a difference of about 50°C. A significant differ­ence exists between the highest temperature recorded at different locations in the same plate (Fig. 5b ).

From these measurements, it is possible to calculate the flow velocity in a plate. Forthe3/4-in. plate, the average flow velocity was 128 mm/sec and 138 mm/sec for the 356 alloy,. without and with Sr. The filling time was, therefore, 2.11 sec and 1.96 sec, respectively. For the 319 alloy, the corresponding values were 102 mm/sec and 114 mm/sec, or filling times of2.65 sec and 2.37 sec.

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Influence of Mold Design

The mold temperature is not uniform and continues to fluctuate, even after the so-called steady-state regime is r~ached. At this point, the casting cycle is held constant by ilie operator, in terms of pressure time and cooling time, and the mold opening time is kept as short and constant as possible, in order to minimize temperature loss. Tem­perature measurements were made with an infrared laser gun on the outside surfaces of the mold for several casting cycles in the steady­state period. At a given location, the temperature varies slightly with thecasting cycle. The temperatures for the mold section correspond­ing to the thin plate were significantly lower than what was recorded from the other sides of the mold. A casting geometry, as used here, with the thickest plate being six times thicker than ilie thinnest one, requires a nonsymmetric mold, and this nonsymmetry affects the thermal equilibrium. Consequently, the thin cavity was not always completely filled, as observed from the 200 castings produced.

Casting Soundness and Porosity

For each production run, several castings were randomly selected from among those produced during the steady-state r'egime. The plates were separated from the gates and radiographed, in order to obtain a general idea of the soundness and overall porosity distribu­tion. In general, when defects were present, they were observed in the middle of the plate, along the length. The results obtained from the porosity determination are in good agreement with those obtained by radiography.

356 Alloy Figure 6 presents some x-rays of 112-in. plates of 356 alloy, cast without and with Sr, at two gas levels. Strontium is neutral in terms of soundness, while degassing promotes larger gate defects in the plates. No significant difference was observed from plate to plate (i.e., casting to casting). This was confirmed by the porosity distribu­tion curves determined from density measurements of small blocks taken from the middle part of the plates along the length. For each condition, six plates were used. Individual curves were frrst obtained, and the average values were calculated.

Figure 7 compares the variation ofaverage porosity, according to the distance from the feeding end. With the exception of the 3/4-in. plates, where some plates exhibit a gate defect and some do not, a good r;eproducibility was obtained from curve to curve (i.e. plate to plate), and the average curves reflect well the general behavior. With the exception of the first 75 mm, where degassing promotes a feeding end defect, it can be observed that degassing lowers the porosity.

Strontium was neutral in terms of porosity, but it did have a microstructural effect, producing good modification, except in the thick plate where uneven structures with areas of fine modified and areas of partially modified silicon were observed.

319 AUoy Average porosity curves are presented in Fig. 8. Much higher porosity levels than in the previous alloy were found, due to the longer freezing range of the alloy. Degassing generates or accentu­ates a gate region defect. As shown in Fig. 9, the addition of Sr has significantly improved the soundness of the 1/2-in. plate, making the porosity fine and dispersed. From the structure of the porosity, it is evident that the spongy areas are due to an inability to feed this long­freezing range alloy. In addition to its positive effect on porosity, strontium partially modified the silicon and caused the CuAh to be more massive.

795

(I) (d)

780---------------------------------------- 780,.-------------------------------------..... "MO I ~~~~ $1: :::::::::~=~~~~~~'.""~~~--:.,_ ---:--:::::::::::::

....,.,.. -;;,(~--· c··---·---·------------···----------------------------· : -'····- 1·;;;··-----·······-------------------------------------···-------

1'40 --------------------------------------·-···-----------------------------

7f01

Ml P' 7l0i ........

I~ -------------------------------------------------·1:i.··-----------------

t:.~--~-"'U'"· ----------·---------A---~~·-------A--·-····--·------····-··

--4- o a __________ """.~t._i;;i ____ _ ,,,.,,,. t:.

A ,

-~~-.ti..:;:? .. ~:::::::::::::::::::::::::::::::::::::::::::::::::::::

1~, ~-----~:-::::::::~:::j:~~::~~:(::::::::::::::: 540 ..... ______________________ ._ ________ ...... __ __

0 20

(b) 780------------------------------------------. 710r---------------------------------------..

760 --------------------------------------·······--·-····-------·---------A .·~A

-- ••• - • - --- ---- •.•. -- •• ·-· - -- -- --l!.- ~--~-,-e-,r--. ~-- .. -- . ------ . ·-- 'I: ---·-------------·;;~~;;---------------·····-··'------·----------·

~,,,,.

