Hot Extrusion of Al SiC Powder

7/27/2019 Hot Extrusion of Al SiC Powder

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the main parameters that are required to obtain products with high-strength welds were determined in the

example, using appropriate high temperature, and possibly high pressure for possibly a long time, and withthe suitable high values of shear strain in the area of weld formation. The lower those parameters are

(temperature, pressure), the easier the material can be welded in the extrusion conditions.

The most common methods of preparation of metal powders is the milling and grinding in solid state,

and spraying and granulation of the liquid phase, as well as the reduction of metal oxides method.[8, 9]Preparation of charge for the extrusion of metal powders can be accomplished through direct pouring of

loose powder into the container, cold compaction and sintering by hot compaction, and by cold compaction

in the cartridge.[8, 10] In the case of pouring material into a container in the form of loose powder, for which

particles are usually large in size, the most often used method is an extrusion with cork, which allows the

initial compaction of the material. Cork can have the form of a cone mounted on the outside of the die, orthe form of plate on the inside of the container, placed on the front surface of the die in such way as to

prevent tipping of the material through a die. After reaching a certain degree of compaction and a certain

level of pressure, the cork is pushed out of the die, or is extruded with the material. Thanks to this

operation, loose powder won't fall out of the container, and the extrusion process can be realized.[8, 11]Another way to prepare the charge for extrusion is by cold compaction and sintering, or hot compaction. In

those methods, a charge with a higher density than in the loose powder extrusion is obtained. This reduces

the path of the punch and the height of the container needed for the production of the product of requiredlength.[8, 12] A more complex approach to the preparation of charge is a method of cold compaction in the

cartridge. The powder is initially compacted directly in the cartridge and the cartridge itself can be closed,

for example, to ensure the appropriate atmosphere for a powder, or remain open during the extrusionprocess.[8, 10, 11]

COMPACTION OF POWDER MIXTURES

Extrusion of powder materials is usually performed from ingots that are pre-compacted at ambient

temperature, and then sintered or compacted at elevated temperatures. It is also possible to put loose

powder into a container; however, this solution requires securing of the die exit to prevent spilling ofpowder. The influence of SiC addition on a degree of compaction of the ingot at an ambient temperature (T

= 25C), and at elevated temperature (T = 380C) was studied. Powder mixtures were prepared (Figure 1),

based on aluminum powder reinforced with the addition of two, four, eight, and 16 percent SiC. Aluminum(Al) powder chemical composition is given in Table 1.

Figure 1. Aluminum-based powder mixture with 2 percent SiC.

Table 1. The chemical composition of Aluminum (Al) powder.

Material Content [%]

Al Min 99,7

Fe Max 0,20

Si Max 0,15

Cu Max 0,01

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The compaction process was carried out on a vertical press with a pressure of 400kN with a moving

punch. The prepared powder mixture was carefully weighed on a laboratory scale before putting into thecontainer. All compaction tests were performed for a fixed value unit pressure of 250MPa. When the load

reached requested value, the press was stopped and the pressure was maintained at this level for 20

seconds. Three samples for each of the variants of SiC addition and temperature were prepared. The densitywas investigated on an analytical laboratory scale equipped with tooling that enables density measurement.

Measurements were made based on the Archimedes law implemented into scales software. The results of

measurement of density and degree of compaction for ambient temperature densification shown in Table 2

are average values.

Table 2. Summary of results of measurement of density and degree of compaction for densification at

temperature T = 25C.

SampleUnit pressure

[MPa]

Density

[g/cm3]

Temperature

[C]

Degree of

compaction

[%]

Al powder+ 2% SiC 250 2,614 25 91,65

Al powder + 4% SiC 250 2,614 25 90,13

Al powder + 8% SiC 250 2,487 25 88,75

Al powder + 16% SiC 250 2,485 25 82,54

Preparing extrusion ingots by compacting metal powders at elevated temperatures is a relatively rarely

used process. Table 3 shows average results of measuring the density and degree of compaction for

densification at temperature T = 380C.

Table 3. Summary of results of measurement of density and degree of compaction for densification attemperature T = 380C.

SampleUnit pressure

[MPa]

Density

[g/cm3]

Temperature

[C]

Degree of

compaction

[%]Al powder+ 2% SiC 250 2,614 380 96,10

Al powder + 4% SiC 250 2,613 380 96,07

Al powder + 8% SiC 250 2,487 380 91,43

Al powder + 16% SiC 250 2,484 380 91,32

The results of compaction tests carried out at ambient (Table 2) and at elevated temperatures (Table 3)

indicate a decrease in degree of condensation with increasing SiC addition content (Figure 2). Method ofcompaction at elevated temperatures significantly improves the consistency of compacts, and allows

increasing the degree of compaction at the unit pressure value equal to one used for ambient temperature

compaction.

Consistency and surface quality of the obtained compacts were evaluated. Compacts made of powdermixtures based on aluminum with the addition of two, four, and eight percent SiC, compacted at ambient

and elevated temperatures maintained good consistency and good surface quality. Compacts with 16

percent SiC additive compacted at elevated temperature showed poor consistency, and those compacted atambient temperature were very brittle.

