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S U P P L E M E N T TO T H E W E L D I N G J O U R N A L , J U N E , 1 9 7 2

ABSTRACT. A study was made of the effect on hot tearing in Al -Zn-Mg alloys when varying such properties as chemical composit ion, degree of purity of the aluminum base, casting temperature, aged condition of the al­loy, dissolved gases and different amounts of zirconium as a grain re­finer. For hot tearing evaluation, the parameter used was the area be­tween the curves for comparative breaking elongation and l i near shrinkage as a funct ion of temper­ature in the solidification range.

High Temperature Elongation — Definition and Measurement

Aluminum is f inding ever increasing uses in industry. Because it lacks high mechanical properties alloying is nec­essary, although in many cases the al­loying elements often create cracking and corrosion problems

in the present paper we shall con­sider the problem of hot cracking of Al-Zn-Mg alloys which frequently ap­pears during casting and weld ing.

The term hot cracking involves two different though directly related phe­nomena: (a) the appearance of cracks formed during solidif ication as a result of normal contraction when solid grains coexist w i th the last remaining drops of l iquid and (b) the cracking which takesplace between the solidus temperature and room temperature either during subsequent cooling or similar heat cycles.

In this paper w e shall discuss the first of these points, designated "hot tearing, ' ' wh ich refers only to crack­ing produced by thermal contraction during solidif ication at temperatures ranging from the solidus to the l i ­quidus.

Archbutt, Grogin and Jenkins1

were the first authors to study these

problems. They observed the sharp variation in impact strength as the solidus temperature is reached and also that alloys which rapidly recover impact strength when the temper­ature falls below the solidus are less prone to cracking than other alloys where the increase is more gradual.

The f irst theory to elucidate these phenomena is due to Pumphrey2 and Jennings3 who established cracking temperatures between single points in the graphical representation of the ultimate tensile strength — tem­perature relationship. It is a theory that although it serves to explain such effects as grain size it does not clear up some others such as the ef­fect of small additions on hot tearing.

In Pellini's much later theory 4 - 5 - 6

it is the solidified grains wh ich , as their volume is reduced, set up stresses on the liquid pockets which f inally break. By means of this the­ory7 the influence of each parameter on hot tearing can be established. A modification to this theory is that by Saveiko8. In Patterson's theory9 it is the liquid drops which act as notches facil i tating fracture of the solid grains.

The theories by Rogerson and Bor­land and that by Prokhorov are more recent.

Borland10 gathers the opinions of previous authors in that the shape of the liquid pockets has a decisive in­fluence on the hot tearing phenom­ena and arrives at the conclusion that as the temperature of the alloy increases along the solidif ication range on account of the reduction of surface tension of the melt, the im­pingement angle '1 is reduced and the grain is f inally coated w i th a liquid layer wh ich causes total deco­hesion of the solid structure12 and re­sultant cracking.

Investigation of Hot Tearing in Al-Zn-Mg Alloys

Review of previous work leads to a method of separating the factors which do and do not affect hot tearing susceptibility

B Y J . HERNAEZ

A N D

A. M A D R O N E R O

This work was carried out in the laboratories of Centro Nacional de Investigaciones Metalurgicas (CENIM). Madrid, Spain.

W E L D I N G R E S E A R C H S U P P L E M E N T ! 281 -s

Temperature 'C

Fig. 1—Hot tearing tendency of an aluminum alloy. (1) Comparative breaking elongation versus temperature: (2) linear shrinkage versus temperature. Ts = solidus temperature of alloy: Tt = liquidus temperature of alloy: T, - lower limit temperature in brittle tempera­ture range; T2 - upper limit temperature; t - transition temperature of rupture mechan­ism from Borland's to Prokhorov's (ref. 16). Shrinkage starts at this temperature

5I0 520 530 540 550 560 570 580 590 600 6I0

Temperature °C

Fig. 2—Hot tearing tendency of an ultra-high strength aluminum alloy (8.50% Zn and 3.05% Mg; 99.75 purity)

Prokhorov13*14 explains the frac­ture phenomenon as spinning of the grains wi th in the liquid envelope due to shrinkage stresses set up by solid­if ication. When this spinning cannot take place, cracking results.

In a recent paper15 both of these mechanisms have been verif ied in Al -Sn alloys. Both theories are in our opinion true. Borland's theory is valid at temperatures just above the solid­us whi le that by Prokhorov holds good somewhat below the liquidus.

These matters are dealt w i t h in greater detail in one of our previous studies16.

None of the above theories ful ly ac­counts for these phenomena nor do they explain quantitatively the in­fluence of various parameters. In practice, tests designed to assess hot cracking of the various alloys ig­nore almost completely these the­ories and limit themselves to trying to reproduce the experimental condi­t ions under wh ich cracking occurs in practice by means of standard cast­

ing or welding tests. The number of cracks formed is determined macro-scopically.

There are in the technical liter­ature two classes of tests for hot cracking evaluation: (a) casting tests w i th restrained shrinkage and (b) we ld­ing tests. On an evaluation of the test results, it should be noted that none take into account either the temperature at wh ich cracks are formed and propagated or their met­allographic or micrographic appear­ance.

Pouring Tests

Pouring tests involve the casting of test bars in dies or sand molds in such a way that the bar cannot con­tract freely on freezing so that con­traction stresses are set up, causing the test bars to crack.

There are two types of pouring tests according to the manner in which they are assessed. In the f irst the test bar dimensions are kept con­stant and the tests are repeated by varying the parameters whose in­fluence is to determined, and the length of cracks wh ich appear under various circumstances is observed. In the other type one of the d imen­sions of the test bar is modified until incipient cracking is produced no matter how slight the contraction stresses may be. The maximum d i ­mension (when modifying the thick­ness of test bar) or min imum (when changing its length) for this tearing to start provides an evaluation index of the hot tearing tendency of the a l ­loy.

There are in the technical liter­ature numerous instances of both types of test. Of those in which the test bar dimensions are unchanged the ring test bar by Singer and Jen­nings17 is a typical example and is, undoubtedly, the test most widely used for evaluating the hot tearing susceptibility of light alloys. When fixing the chemical composition for light alloys practically all procedures reduce the proportion of alloy ele­ments until the hot cracking (tearing) produced in the Singer test is not ex­cessive.