-----·---~--6·z::;-·······---------·-·---- - -tl······-------,~ 0 I 1CJ

---~-------------··c:r·-- ---!---································--' 0 'I·---------I~

1: I:!

.,1

6401

I~, :::;_:~:;::;;;:;;::-t-~~-·~~t::;:;;::::::;:;;;:;:

- ------ .• -- -- ---- -- . -- .. - •.. - ·- --- -- -- ..•..• ---·· ;:s· ---- -.. - ... -- . -...

A A -...Q.--..:!" .. --. -- .... -- . -----. · >:.:~·.&i--~ .... "A __ . - -- . ---- - •. - .•.•...• --

--.!"1!. (!;. • ·-···· ..,_111'.1;, ••••••••••• - ••. • D ·er· .... A ,!5. 6 c _,~4 ...... ····c:i-:~ .

540._ ______________________________ ...... ____ __,

'40--------------------------------------....,, 0 s 10 15 25 0 5 10 15 20 2S

casting aumber casting aumber (c)

'180..--------------------------------------.. 781 ::::::::::::::::::::::::::::::::-:::::::!~~:~~:!: a .,. - .

t... '120 I ---------------------------------····----------····---------------- - ---·

700 --------------·······--------···········-·6·-;.;-..~'~-------------... -"!S

1680 I ·········--············------~---·······················---····· ,.-""~ 6'0 ..... ------ .. ·;'J-" ____ .. "ii.,""-----. -- ........ --· .. -- ----- ... --·- •... -

d40 ---------fr-!'.·----------········----· .. ······-···--------------------··· , szo ---?~---------·------------------·········-······-··-··············-

- --1------------······-----------------·····----------------------------

SIO A, _______ ···· -- ·-- --- ·-· -· ·- -· · --· -o---- - ··cr· ·· - --· --- · - --- -··· · · · - · , iabq1Dg PD acb

'80 :D_.,,-e-cr·et-,c:r· --- --- · --- · ·· -- ·· · -- · · · -· ·· · · · ·· · · · ···· ·· --- · --- · · · · ·

I I

(f) '1801..---------------------------------------6: ::::::::::::::-::::::::::-::::::: ::::j:~~~~t

0 120 ........ I

I~ J:

580

t:. t:. A

... ---·- -- -- ... -. -. -... --· -- .. -- -- ·--'-'·-~~~':":~ - . -- -.... -. -. -~~----..a-- A

---------,~----·····-------6---·············---------------·····-A ,,. ---P-rs.-A--·-·····-·--------------------------·-----·----·-----------,

~··~· ~:::::·:~:~:~:::-~:": q-"-~ -~-·_-:::::.:: :: a

560 ,., _______________________________________ __ 0 5 10 15 20

540' ______________________________________ ....,,

0 5 10 15 20 casting mmber casting mqnber

Fig. 4. Highest temperatures recorded for the 356 alloy: (a) no Sr, 314-in. plate; (b) no Sr, 112-in. plate; (c) no Sr, 114-in. plate; (d) with Sr, 314-in. plate; (e) with Sr, 112-in. plate; (f) with Sr, 114-in. plate ..

796 AFS Transactions

7llG ······•····· ·· ········· ·············· ........... ........ .... ·· · · ·· · · ·· . .........•

6 'JOO

(a) t,,i : . "0 Q ~ ~;:;_~~~j-~~~~:?

- ----- ---- --········.0:6 4-:;.g.;.·-r· ····-- ----- ---------------------···· · ---·--~ -- ~

I: ···· · · ~~,,./ ... .. .... ................ ............... . "" l •·:··:· .. r~:::~::i:+··:::: ·:::::::::.: ... n

5S)~~=-~_._~~ ........ ~~ ........ ~~...i....~~..._~~..._~___, 0 10 25

740

.,..,

a100 0 -

(b) E I:

580 ········ · ·f - -~~ -- -~~-- 1 ---···········--------·--··--· -~

0 5 10 l!S 20 2S 30

casting number

Rg. 5. Highest temperatures recorded for the unmodified 319 alloy: (a) location 3; (b) 314-in. plate.

Hot tearing was observed in the 319 alloy. This began during cooling, and crack growth continued as the casting cooled after removal from the mold. In general, all the cracks fonned at about the same location in the plates, near the feed end, perpendicular to the length of the plate. The percentage of plates having a crack was determined, and the crack size was measured as the linear distance from one end of the crack to the other. An average crack size was then calculated for each condition.