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Figure 2. The degree of compaction, depending on the amount of SiC additive.

EXTRUSION OF POWDER MIXTURES

Tests of the direct extrusion process were made with the use of ingots prepared from powder mixtures,

based on aluminum powder with the addition of two, four, eight, and 16 percent SiC by weight. Those

ingots were compacted at ambient temperature T = 25C, and at elevated temperature T = 380C, and with

unit pressure of 250MPa. It was carried out on a vertical press with pressure of 3MN. A special tool kit

consisting of a container, punch, transfer mold, and die was designed and manufactured for the needs of

those tests.

Figure 3. Tool kit for extrusion of powder mixtures.

In the first stage of the study, extrusion of rod tests were performed with the use of a porthole die to

analyze the effect of SiC content on the effectiveness of welding. The prepared ingots were heated up to

500C in a resistance furnace. Then, 10mm- and 12mm-diameter rods were extruded at temperature T =

500C, with punch velocity of 55mm/min and 150mm/min. Regardless of the method of ingot preparation

in the first phase of the process, the ingot is split into two streams and then compacted and welded in the

T = 380C

T = 25C

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welding chamber. The extrusion process was stopped after the transfer mold reached the mandrel;

afterwards, the set of tooling with the rod was pulled out of the container (Figure 4).

Figure 4. Tooling set after extrusion process: transfer mold, mandrel, and die with extruded rod.

Because of the difference in the degree of compaction between ingots compacted at temperature T =

25C and those compacted at temperature T = 380C, the force required to split the ingot on the back of the

mandrel is lower for the ingots with a lower degree of compaction, and because of that the force of the

extrusion process is lower. Good seam quality was acquired for rod extruded using Al powder with two,

four, and eight percent of SiC additive. Quality of seams did not depend on the method of ingot

preparation, extrusion ratio, or extrusion velocity (Figure 5). The increase in content of SiC up to 16wt%

led to not welding streams of material in the welding chamber, and separation of the rod on the press exit.

Figures 5a), 5b), and 5c). Rod extruded form powder mixtures with the addition of a) two percent SiC; b)

four percent SiC; and c) eight percent SiC.

The cross-section of the welding chamber presented in Figure 6 shows that the degree of compaction

can be different in different places of the welding chamber. The highest degree of compaction is in the area

of the die land, and that is the place where two streams of material are welded.

Figure 6. The welding chamber cross-section.

In the extrusion process the ingot, after splitting on the back of the mandrel, fills the inlet ports. The

path of flowing of the material is shown in Figure 7. Differences in the flow material are caused by

differences in friction between the container and powder mixture particles (Figure 8). The particles,

a) b) c)

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because of their contacts with the container, are being slowed down and are moving much slower than in

other areas. The problem of friction appears also in the lower part of the mandrel, where the forms of

unmoving powder are created.

Figure 7. Macrostructure of material flow in cross-section of the stream of the powder mixture.

Metallographic analysis of the welding chamber cross-section (Figure 8) shows the areas of

compaction and welding. The stream of the powder mixture that flows into the welding chamber

completely fills the space under the mandrel (Figures 7 and 8). The nature of the flowing of streams in the

welding chamber, as well as their depth, indicates that the area of compaction and welding is large.

Figure 8. Metallographic specimen of the welding chamber cross-section.

Weld quality assessment was based on analysis of the macrostructure of the extruded rods cross

sections (Figure 9). Regardless of the amount of reinforcing additive, the nature of the flowing of powdermixtures is connected with placement of the welding zone in the middle of the samples. The visible

difference in flowing of material streams is caused by various conditions in the inlet ports, which are

caused by the difference in particle size of Al powder, and SiC powder distribution.

Compactibility and weldability was also evaluated in the static tensile test, in which the value of the

tensile strength Rm was assessed (Figure 10). The samples for the stretching test were prepared from theextruded rods containing the reinforcing addition of two, four, and eight percent SiC. The tensile test results

confirmed the effectiveness of compaction and welding, and demonstrated an increase in strength for barswith a higher content of SiC additive.

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Figure 9a), 9b), and 9c). Macrostructure of the flow of the material, a) two percent SiC; b) four percent

SiC; and c) eight percent SiC.

Figure 10. Tensile strength of extruded bars, depending on the amount of SiC additive.

Based on experience gathered during porthole die rod extrusion tests, aluminum powder ingots

reinforced with two, four, and eight wt% of silicon carbide were prepared and were used for tube extrusion.