Among the tests that measure hot tearing susceptibility by the min ­imum test bar diameter wi thstanding the test wi thout signif icant cracking, mention should be made of the Za-kharov test18. Among tests wh ich measure the maximum elongation sustained without cracking are those by Gamber19 and Rosemberg20. More recently, tests have been developed in which several test bars of increas­ing length are poured s immultan-eously and the hot tearing suscepti­bility of the alloy is measured by eval-

282 -s I J U N E 1 9 7 2

Table 1 —Three

Type of alloy

Weldable Intermediate Ultra-high

strength

Alloy types invest igated

Chemical composit ion, percent

Zn

3 6 8.5

Mg

1.7 2.5 3

Solidif ication range, deg C

Solidus Liquidus temperature temperature

6 2 0 645 585 637 550 632

Pouring temperature,

deg C

745 737 730

Table 2—Indexes of hot tearing strength fo

Al loy

Ultra-high strength Intermediate Weldable

Fig-

2

3 4

T2 , °C

602

624 631

T , °C

542

573 619

r Figs. 2

(T2 -T), °C

60

51 12

3 and 4

h taken

°C

572

598 625

h,

mm

12

22 26

c= h

VT 0.200

0.431 2.166

5 8 0 590 6 0 0

Temce ro t u re

Fig. 3—Hot tearing tendency of an intermediate alloy (6.30% Zn and 2.31% Mg). Alumi­num-base. 99.75%, purity

uating the amount of cracks formed in various lengths, in accordance w i th a previously established table giving the hot tearing susceptibility values. The tests by Tatur21 and Prok­horov13 are typical examples of this type.

Welding Tests

Welding tests can be said to have started wi th the restrained plates of Singer and Jennings1 7 wh ich have been discussed by numerous au­thors. The next phase was to replace the two restrained plates by a single one in which weld metal was depos­ited in a groove on the axis of sym­metry. Mudrack22 proposed an orig­inal criterion for assessing these tests. He defined as hot tearing sus­

ceptibility the quotient between the length of crack produced and 80 percent of the groove containing the weld; that is, when the crack reaches a length equivalent to 8 0 percent of the groove length, hot tearing susceptibility is 100 percent. For longer cracks, the hot tearing susceptibility is over 100 percent. This author made an attempt to im­prove the test by adding transverse cuts in the test bar to provide uni­form cooling condit ions along the weld bead.

On this line Houldcroft23 proposed his " f ishbone" test wh ich has been widely discussed. This test was de­veloped by Evrard24 w h o established more suitable test bar dimensions for each plate thickness. The sensi­tivity of this test is so marked that it

has enabled the influence of very small amounts of grain refiners as wel l as other factors to be estab­lished by means of rigid statistical analysis.25*26*27 This test should not be considered decisive despite its ex­cellent sensitivity because it does not possess the required proportion­ality between length of crack and hot tearing tendency of the alloy. The work of Rogerson, Cotterell and Bor­land28 give good evidence of this.

From calculations in the latter re­port the so-called function " inherent crack sensit ivi ty" is defined which does truly express hot tearing of the alloy and accounts for the fact that the proportionality between the crack and the hot tearing tendency of the alloy does not hold. They also explain the influence of welding rate and other welding conditions on the results. In spite of this we consider the " f ishbone" test the most tho­rough of all. Some attempts to im­prove this test have been made, among which is the test by Hirsch-field and Scilley29. In the work by Borland30 some twenty examples are given.

The latest trend in these tests makes use of two plates which are welded together and tested using highly automated equipment. These plates are subjected to tension stresses whi le being welded simul­taneously under severe condi­t ions. 3 1 . 3 2 . 3 3* 3 4 In fact these tests attempt to measure not only the cracking produced during solidifica­tion but also the cracking which ap­pears on welding. A more exhaustive bibliographical review is given by Madronero.35

Summing up, it can be said that no universally accepted test is available to estimate the hot tearing tendency of the various alloys.

Evaluation Of Hot Tearing By Ductility Measurement In The Solidification Range

As indicated the usual tests for de­termining the hot tearing tendency of light alloys consist of welding or casting test bars of various sizes in which cracking is measured. Cracks that could have been formed at tem­peratures below the solidus are not taken into account, nor is a distinc­t ion made between cracks that are due more to the embrit t lement of the alloy than to particular conditions of the test. Furthermore, at the t ime of evaluation only the length of the cracks produced are taken into ac­count.

Hot tearing is a fracture phenom­enon which appears in metals due to the solidification process producing some contraction stresses which can exceed the ability of the alloy to w i t h -

W E L D I N G R E S E A R C H S l l P P i P M C M T I O O O

stand them. To determine the hot tear­ing susceptibility inherent in each alloy, in the least arbitrary manner and wi th in the solidification range, a test should relate the ability to sustain stresses w i th the tendency of the al­loy to set up contraction stresses. The relation should clearly indicate the variations in the values of both characteristics of t h e m a t e r i a l throughout the entire solidif ication range.

The immediate problem is to con­sider which measurable properties are more apt to represent, in the solidification range, the ability of the material to wi thstand stresses w i t h ­out fracture and to set up contraction stresses. Both should be homologous since the objective is to establish their difference throughout t he whole solidif ication range.

The first step is to evaluate the ten­dency of the alloy to set up contrac­t ion stresses by suitably varying the

temperature. It is wel l known that in the temperature range just above the solidus, there is a correct proport ion­ality between the relative elonga­tions and the stresses wh ich pro­duce them. Since in the casting of an alloy the effect of a reduction in tem­perature is a shortening of the length of test bar, it is just this short­ening wh ich sets up the contraction stresses when the movement of the test bar is restricted. We therefore consider a graphical representation of linear shrinkage versus temper­ature to be appropriate for represent­ing the inherent tendency of the al­loy to set up contraction stresses throughout the entire solidif ication range.

The curve provided by a high preci­sion dilatometer has the shape (par­ticularly w h e n the solidus temper­ature is reached during a quick cool­ing started at a temperature very slightly below the liquidus) of a parab-

£ o o

580 5 9 0 6 0 0 610 620 630 640 650

"em pe ra ture , °C

Fig. 4—Hot tearing tendency of a weldable alloy (2.60%> Zn and 1.75%> Mg). Aluminum-base, 99.75%o purity

Table 3—Purity of three of four aluminum bases used in preparing weldable alloy specimens. Chemical compositions of the aluminum with different grades of purity*

Element

Fe Cu Si Zn Mg Cr Pb M n Ni

Aluminum, 99.3-4%

0.30 0.02 0.065 0.019 0.004 0.01 0.02 0.01 0.01

Aluminum, 99.5%

0.20 0.005 0.065 0.0065 0.001 5 0.01 0.01 0.01 0.01

*The chemical composition of the 99.75%> purity aluminum was

Aluminum, 99.99%

0.001 0.005 0.001 0.002 0.001 0.0003 0.001 0.001 0.0005

given previously. .

ola whose slope can he determined by straight lines tangent to it, especially at temperatures slightly above the solidus where hot tearing takes place.16 Thus, we correlate linear shrinkage to a straight line whose slope is the tangent of the curve re­corded by a precision absolute dila­tometer. in 3 6 can be seen a more de­tailed justi f ication of these reasons as wel l as a description of a device for the appropriate measurement of linear shrinkage.