As can be seen in Table 3, Sr has a very strong positive effect on elimination of hot tearing. In the presence of Sr, and without degassing, no 112 in. or 314-in. plate contained a crack, while in the absence of Sr, 13% and 27% of the plates had cracks, respectively. The worst case occurred for the 114-in. plate, where 87% contained cracks if no Sr was present, compared to only 7% for the Sr­containing alloy. In addition, a significant difference in crack size is seen.

When the melt was degassed, more plates did crack, but Sr continued to have a positive effect on hot tearing. Cracks in p1ates containing Sr were shorter, narrower, and more shallow that those in the absence of Sr. Frequently, the cracks in Sr-free pfates extended through the entire thickness of the plate, from the top to bottom surface (Fig. 10). Such deep cracks were not observed if the melt was strontium treated.

AFS Transactions

(6a) no Sr, normal gas

(6b) no Sr, degassed

(6c) w;th Sr, normal gas

(6d) with Sr, degassed

Fig. 6. X-rays for the 112-in. plates for the 356 alloy.

797

' -------------- --- --- --- -- --- -- --- --· ----- ----- ------ --- --- --------- ---- .. ;r'"'\ lilt I ---- -~ ·-. --r-··· ... --- ...... , .......... ·-· ---- ------ --- --- ·-- ---- ·- ---- ·-·t 7: il/ __ -- . -· \-· .. -- ---. ----., .. ----- --- -- ........ -. -. --. --.. ---- ----.. ---

: ::::::::::··· t::: ::: :: :::: ::::::: ::::::::: ::::::::::::: ::::::::::::::: ~ • 4 ---- "-··-·-··-······························----··-·-··-··--·-

' 2

I

01...1....&.. ....... ..i....-.i. ..... ...i.. ...... ~liC.:IE;....&.-i...Jlll.:l~.-.1!11(6111 ............ ,o 25 '° 75 100 125 150 175 200 m 250 21.s 300

distaDM from feeding end (mm)

(b)

13

: :>0:·::::::._+~:::~:::[_:::::_:>0-::.: 9 -4····'·····················-····················--····-···-·············

\ Wt 8 ·····\--·-··--·--·--·--···························-··----···-············

-17 --· -.. ,. ·-- .... -- -·. --· -- .. --. --- -·· ..... --·-. --· -· -... -..... -·----- ·--· :a:::::::::\:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

' 4 -····1:················································-······-···-· ' ··--·-- ---\----············································-············

:z 1 o.._...,.. __ ,,_....,. __ '1'-__,_.ar!!P-l!f--T..;.-i...._dil~ ... __,

10 2S 50 75 100 125 1'0 175 200 225 2SO 275 300

distance from feeding ,eJJd (mm)

(c)

0---------------------------------------: :::::::::::::::+~:::~:::j::::::::::::::: 10 .................. ' . ' . . ..... - -- ····················

9 ·············--·-····························-·-····························

lit. I ..... , ..................................................................... .

·1:,· ···t,···-··-·-····································-·····----··-············

::[:\::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: f ' 4 --1-·····\-················-·-·········------·-··········--·-··--·-·······

, -1--------~---························--·-·-·--·-· .. ······················ 2 1. .lo

I 1 ....... ·····-\"'·· ;:/Ji •, O~....L..--""---'---'---.i..-....i;;zi...&....-..1..__,__.;;.i:._.-.ii.--~

0 25 so 1s 100 125 150 11s 200 225 250 ns 300

distance from feeding 1end (mm)

(d)

1s---------------------------------------: :::::::::::::::::j::~:::~::f:::::::::::::::: 10 --·····,··-··········--····-·-----··-····-·-··-·····-·----------·-··-·······

9 ···················---·-··,······--········-------------------------·····---

tit I !t., .......... , .......................................................... . Ii;,.. ·t' -,&: .... , •••••.••••••.•..•••••.....•..•.••.••...••..••.••.••.•.•••.•.•.•

,5 I

5

4 ...... ., ........ -··················-······························-·-·-----

2

1 1Q.._ ..... ____ --'---'---.__C:.a.;;.....i....__.;; .... w..'--'M-11~._~

0 25 50 15 100 125 1'0 175 200 225 250 2'75 300

distance from feediog end (mm)

(e)

1s,..--------------------------------------

:-\,:·::-:::J-·~:-.:~-----t:_-:::-::: :::: ::::: 9 ·-·----~----·········,··········-·········-···--····,··········-·············

' Wt,I ·-----\-····,····,······--------·············----------····················

.mt -- -----\----------------------······---------········---------····---·-·· .... --·-t----------·-···················-·--···············--·--·-··-····'•

' ...... ..\ .................................... ···--··· --···-···· ..... ···-· I l

4 ··-----· .\-·-···································--·-·········-············

s ........ , .. ,, ·······························- ------ ....................... , .. . 2 ···········\~-et-s.....;,.;..,.e~

1 .............. ~,---···········""····· .... ....., ... , .... ············--·---~b-·'llt" '& ~

oL-....1........J~~~--...1........a.i::lltl:.:~....J=.:=~::.:!t:s;~_J 0 25 50 15 1!00 125 150 175 100 225 250 'Z15 300

distance &om feeding end (mm)

Ur-------------------------------------

: ::-:::::•J:~:·:·~:J::::::_:::::.:::::::: ' -----··-··-······ ····- .......... , ........................ , .................. .