A sealed container was filled with powder mixture, filling both welding chamber and inlet channels. Then,the prepared container was heated in resistance to a temperature of 500C. Tubes with the outer diameter of

10mm and a wall thickness of 1.5mm were extruded. In the first stage of the extrusion process, powder was

densified until the moment when it broke through the aluminum plate, which was sealing the die exit. The

flow of material in the extrusion process was preceded by pouring out of some powder from the weldingzone. A good product was created with the use of aluminum powder reinforced with two percent SiC

(Figure 11a). The same situation appeared when four percent SiC aluminum powder was used (Figure 11a),

while extrusion of tube made of eight percent SiC aluminum powder ended with tube not being welded in

the welding chamber during the process (Figure 11c).

a) b) c)

a) b) c)

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Figures 11a), 11b), and 11c). Tubes extruded from aluminum powder mixture with a) two percent SiC; b)

four percent SiC; and c) eight percent SiC addition.

During the extrusion process temperature was measured as well as extrusion force, as a function ofpunch displacement. The values of maximum force are given in Table 4. The increase in SiC content

increases the maximum force in the extrusion process.

Table 4. Maximum force values during the extrusion process.

Sample Fmax [kN]

Al powder + 2% SiC 760



In order to assess the impact of reinforcing ingredient content on the weld quality and mechanical

properties of samples, tensile tests were carried out, and tensile strength of samples was evaluated. Tube

made of pure Al powder was extruded to emphasize influence of reinforcing additive on material properties

(Figure 12). Pure Al sample was marked as the zero percent SiC sample. Tubes reinforced with the SiCingredient have higher tensile strength then tube made of pure Al powder. The relationship between the

amount of reinforcing ingredient and the tensile strength is shown in Figure 12.

Figure 12. Tensile strength of extruded tubes, depending on the amount of SiC additive.

Based on the analysis of the extruded tubes cross-section microstructure (Figures 13 and 14), the

quality of welds was evaluated. Two welds are visible, but their amount depends on the construction of the

porthole die. Tubes made of aluminum powder, reinforced with two percent SiC (Figure 13) have weldsthat ensure a good connection between two streams of material.

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Figure 13. Cross-section macrostructure of extruded tube of Al powder with two percent SiC additive.

When the content of SiC additive is increased up to four percent (Figure 14), mechanical properties of

extruded profiles increase, but the weld area becomes more visible. However, the weld quality is

satisfactory and provides a good connection.

Figure 14. Cross-section macrostructure of extruded tube of Al powder with two percent SiC additive.

A further increase in the quantity of the reinforcing ingredient increases tensile strength, but makes

welds weaker and less susceptible to welding, which leads to cracking and splitting of the material in

welding zones. Such a situation can be observed in the case of the sample with eight percent SiC content.

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CONCLUSIONS

Studies on the influence of the amount of reinforcing ingredient (two percent, four percent, eight

percent, and 16 percent SiC) on the degree of compaction showed that the increase in the reinforcing

ingredient decreases the degree of compaction. In the case of samples with 16 percent SiC, the degree ofcompaction in ambient as well as in elevated temperature was lowest, which resulted in low consistency of

samples. Compaction at elevated temperature T = 380C, and not changing any other parameters leads to a

higher degree of compaction than in compaction at ambient temperature T = 25C. The method ofcompaction at elevated temperatures can increase the degree of compaction, even when unit pressure is the

same as in the ambient temperature compaction method, which improves the consistency of extrudates.

Tests of direct extrusion of bars through a porthole die made from ingots prepared from powder

mixtures, based on aluminum powder with the addition of two, four, eight, and 16 percent SiC, compacted

at temperature T = 25C and at elevated temperature T = 380C at unit pressure 250MPa showed that the

force required to separate the ingot on the ridge of the mandrel is lower for ingots with lower degree ofcompaction. The extrusion of ingots with two, four, and eight percent SiC content allowed the obtaining of

bars with a good weld quality for all investigated variants of densification temperature, extrusion ratio, and

extrusion speed. The good quality of welds was confirmed in cross-section macrostructure analysis. Thestreams of the powder mixture incoming into the welding chamber fill the area under the mandrel

completely, and compaction and welding take place over a large area. The degree of compaction is highest

in the area of the dies, and here comes to the welding of streams of compacted powder mixture. Tensile

tests showed an increase in tensile strength for bars with a higher content of SiC addition. Only during the

extrusion process of ingots with a higher content of silicon carbide (16 percent SiC), the streams of materialhave not been welded, and separation of the rod was observed on their exit from the press.

In the extrusion process of tubes made of aluminum powder reinforced with two and four percent

weight of silicon carbide, a product with good weld quality was obtained. But when the addition of SiC has

risen up to eight percent weight, the weldability decreased, and material was not welded in the welding

chamber, and separation of tube was observed. Macrostructure analysis of the tube cross-section confirmedgood quality of welds for tube with two and four percent SiC content. Tubes with the reinforcing ingredient

have higher tensile strength then tubes made of pure Al powder. The four percent of SiC content increases

the tensile strength of tubes by more than 20 percent, compared to tubes made of pure aluminum powder.

REFERENCES

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6. Galanty M., "Wyciskanie na zimno metalicznych materiaw porowatych," Kompozyty (Composites),

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Hot Extrusion of Al SiC Powder