The next step is to describe the method chosen to measure the capacity of an alloy to sustain stress­es wi thout fracture. It seems logical to apply a tensile test to the alloy and to choose a parameter wh ich would show us how near the alloy is to breaking point. The tensile test measures stresses and elongations. Stresses as measured show an ex­cessive degree of dispersion2*3 ; they are also of different dimensions than the linear shrinkage. It is, therefore, logical to choose the relative elonga­tion as this is measured in the same units as linear shrinkage. As at these temperatures it is impossible accu­rately to determine from the tensi le-stress plot the elastic l imit, although the ult imate tensile strength is deter­mined, w e choose the relative break­ing elongation to represent the abil­ity of the alloy to sustain stresses without fracture.

At temperatures at wh ich the rel­ative breaking elongation is lower than the linear shrinkage, tearing of the material wi l l take place, whi le at temperatures where the relative breaking elongation is higher than the linear shrinkage the alloy wi l l be able to wi thstand the stresses w i t h ­out tearing, w i t h a probability propor­tional to the difference between these two values. Thus the not tear­ing tendency of an alloy w i l l be given by the difference between the rel­ative breaking strength and the l in­ear contraction throughout t he whole solidif ication range.

From the foregoing it appears that the probability for an alloy to sustain this state wi thout fracture is propor­tional to surface area S 0 in Fig. 1. The correct procedure for a study of the influence of each of the param­eters which affect the hot tearing tendency of an alloy is to observe how they alter values of S0 , bearing in mind that any increase in this sur­face area created by an increase in the relative breaking strength or a re­duction in the slope of the linear shrinkage wi l l be considered to favor the hot tearing strength of the alloy so that they must be added to S 0 , whi le increases in surface produced by a rise in temperature T2 or a re­duction in temperature T should be

284 -s | J U N E 1 9 7 2

subtracted from So since they in­crease the probability of the alloy to undergo hot tearing.

Thus the initial hot plasticity coeffi­cient of the alloy is defined as:

P = S 0

When the characteristics of the al­loy are modified, its hot tearing ten-denci diagram also undergoes modif i­cation, so that the value of *P be­comes

P = S0 + AS, + AS2 + AS3 + AS4

TheAS values are either positive or negative depending on whether they represent an increase or reduc­t ion in the ability of alloy to w i th ­stand hot tearing, as shown in Fig. 9.

Wi th this we can study the devel­opment of the hot tearing tendency of an alloy w h e n its ductil ity changes due to the variation of a given factor (grain size, gas content, proportion of stabilizers, such as Cr, etc.).

To compare the hot tearing of com­pletely different alloys (more correct­ly, of alloys which have a quite differ­ent hot tearing diagram) it is more convenient to use the coefficient re­sistivity to hot tearing:

9 10

Z i n c %

<T2 T,)2 T2 - T

Fig. 5—Hot cracking curves in Singer test of Al-Zn-Mg alloys (ref. 38). W - weldable alloy (2.60% Zn and 1.75% Mg); I = intermediate alloy (6.30% Zn and 2.31% Mg); U = ultra-high strength alloy (8.50% Zn and 3.05% Mg)

The lower the value of C in a alloy, the greater w i l l be its hot tearing sus­ceptibility in the solidification range.

The reader wi l l f ind this criterion fully justif ied in our previous paper36

together w i th a complete description of the operational procedures devel­oped for their use.

We wish to point out that by using this criterion we measure the inher­ent hot tearing tendency of the alloy, regardless of the particular welding or casting conditions wh ich give rise to the fact that the alloy undergoes a certain amount of cracking in these processes.

Evaluation of Hot Tearing In A -Zn-Mg Alloys

General. Once the criterion to be observed and the operational proce­dures have been developed we go on to determine hot tearing in A l -Zn-Mg alloys. To this end we select three al­loys in the Al-Zn-Mg system, each representing the three large groups into which these alloys are divided; namely, ultra-high strength alloys, weldable or self-hardening alloys and intermediate type alloys as shown in Table 1. The ultra-high strength alloys contain a high propor­t ion of alloying elements, their strength is very high and when they

Table 4—Comparison of test results obtained from weldable alloy specimens pre­pared from four levels of aluminum base purity

Diagram of Fig.

6 7 4 8

Purity of aluminum base, %

99.35% 99.5 99.75 9999

Chemical | i composition % |

Zn

2.84 3.23 2.60 2.85

Mg

1.75 1.72 1.75 1.75

Zn-Mg

4.59 4.95 4.35 4.60

T 2

°C

629.5 628.5 631 629.5

T °C

615 612 619 616

h, mm.

25 22 26 25

C = ^ T T2-T,

1.724 1.333 2.166 1.293

Brittle area. mm 2

679 682 537 652

also contain copper possess the high­est mechanical strength values in light alloys. To represent this group we have chosen an alloy whose chemical composition is 8.5 Zn-3.0 Mg. The weldable or self-hardening alloys are unique in that they do not lose the mechanical properties ob­tained through heat treatments on air cooling, that is, they are self-hard­ening. They have some interesting cryogenic uses and are undoubtedly the group of alloys in the Al -Zn-Mg family showing more promise for the future. As representative of this group we selected an alloy of compo­sition 3.0 Zn-1.7 Mg. Finally, we chose an alloy of composition 6.0 Zn-2.5 Mg to represent the intermediate type.

Apart f rom the case discussed later under the influence of pouring temperature, these alloys wi l l be cast w i th a pouring temperature 100 C higher than its liquidus temper­ature which together w i th the fact that none of these alloys possess in ­termediate phases as they are in­cluded in the aluminum solubility cor­ner of the equil ibrium phase diagram for the Al-Zn-Mg system, make them have an homogeneous structure so that the results of the measurements are perfectly comparable.