Wt 8 ................................................... , ....................... .

II 7 ----·-- .. .,.- . .,.•,.••--•-••··-·~-•w•••·~-•-••••••-•••••·-·•••••••-••••••••••••·•-

0: ' ···ft~·-· · ·-····-----·-············-··--------·······-·-··-----···········, ! 5 .. .l---\····················· ··-······································--··-'

I \ !

4 J·····\••·····-·--·-··················--······················--········1 3 -A,.-···-···.,-·-------···············-··- -············· ····------- ---- ·······'

\ 2

I

~ 1L...-·. _-. J...-. _--_-- J..A..: __ .J.._---1--1---.l.Bl~.:.......L........J~::l!t.:illl:li:L-...I ,0 2S SO 75 100 125 lSO 175 200 225 250 'Z15 300

distance &om feeding end (mm)

Fig. 7., Porosity distribution for the plates of 356 alloy: (a) no Sr, 314-in. plate; (b) no Sr, 112-in. plate; (c) no Sr, 1/4-in. plate; (d) with Sr, 314-in. plate; (e) with Sr, 112-in. plate; (f) with Sr, 114-in. plate.

798 AFS Transactions

(a)

15--------------------------------------: :::::::::::t~::::::::~::::~::::··::.:::::::::::: ll ···-----·············---······································· ······ · · · ··

10 ···········-·-·················· ., ···· · ··· ·······-·--··-·····-·········

"'' E 5

'oo 25 50 75 100 125 150 175 20liJ1 225 250

dUUnce .from feeding end (mm)

(b)

15 14

1J

l2 ti

10

"'' -t:

s

' ' l

0 0 25 275 300

u--------------------------------------. : :·:-:::_: .. ::::+~~::::~::i::::·-:···· 11 10 .. • .. ··········································· ...... ···· · · ·· ······

Wt 9

u ....... ····················· ............... . 4

' 1 0'--_._ __ ._~.__._.....1 __ ...u~r...... .... __ .J.-...-:=:;a::::-...-..1

0 2S 50 75 100 125 150 175 200 225 250 'Z15 :JOO

distance from feeding end (mm)

(d)

~---------------------------------------.

11 _., ~9

i: 6

5 4

l

1 o...._ ....... __ ..._ __________ .............. ..__._ __ ..__... __ ...__.

0 25 50 75 100 125 150 175 200 225 250 275 JOll

distance from feeding end (mm)

(e)

15.------------------------------------:' :::::.::.J:~:=?*"::··:~·:.t:· · :::·. ·:·: 11 ·--*···-······-············ ········ ···· ······ ······ ············-· ····· 10 ··:\········-·························································

"" 9 ··1··\·········· ···--······················· · ····-····················· ·t ,; ·1---~············ ··· · ·· ········· ······················--··············

i ~::::\~::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::: :::: s ········\····················-······ ···························-········ "' ·-·······\··············································-·· ·· ·········· ' ··········'··· ·············· ················ · ·······················

2

1 0'-....1---....___,_ __ ...__,_ __ .J.--'----'--'---.J.---L~

0 2S 50 75 100 125 150 175 200 225 250 1:15 '°° distance from feeding end (mm)

~------------------------------------

: ::::::::::r~~:::::~::1:::::: ::::: 12 ···········. .····················

11 ······- ·· · ··· .................... ----- · -·· ·· .......................... .

10 ······· ··· ··· · ··············-···· ·· · ··· ····· ·· ···· ········ · ·- ---- --- ···

'Wit; 9 ---l·············· ····-·----················ · ·············-············

·t7·,· ··f\··········-········································· ··· · ·········· ··1··-\··--················ ··········· · ················· ·· ··············

I \ .. , ... , ................. . ... . -·. --- --. -·-- ...... -.. -· ..... -.... -- -- -.. .

' ' 5 ., .. .&.--·-··--· · ·········· .... ...... ············· ................ ..... .