To prepare the alloys two master alloys were produced, one contain­ing 20 percent Zn and the other, 10 percent Mg, pure aluminum being added to the crucible. This aluminum was of the same degree of purity as

W E L D I N G R E S E A R C H S U P P L E M E N T ! 285 -s

E o o

580 590 600 610 620 630

Temperatu re , °C

6 4 0 6 5 0

Fig. 6~Hot tearing tendency of a weldable alloy (2.84%, Zn and 1.75%, Mg). Remelt alu­minum base

5 •

\

\

- \

1

E o o

580 590 6 0 0 610 620 6 3 0

Te mperature, °C

5 4 0

Fig. 7—Hot tearing tendency of a weldable alloy (3.23%, Zn and 1.72%, Mg). Aluminum base, 99.5%, purity

Table 5—Values of the coefficient P for Figs. 6, 7, 4 and 8* Diagram Brittle ASj AS 2 AS3 AS* 'P = S o + s A S , of Fig. Area, mm2

6 679 — — — — 679 7 682 - 2 0 + 1 7 - 1 3 8 538 4 537 — - 2 8 — + 200 851 8 652 — — — + 48 727

* AS signs and values have been taken according to Fig. 9.

that used in the preparation of the master alloys.

The melt was poured into a bento-nite-bonded green sand mold. From each cast enough melt was obtained to fi l l three molds, eight identical in­gots being obtained from each mold. These ingots were later machined and a tensile test specimen was pro­duced from each one of them. From each mold ten dilatometry test spec­imens and chippings for chemical analyses were also obtained. The chemical analyses were made by atomic absorption. The Zn and Mg used for making the master alloys were of high commercial purity.

Under these operational condi­tions practically all of the ingots w e obtained had an equiaxed as-cast structure. This is of great signif­icance as " . . . w i th the columnar structure . . . conditions are less fa­vorable for intergranular deformation than w i th the equiaxed structure. Ac­cordingly, the solid-liquid ductil ity of the equiaxed structure is at all tem­peratures higher than that of the col­umnar structure."37

When plotting the relative break­ing elongation and linear shrinkage on the hot tearing diagrams we made sure of always using the same scale so that comparable results would be obtained at all t imes.

Hot Tearing Susceptibility of the alloys shown in Table 1

The three alloys were prepared wi th an aluminum base which f rom now onwards wi l l be known as 99.75 aluminum. Its chemical compo­sition (in weight percent) according to the analyses carried out in our lab­oratories was as fol lows: 0.09 FE, 0.01 Mn, 0.01 Ni, 0.03 Zn, 0.085 Si, 0.01 Cr, 0.01 Pb, 0.005 Cu, 0.002 Mg.

Hot tearing diagrams are given in Figs. 2, 3 and 4 and indexes of hot tearing strength in Table 2.

These results show that the ultra-high alloy is much less resistant to hot tearing during solidification than the weldable alloy whi le the hot tear­ing susceptibility of the alloy desig­nated intermediate lies between that of the other two. It can easily been seen that the difference between the various values of C is basically due to the amplitude of the britt le temper­ature range and to the height of the comparative elongation plateau. L in­eal shrinkage is practically the same for the three alloys.

These results are logically predict­able since, as the amount of alloy elements added to the aluminum base increases, it can be expected that the possibilities for undergoing hot tearing wi l l increase.

Figure 5 gives the results obtained

286-s I J U N E 1 9 7 2

by Patterson38 w i th the Singer test. It w i l l be seen that his results differ considerably from ours, in particular so far as the ultra-high strength alloy is concerned. We believe this is due to the fact that, we explained in our previous paper16, the cracks which appear in the Singer test of A l -Zn-Mg alloys do not form at temper­atures above the solidus but under it. As the ultra-high strength alloy has a greater strength than either the weldable or the intermediate alloy it w i l l , in spite of its higher hot tearing susceptibility, resist hot cracking at temperatures under the solidus bet­ter than the other two.

Owing to the great similari ty be­tween the diagrams for the hot tear­ing tendency of these three alloys, we shall f rom here on study the in­fluence of the various parameters on one of them and shall extrapolate the results to the other two. Of the three alloys we shall select the weld­able one since it represents the most important group of these alloys and also because its range of solidif ica­t ion temperature is the narrowest, so that a smaller number of tests are re­quired in order to determine its duc­ti l i ty throughout that range. We shall assume the results obtained in these tests to be valid for the other two al­loys, the ultra-high strength and the intermediate ones.

Effect Of Aluminum Base Purity On Hot Tearing Susceptibility

The influence of the grade of purity of the base alumimun on the hot tearing susceptibility of A l -Zn-Mg al­loys was considered from the view­point of cost to determine whether the use of high purity and expensive aluminum would relieve th is great technical inconvenience of A l -Zn-Mg alloys.

We therefore investigated the hot tearing susceptibility of the weldable alloy prepared w i th aluminum of four different grades of purity, as shown in Figs. 6, 7, 4 and 8. The chemical compositions of the aluminum bases are given in Table 3. Test results are shown in Table 4.

The problem is to deduce from the results of Table 4 whether the vari­ation in britt le area (area between the curves for relative breaking elongation and linear shrinkage and the ordinates corresponding to T, andT 2 ) is due to the purity of the base aluminum or to the chemical composit ion. Of course, the ideal would have been to test four alloys of exactly identical chemical composition. That was impossible since (a) the base aluminums dif­fered and (b) the alloys were pre­pared in different casts making it practically impossible to obtain iden­tical composit ion.

5 •

3 -\

580 590 600 6I0 620 630 640 650

T e m p e r a t u re , °C

Fig. 8—Hot tearing tendency of a weldable alloy (2.85%, Zn and 1.75% Mg). Aluminum base, 99.99%,purity

P= S0± A S , - A S 2 - flS3i A S 4

o a

u Te m p e r a t u r e , ° C

Fig. 9—Rule for applying positive or negative sign to values of S and A S in determining P from hot-cracking diagram

Comparing the Data of Table 4 w i th Table 2, it seems at first that the dif­ferences are simply due to variations in chemical composition.

It can be seen that in the alloys of Table II the variation in chemical composition influences the value of C. The average amount of alloying elements (% Zn + % Mg : 11.55, 8.61 and 4.35) is 8.17 t 2.08 ( 1 25%). Average value of C is (0.200, 0.431

and 2.166), 0.932 ± 0 . 6 2 (+_66.50%). The relationship between both per­centage errors is

66.5 = 2.66

25 Let us do this same thing for re­

sults in Table IV. The average amount of alloying elements (% Zn + % Mg: 4.59, 4.95, 4.35 and 4.60) is 4.62 ± 0.12 ( ± 3%). The mean

W E L D I N G R E S E A R C H S U P P L E M E N T ! 287 -s

c o

4 -

_. N

580 590 600 610 620 630 640 650

Due to the small difference exist­ing between both relationships, it may be presumed that the influence of the degree of purity of the alumi­num base is completely masked by the small variations in the chemical composition, unavoidable when hav­ing to make various casts.

To operate even more strictly Figs. 6, 7, 4 and 8 should be interpreted by means of coefficient P, as we are dealing w i th very similar britt le di­agrams.36

Table 5 gives the coefficient P for the britt le diagrams of Figs. 6, 7, 4 and 8, taking as S 0 values correspond­ing to Fig. 6.

As may be seen, the mean of P values (679, 538, 851 and 727) i s , 6 9 9 + 64 (+9.1 5%), so that

^ l 5 - = 3.05

T e m p e r a t u re, °C

Fig. 10—Hot tearing tendency of a weldable alloy (3.00% Zn and 1.80%, Mg). Aluminum base, 99.75%, purity. Pouring temperature 700 C.

Tem p e r a t u r e , °C

Fig. 11—Hot-cracking tendency of a weldable alloy (3.00%, Zn and 1.60%, Mg). Aluminum base, 99.75%, purity. Pouring temperature 800C

value of C is (1.724, 1.333, 2.166 and 1.923) is 1.786 + 0.17 (+9.51%). The relationship between both mean

square errors is: 9.51 3.17

is valid since it f i ts in perfectly w i th ourforegoing conclusion.

It therefore appears justif iable to interpret the variations wh ich the hot tearing susceptibility of an alloy can experience due to the variation in the purity of the base aluminum as being negligible in terms of the var­iation it undergoes due to the least minimum change in its chemical composition. That is, the influence of the purity of the base aluminum may be considered of no importance.

The only tests we know of per­formed in th is respect are those by Guilhaudis and Develay39 who tested Al-Zn-Mg alloys using the welding test on restrained plates. They showed it was likely that when the base aluminum passed from 99.5 to 99.7% the hot tearing susceptibility seemed to increase and this in­crease was of such litt le importance that the authors did not venture to state categorically that this slight in­crease be significant. We further be­lieve that the dispersion in their re­sults was higher than in ours.

Effect of change in grain size due to variation in pouring temperature.

In order to determine the influence of grain size we measured the hot tearing susceptibility of the weldable alloy w i th 99.75 base aluminum and

Table 6—Comparison of test results obtained from weldable alloy specimens poured at three different temperatures

Diagram of Fig.

10 4

11

Chemica % Zn

3.00 2.60 3.00

composit ion % Mg

1.80 1.75 1.60

Pouring temperature,

°C 7 0 0 745 8 0 0

T, °C

618.5 619 621

B ittle range T5

°C 630.5 631 632.5

T; —T °C 12 12 11.5

h measured at (°C)

624.5 625 627

h, mm

34 26 20

Parameter C

2.833 2.166 1.739

288-s I J U N E 1 9 7 2

va r i ous g r a i n s izes. G r a i n s ize is u s u ­a l l y c o n t r o l l e d by a d d i n g g r a i n r e ­f i n e r s o r v a r y i n g t h e p o u r i n g t e m p e r ­a tu re . To o b s e r v e t h e i n f l u e n c e of g ra i n s ize w i t h o u t i n t e r f e r e n c e s f r o m o the r e f fec ts , w e p r e f e r r e d v a r y i n g t h e p o u r i n g t e m p e r a t u r e o n l y b e ­cause g r a i n r e f i n e r s can at f i r s t a l t e r t h e p r o p e r t i e s of t h e l i qu i d p h a s e apa r t f r o m r e d u c i n g g r a i n s ize.

W e t h e r e f o r e c a r r i e d ou t t h e t e s t s , us i ng t h e c o n d i t i o n s d e s c r i b e d in F igs. 10 , 4 a n d 11 ( the a l l o y c o r r e ­s p o n d i n g to F ig. 4 w a s p o u r e d at 7 4 5 C). The resu l t s a re g i v e n in Tab le 6.

The i n c r e a s e in t h e b r i t t l e r a n g e of t h e a l loy is d u e to t h e r i se in t h e p l a ­teau of t h e re la t i ve b r e a k i n g e l o n g a ­t i o n .

A s c a n be s e e n t h e r e s u l t s a g r e e c o m p a r a t i v e l y w e l l w i t h t h o s e g i v e n by Ta tur 2 1 (page 2 2 5 5 ) . In ou r case it is poss ib le to o b s e r v e no t o n l y h o w t h e ho t t e a r i n g t e n d e n c y of t h e a l l oy va r i es bu t a lso t h e r e a s o n fo r it. In t h e f i r s t p lace (Fig. 10) it w i l l be a p p r e ­c i a t ed t h a t w h e n t h e p o u r i n g t e m p e r ­a t u r e is ve r y l o w ho t t e a r i n g is g r e a t l y r e d u c e d d u e t o t h e h i g h d u c t i l i t y ex ­h i b i t ed by t h e a l l oy at t h e ho t t e a r i n g range t e m p e r a t u r e s . If w e i n c r e a s e t h e p o u r i n g t e m p e r a t u r e t h i s d u c t i l i t y w i l l dec rease bu t if w e i n c r e a s e it ex ­cess ive ly , apa r t f r o m d i m i n i s h i n g , t h e g r a i n u n l o c k i n g zone w i l l d i s a p ­pear ( w e re fe r t o t h e s h a r p d r o p i n duc t i l i t y w h i c h s e p a r a t e s t h e P ro ­k h o r o v zone f r o m t h a t of B o r l a n d ) b e ­cause due to t h e i r s ize t h e y ve r y eas i l y i n te r l ock , P r o k h o r o v ' s zone t h u s be ing r e d u c e d to a m i n i m u m . 1 6

Al l t h e s e resu l t s c o n f i r m t h e usua l ru le in m e t a l l u r g y t h a t a m a t e r i a l w i l l e xh ib i t i ts bes t p r o p e r t i e s w h e n i ts g r a i n s ize is t h e m i n i m u m poss ib le .

Ef fec t of Grain Ref iners, Especial ly Z i rcon ium

S p e c i a l a t t e n t i o n s h o u l d be g i v e n to t h e w a y g r a i n r e f i n e r s a t t e n u a t e t h e ho t t e a r i n g t e n d e n c y of l i gh t a l ­loys. A l t h o u g h eve ry a u t h o r a g r e e s t ha t t hey reduce ho t t e a r i n g e f f i c i e n t ­ly, it is no t c lea r h o w a n d w h y t h e y d o so . A l t h o u g h t h e m a j o r i t y i m p u t e t h e e f fec t s i m p l y t o a r e d u c t i o n in g r a i n s ize, t h e r e a r e o t h e r s w h o c o n ­s ider it due to a v a r i a t i o n in t h e a m o u n t of gas d i sso l ved in t h e me l t . 4 0 S t i l l o t h e r s f i n d i n s t e a d t h a t t he e f fec t is due m o r e to t h e i m p r o v e ­m e n t m a d e in m i c r o p o r o s i t y a n d t h e d i s t r i b u t i o n of t h e i n t e r m e t a l l i c c o m -

A

- \

E o

CJ

5 8 0 5 9 0 6 0 0 610 6 2 0 6 3 0 6 4 0

Tem p e r o t u r e , °C

Fig. 12—Hot tearing tendency of a weldable alloy (2.63%, Zn and 1.61% Mg) with 0.1% Zr. Aluminum base, 99.75% purity

\

\

\ .

i / \

E o u

580 590 600 6I0 620 630 640 650

T e m p e r a t u r e , ° c

Fig. 13—Hot tearing tendency of a weldable alloy (2.84% Zn and 1.62%, Mg) with 0.2% Zr. Aluminum base, 99.75% purity

pounds 4 1 (p. 72 ) . The f i r s t p r o b l e m is to c h o o s e fo r

s tudy t h e m o s t su i t ab le g r a i n re f i ne r . The l i t e ra tu re p rov ides n u m e r o u s ex ­a m p l e s m a i n l y f o r t h e use of Zr , T i , M n a n d B.