' : :.:::: .. ·::::::::::::::~~:::::::: : :::::::::::)\::::::::::::::::: I ~ \

2 11

o.___,,_ __ i...-_._ __ ,.__-'------.lo.--"'--i.--"'---=:..-J 0 25 50 7S 100 125 150 175 7nO 225 2SO Z15 300

distance from feeding end (mm)

Fig. 8. Porosity distribution for the plates of 319 alloy: (a) no Sr, 314-in. plate; (b) no Sr, 112-in. plate; (c) no Sr, 114-in. plate; (d) with Sr, 3/4-in. plate; (e) with Sr, 112-in. plate; (f) with Sr, 114-in. plate.

AFS Transactions 799

(9a) no Sr, normal gas

(9b) no Sr, degassed

(9c) with Sr, normal gas

(9d) with Sr, degassed

Fig. 9. X-rays for the 112-in. plates for the 319 alloy.

800

Casting Yield

Higher metal yield, easier cleaning of the casting, fewer scrap castings, better surface definition and consistent dimensional accu­racy on production runs are advantages to all of the permanent mold processes. The low-pressure casting process is a process, both technically and economically, that bridges the gap between the gravity (permanent mold) and high-pressure die casting processes. Its advantages over permanent mold casting are twofold. First, it allows a relatively nonturbulent filling of the mold or die cavity, reducing defects such as oxide inclusions and air entrapment. Sec­ond, since the excess of metal in the feed tube drains back into the furnace, the casting yield is significantly greater and the need for bulky risers is eliminated.

The casting yield for the production runs of 356 and 319 alloys done in this study was found to be about 89% in all cases. An increase of 2-3% in the casting yieldl was obtained after the transient period was over.

Table 3. Hot Tearing ObseNed in Plates of 319 Alloy

Condition 'Thickness Average crack siz.e (mm) % of plates with crack

118" - 0

no Sr 1148 26.3 ± 9.4 87

nonnal gas lfl9 16.8 ± 8.2 13

3W 17.8 ± 12.4 27

118· 8 3

with Sr 1149 8.5 ± 3.5 7

normal gas 112· - 0

314• - 0

1/8" - 0

no Sr 114" 30.1 ± 9.5 90

degassed 112· 14.7 ± 7.4 67

3149 15.5 ± 6.7 37

1/8" - 0

with Sr 1/4" 18.7 ± 7.9 63

degassed 112· 20.4 ± 7.1 23

314• 5 3

Fig. 10. Portion of hot crack formed in 114 in. 319 alloy plate without strontium. Total crack length: 35 mm.

I

I

AFS Transactions

SUMMARY

Although the LPPM casting process is an automated process in whlch the casting is ejected from the die after solidification, it has been possible to instrument the unit for therma] analysis. Tempera­ture profiles with time dluring the solidification process for all castings produced were obtained in-situ and! provided useful infor­mation. Typical parameters, such as eutectic temperature, solidus and liquidus tempe.!iature and solidification time, were measured and can be used foF the:rrnal modeling.

On the other hand, information on the operation of the LPPM machine was obtained: a transient period was well identified., in terms ofthermal behavior of the metal. Moreover, the temperature at any step of the process could be determined at any location of the thermocouples. Porosity profiles indicate that Sr has a neutral effect on the soundness of 356 alloy, but dinrinishes porosity in the 319 alLoy and can significantly reduce hot tearing in tlris alloy.

AFS Transactions

ACKNOWLEDGMENTS

The authors wish to acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC), Grenville Castings, Ltd. and Timminco Metals, a division of Timminco, Ltd.

REFERENCES

1. J.E. Gruzlesk:i and B.M. Closset; The Treatment of Liquid Aluminum­Silicon Alloys, American Foundrymen's Society, Des Plaines, Illinois, 1990.

2. D. Emadi and J.E. Grusleski; "The Effects of Casting and Melt Variables on Porosity in Directionally Solidified Al-Si Alloys," AFS Transactions, vol W2, 1994.

3. G. Laslaz and P. Laty; "Gas Porosity and Metal Cleanliness in Alumi­num Casting Alloys," AFS Transactions, vol 99, 1991, pp 83-90.

4. D. Emadi; "Porosity Formation in Sr Modified Al-Si Alloys," Ph.D Thesis, McGill University, Montreal, Canada, Feb 1995.

5. W. La-Orchan, M.H. Mulazimoglu and J.E. Gruzleski; "Constant Vol­ume Risered Mold for Reduced Pressure Test," AFS Transactions, vol 101, 1993, pp 253-259.

6. W. La-Orchan; "The Quantification of the Reduced Pressure Test," Ph.D Thesis, McGill University, Montreal, Canada, Sep 1994.

801