E n o u g h is n o w k n o w n abou t t h e m a n n e r in w h i c h g r a i n r e f i n e r s b e ­have. H a v i n g d i s c a r d e d t h e h y p o t h ­es is of t h e c o n c e n t r a t i o n g r a d i e n t , 4 2

it is b e l i e v e d t h a t g r a i n r e f i n e r s b e ­l o n g i n g to t h e t r a n s i t i o n e l e m e n t s

Table 7—Ef fec t o f Zr addi t ions to we ldab le al loys having 9 9 . 7 5 % pure al loy base

Diagram Chemical composit ion of Fig. % Zn % Mg % Zr

4 2.60 1.75 12 2.63 1.61 0.1 13 2.84 1.62 0.2

Britt le range °C T, T2

619 631 618 631 617 631

T 2 - T , 12 13 14

h measured at °C 625C 624C 624C

h in mm 26 26 37

: = h

T2— T, 2.166 2.000 2.642

W E L D I N G R E S E A R C H S U P P L E M E N T ! 2 8 9 - s

and to the lanthanides, due to their electronic layers,43 form some few degrees above their liquidus temper­ature "quasi-molecules."These molec­ular aggregates of grain refiner atoms are related to aluminum atoms and behave as solidification nuclei. Ac­cording to this theory grain refiners are, in order of efficiency, Zr, Ti , Hf, Ta, V, W, Nb, Mo, Re, Fe, M n , Cr, Co and Ni. As some of these bodies are "donors" and others "acceptors," it is possible that two of them in a cer­tain amount could nullify their effect as grain refiners. Thus, for instance, Ti + Zr, Ti + Fe and Ti + Cr may be cancelled. Other times, on the con­trary, their effects are additive and this explains the known results44: that by relating two of them (Ti and B) an optimum reduction in grain size is achieved.

The effect of grain refiners on hot tearing has been examined tradi­tionally by means of welding tests. Ti and Zr are undoubtedly the two grain refiners more often studied. Zircon­ium was studied by Schoer and Gruhl45

who recommend 0.20% as an opti­mum ratio, by Dudas46 who suggests its use in a ratio of 0.10-0.15% and by Sugiyama and Fukui.26 The latter give a curve of the variation in hot tearing which seems to show that up to a ratio of 0 . 1 % , Zr does not have any influence on cracking; f rom this ratio onwards the hot tearing sus­ceptibility starts to diminish, until f rom 0.2% upwards its influence does not increase further. They also found that Ti and B, although they refine the grain, exert only a very slight influence on hot tearing sus­ceptibility.

Chevigny and Develay go so far as to eliminate the hot tearing of an A l -Zn-Mg alloy by putting the Zn and Mg into an approximate relationship of 2 and by the use of Zr in a 0.1 to 0.5% ratio.

The effect of Zr is perhaps better studied in a paper by Develay and Croutzeilles.26 In this paper it is shown that Ti produces similar ef­fects and that when -it is added to­gether w i th Zr there is an optimum reduction in the hot tearing suscepti­bility.

In the light of the foregoing w e chose Zr for study as the exclusive grain refiner in the ratios of 0.1 and 0.2%. The hot cracking diagrams are

given in Figs. 12 and 13. The coeffi­cients resulting from the diagrams in Figs. 4, 12 and 13 are shown in Table 7.

The data in Table 7 are highly significant. They clearly show that the effect an 0 . 1 % Zr addition can have is below the measurement er­ror, so that we consider it has no in­fluence. Observing the effect of an 0.2% Zr addition we see a reduction in the hot tearing susceptibility and if we compare Figs. 13 and 10 it w i l l be seen that the difference is due to an increase in the height of the pla­teau typical of a fine size grain. This increase could have been greater if the chemical composition instead of being 2.80% Zn and 1.70% Mg had been 2.60% Zn and 1.75% Mg.

There is a special effect in the Zr addition which is seen on studying Fig. 13. There is a broadening of the Prokhorov zone which is hardly per­ceptible in the alloy w i th 0 . 1 % Zr and wh ich seems to be produced by an increase in the liquidus temperature, this being in perfect agreement w i th the Al-Zr diagram shown in Fig. 14.

Thus the effect of Zr appears to be accounted for. When the ratio of Zr is less than 0 . 1 % it does not in­fluence significantly the hot tearing susceptibility of the alloy. But when its proportion is higher (when there is enough Ar so that, according to Sam-somov's theory43 some molecular aggregates are formed in a con-siderableamountand, therefore, grain refining takes place) apart from in­creasing the ductil ity of the alloy, a widening of Prokhorov's zone is pro­duced due to the increase in the l iq­uidus temperature. We believe this broadening bears no influence on the hot tearing susceptibility as in this zone the alloy can sustain signif­icant elongation without cracking and should th is happen there are many probabilities that at lower tem­peratures the cracks could be "cured."4 8

Novikov37 obtained similar conclu­sions when he showed that small amounts of M n and Ti increased the relative elongation of an AI-7.5% Cu alloy.

That a reduction in grain size al­ways produces an increase in pla­teau height, whether it is caused by a reduction in the pouring temper­ature or whether it is produced by

the addition of grain refiners, sug­gests that this increase in ductil ity is due to the cracks being "cured" 4 9

more readily by the remaining mol­ten metal in Borland's zone. This can occur for two reasons: (a) because the cracks to be fil led are smaller (we must remember that a crack is the space left by two adjacent grains which have been torn apart and, therefore, the finer the grains the shorter w i l l be the cracks) or (b) be­cause the surface tension of the melt remaining is reduced by the addition of the grain refiner. If the two effects are added the influence of the grain refiners wi l l be strikingly great. In the technical literature we f ind, for instance, that Saveiko50 supposes that Cr attenuates hot tearing of steels by reducing the surface ten­sion of melt.

We do not believe this happens in the case of light alloys, since if the two effects (reduction in grain size and variation in the surface tension of remaining melt) were to be added the increase in height of the plateau of the relative elongation would be much greater in Fig. 13 than in Fig. 10 which, in spite of the differences in chemical composition does not happen. We believe that, should the variation in surface tension bear an influence, it would be insignificant in terms of the effect of grain size re­duction (or, in other words, of size of cracks).

This is difficult to prove by means of accurate mathematical calcula­tions, for in order to calculate the variation in the surface tension of an aluminum melt containing Zn and Mg w i th Zr additions, an exact knowl­edge of Eotvos's constant would be required and, although w e have car­ried out an exhaustive bibliograph­ical review we have been unable to f ind it. It was also impossible to f ind data on experimental measurements concerning the influence of Zr addi­tions; in addition his experimental determination is beyond the scope of the present work due to its excessive complexity.51

Nevertheless, it is not surprising that the additions of Zr in these pro­portions hardly reduce the surface tension of melt, since in an early paper by Pelzel52, in wh ich a exper­imental study of the variation in sur­face tension of Al due to individual

Table 8—Effect of Cr Addition to Weldable Alloys Disregarding Purity of Aluminum Base

Diagram of fig.

4 17 18

Chemical composition, % Zn Mg Cr

2.60 268 2.70

1.75 1.74 1.71

0.24 0.24

Range of brittle temperatures, °C T, 72 T, —T2

619 608 609

631 632 632

12 24 23

h taken at °C

I 625 620 621

h in mm.

26 22 22

2.166 0.916 0.956

290-s I J U N E 1972

and simultaneous additions of Zn and Mg is made, the Zn and Mg alter little the surface tension of melt, par­ticularly when in small proportions. Moreover we noted earlier that small variations in the proportion of Zn and Mg have greater effects on the ductil ity of the alloy than small addi­tions of Zr.

In a more recent work by Korol-kov53 it is shown that the effect of an alloying element on the surface ten­sion of A l is almost proportional to the difference between the atomic volumes of the alloying element and of the aluminum (at. vol. of A l = 10, of Mg = 16 .21 , of Zn = 9.90 and of Zr = 14.1). According to this, Zr should have a lesser influence than Mg and almost the same as Zn. It has been proved experimentally52, however, that Zn has an almost negligible in­fluence and that of Mg is very small . In any case, even if the Zr did influence the surface tension of the molten aluminum considerably, it would not affect the surface tension of the last alloy drops in the liquid state located among the grains of the alloy under consideration. Since the Zr atoms remain almost "bu r ied " in the core of the grains due to their grouping into "quas i -mo lecu les " grain embryos (according to Samso-mov's theory43 cited previously), it wi l l be difficult for them to influence the remaining melt when there is not much of it as is the case in Bor­land's zone. They wi l l only have an effect on the melt when the grains are almost decomposed. Because of proximity to the liqffidus temperature the nuclei of the most minute grains, on dissolving, provide a considerable proportion of Zr to the remaining melt. This agrees wel l w i th the

9 50

660,5°

AI+ Zr A l 3

400 0,2 0,4 0,6 0,8

ZIRCONIUM (Wt % )

Fig. 14—Equilibrium diagram of Al-Zr alloys

l,0 1,2 l,6

z- \

^ t

• r \

T

{ j l

=̂=7*

•r

• ft. '. •

f

1 h r

W' *

Fig 15—Structure of weldable alloy with 0.24%, Cr; Aluminum base, 99.5%, purity. As-cast condition (100X) (a) Magnesia polished. Unetched, (b) Mechanically polished and electrolytically etched.

W E L D I N G R E S E A R C H S U P P L E M E N T ! 291-s

y

J,

/r ~-^s \.

i*** ~7 / Fig. 16 — Struc­ture of weldable alloy with 0.24%, Cr after an age­ing heat treat­ment at 450C for 4 hr, (a) Magne­sia polished. Un­etched (b) Me­chanically pol­ished and electro-lytically etched.

3 2

\

\ \

630 640 650

Fig. 17-Cr. Non

Tern peratu re . °C

-Hot tearing tendency ot a weldable alloy (2.68%, Zn and 1.74%, Mg) with 0.24%, aged. Aluminum-base, 99.5%, purity

breakdown zone of the solid struc­ture16 of the curve in Fig. 13 (last part of the curve, at temperatures just below the liquidus).

Thus we conclude by stating that grain refiners produce an increase in the strength of Al -Zn-Mg alloys to sustain hot tearing due only to their reducing the mean grain size. The possible variation they may produce in the surface tension of the melt does not appear to have any appre­ciable influence at all. They also pro­duce a broadening of Prokhorov's zone which does not influence the hot tearing tendency of the alloys sig­nificantly. The minimum amount of grain refiner required to produce this effect can be deduced from the phase diagram of the system.

Effect of Stabilizers, Especially Chromium

One of the main drawbacks of A l -Zn-Mg alloys that sometimes re­stricts their use is the tendency to sustain stress corrosion. Ageing these alloys is the best remedy for stress corrosion. It is easy to do and is also advantageous because the mechanical properties of the alloys can thus be increased. The alloying elements most widely used for this are Ge, M n and above all, Ag and Cr. Thus, for instance, the effect of Cr and Mn are compared in references 54,55,56 a n c j that of Cr, Zr and Ag in 57.

However, monographs on a single stabilizer are more abundant; for ex­ample, the effect of Ge is studied in 58 and that of Ag in 59 to 63 But the one which has been more widely in­vestigated is, undoubtedly, Cr.64

Therefore we are famil iar w i th the phenomena which take place on the ageing 65 to 69 of these alloys wi th various Cr additions. The ageing ki­netics can be seen in 76*70 and the in­fluence of the point defects of the material on ageing when there are Cr additions present is to be found in 71 *72. There are also various papers which report on optical and electron microscopy, and X-ray observations of the precipitates whose formation is due toCr 7 3 80,

Because the ageing process of A l -Zn-Mg alloys is better known when the alloy containsCr additions it seems appropriate to choose this stabilizer study. There are two reports wh ich induce this selection. In 67 it is said that Cr has a much more consider­able effect on the ageing of these al­loys than Mn and Cu. Baba81 is even more explicit. He makes an exhaus­tive study of the effect of possible ad­ditions on the ageing of Al -Zn-Mg al­loys and reaches the conclusion that the most active is Cr, fo l lowed at quite a distance by Mo, V, Zr, Mn ,

292-s I J U N E 1 9 7 2

Cd, Ag, and Cu. We consider it appro­priate to study it in the weldable a l ­loy as it is the type in which the an­nealing temperatures and heating rates have a lesser influence on age­ing.82 Besides we need not worry about the purity of the base of alumi­num because, as it appears to have been proved, it does not influence ageing.83

We have studied the hot tearing tendency of the weldable alloy w i th U.24% Cr in the as-cast condit ion ana after it had been subjected to an anneal-ageing treatment at 450 C tor four hours. Figs. 1o and 16 are micrographs of the alloys prior to and fol lowing the ageing treatment; the electrolytic etchant used was Herenguel's.84 As can be easily seen ageing is complete. The diagrams for the alloys before and after ageing are shown in Figs. 17 and 18 whi le the results of the determination are given in Table 8.

Table 8 compares the weldable al­loy containing Cr w i th the alloy of Fig. 4 (99.75% purity) instead of com­paring it w i th that of Fig. 7 whose base aluminum is of the same purity as those in Fig. 17 and 18. This com­parison was selected because (a) the quantities of Zn and Mg are in more similar proportion and (b) it was pre­viously shown8 3 that the purity of the aluminum base does not influence the effect of chromium on the ageing of these alloys.

The results in Table 8 are easy to interpret. In Figs. 4, 17 and 18 it is seen that the three curves have their plateau almost at the same height and that all the chromium addition has really done is to change the ex­tent of the solidification range. Fur­ther, there is a very slight linear shrinkage increase of the alloy.

These results indicate tnat chro­mium does not alter the ductil ity of the alloys. However, because it widens the range of britt le temper­atures, it increases the hot tearing susceptibility slightly. This effect is independent of whether the alloy is aaed or not.

Effect of Gas Content on Hot Tearing

On this topic the bibliographical data are more contradictory. On one hand there is the general belief that gases reduce the hot tearing susceptibility of light alloys, as expressed by Lees85 and recently by Scheuer, W i l ­l iams and Wood8 6 and Novikov37

who ascribe it to the fact that the gas content reduces linear shrink­age. There are some, however, who consider that the gases increase the hot tearing susceptibility of these al­loys87 and there is always someone who finds that gases have no inf lu­ence 21

lo

A \

M E o o

5 8 0 5 9 0 6 0 0 6 I 0 6 2 0 6 3 0 6 4 0

Te m peratu re

Fig. 18—Hot tearing tendency of a weldable alloy (2.70% Zn and 1.74%, Mg) with 0.24% Cr. Aged at 450 C for 4 hr. Aluminum-base, 99.5% purity

580 590 600 6I0 620 630 640 650

Temperatu re, °C

Fig. 19—Hot tearing tendency of a weldable alloy (2.80%, Zn and 1.75% Mg. remelted). Gas content- 22cm3/100g

The result of our test can be seen in Fig. 19. The amount of gas exist­ing in the alloys tested previously was about 15 c m 3 / 1 0 0 g. The proce­dure in pouring tor this test was s im­ply to take no precaution whatsoever to avoid the dissolution of the gases in the melt whi le in all other previ­ous alloys we had used some degas-ifying products.88 The gas content in­cludes not only hydrogen but all gases that could be produced during the fusion casting process.

Comparing Figs. 6 and 19 it seems that hot tearing susceptibility clearly increases due to the step wh ich the

plateau makes in the zone of the higher temperatures of the Borland range. We believe, however, that this increase is less signif icant than is shown by these figures, for a l ­though at this step the cracking prob­ability is greater, " cu r i ng " of the cracks may take place at the lower temperatures.

It must be noted that the effect of gas content is the most difficult to study w i th these tests, as we can only examine specimens w i th a smaller quantity of gas than would produce porosity; that is, only gassed specimens of zero type and another

W E L D I N G R E S E A R C H S U P P L F M F W T l i>a->-

from the range of gassed specimens by of zero type and another from the range of gassed specimens by Scheuer.86 Even so it was seen that the dispersion of tensile test results was higher than in those made in the preceding tests, to such an ex­tent that we had to repeat this curve many t imes and our tests had to be more carefully controlled. As the gas content is increased, the distr ibution of porosity is never perfectly ho­mogeneous, and as at testing tem­peratures fracture is fully britt le, dis­persion wi l l be excessive on account of the variation in the cross-section of the test specimens. Another disad­vantage is that in the dilatometry tests heating takes place slowly and it is possible that some of the gases may escape from the test bar during the linear shrinkage measurement. It may be for these reasons we never found the amount of gases to in­fluence linear shrinkage, contrary to Novikov's opinion.37

We are therefore inclined towards the belief expressed87 that the hot tearing tendency of Al -Zn-Mg alloys increases w i th gas content, although the tests performed for this determi­nation were the least satisfactory of all.

Conclusions

1. The hot tearing tendency of A l -Zn-Mg alloys can best be studied by measuring their ductility and linear shrinkage along their solidif ication range than by using the welding or pouring tests of a somewhat arbi­trary design wh ich are usual in tech­nical literature.

2. The variation in ductil ity is what determines the influence of a given factor on the hot tearing tendency of Al-Zn-Mg alloys. The variations in linear shrinkage are negligible in terms of the observed differences in ductility.

3. The hot tearing susceptibility of these alloys seems to grow wi th the proportion of alloying elements. On this account, of the three alloy-types studied the one more prone to sus­tain hot tearing is the ultra-high strength alloy type and the less prone to tearing is the weldable alloy type.

4. The degree of purity of the base aluminum does not appear to bear any influence on the hot tearing sus­ceptibility of the alloys examined in this work.

5. Wi th increasing pouring temper­ature the hot tearing susceptibility of the alloys increases.

6. The addition of Cr alters some­what the hot tearing susceptibility of these alloys; it modifies only to a scarcely noticeable degree their

solidification range. We believe to have proved that this effect is inde­pendent of whether the alloy has or has not aged.

7. Grain refiners reduce the hot tearing susceptibility on account of the grain size reduction they orig­inate, wi thout it seems, at least in the particular case of Zr, its possible influence on the surface tension of the molten alloy making up the inter­granular layers, being significant. They also produce a broadening of Prokhorov's zone which does not in­fluence the hot tearing tendency dur­ing solidification of the alloy.

8. It seems that the amount of gases in these alloys increases their hot tearing susceptibility although this effect is more difficult to study wi th our procedures due to the dis­persion in the tensile tests.

A cknowledgments Part of this paper is contained in the Doctoral Thesis by one of the authors (AM ) who wishes to express his thanks to the "Juan March" foundation for an allow­ance granted for carrying it out at CENIM

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vQd.c I I I I N F 1 9 7 J