WO2007027629A1

WO2007027629A1 - Method of press quenching aluminum alloy 6020

Info

Publication number: WO2007027629A1
Application number: PCT/US2006/033561
Authority: WO
Inventors: David A. Lukasak; Thomas J. Klemp
Original assignee: Alcoa Inc.
Priority date: 2005-09-02
Filing date: 2006-08-30
Publication date: 2007-03-08
Also published as: US7422645B2; CN101278067B; EP1926838A1; BRPI0616129A2; JP2009507133A; US20070051443A1; CN101278067A

Abstract

A method of press quenching a 6020 aluminum alloy comprising the steps of providing an ingot or billet of 6020 aluminum alloy consisting essentially of about 0.5 to about 0.6 % silicon, about 0.7 to about 0.8 % magnesium, about 0.55 to about 0.65 % copper, about 0.35 to about 0.45 % iron, about 0.01 to about 0.04 % manganese, about 1.05 to about 1.15 % tin, and about 0.04 to about 0.06 % chromium; homogenizing the billet, cooling the billet, reheating the billet, extruding the billet, quenching the extrusion, and artificially aging the extrusion. The alloy has enhanced productivity, strength, and machinability and can be used as a direct replacement for lead containing alloy 6262 T-6.

Description

METHOD OF PRESS QUENCHING ALUMINUM ALLOY 6020

Field of the Invention

[0001] The invention relates to a method of press quenching a 6XXX series

aluminum alloy, preferably aluminum alloy 6020. This press-quenched aluminum alloy

can be used as a direct replacement for lead containing alloy 6262-T6, thereby addressing

any environmental issues that may be raised.

Background of the Invention

[0002] Aluminum alloy 6020 was developed in 1992 for cold finished product

possessing good machinability. Cold finished products include wire, rod, and bar

applications that have been used in the automotive and commercial industries.

Machinability can be defined as the relative ease with which the material can be

machined. Machining processes include such processes as roughing, finishing, and

milling. Good machinability is difficult to measure, however one ranking system that has

been used for some time classifies machinability based on a letter scale with an "A" rating

being most machinable, followed by "B", "C", "D" and "E" ratings taking into account the

following characteristics:

(1) Chip Size. Smaller chip sizes are more desired because such chips

simplify the machining operation and facilitate more effective heat removal from the tool

work piece interface than larger chips. Chips must not be too small or they interfere with

lubricant recirculation during the overall machining operation, such as by drilling or

cutting. Long, thin chips by contrast tend to curl around themselves rather than break. Such chips, sometimes called curlings, may require manual removal from the machining

area and are less effective than smaller chips at heat dissipation because larger chips tend

to block the cooling lubricant.

(2) Tool Wear. Lower tool wear rates are desired to save money by

increasing the amount of time a tool can be used before prescribed tolerances for a given

work piece are exceeded. Lower tool wear rates further increase productivity by reducing

downtime due to tool changeovers.

(3) Surface Finish. Alloys exhibiting a very smooth exterior surface finish

in the as-machined condition are more desired to eliminate or reduce the need for

subsequent surface finishing operations, such as grinding and deburring.

(4) Machining Forces. Lower machining forces are more desired to: reduce

power requirements and the amount of frictional heat generated in the work piece, tool

and tool head; or increase the amount of machining or metal removal that can be

accomplished with the same power requirements; and

(5) Mechanical and Corrosion Properties. Mechanical characteristics such

as strength, or other properties such as corrosion resistance, may be "optional" with

respect to machinability. They can also be rather important depending on the intended end

use for the work piece being machined.

Although this "A" through "E" rating system is based on the five parameters discussed

above, the relative importance of each parameter changes as a function of intended end

use for any given alloy. [0003] The desire to get lead out of the alloy for environmental reasons drove the

development of aluminum alloy 6020. It was desired to extend this alloy to a press

quenched product to also address environmental issues related to press quench aluminum

alloy 6262-T6. A press quenched product is one that has been rapidly cooled from an

elevated deformation extrusion temperature by immersion in a liquid bath, such as oil or

water, so as to withdraw heat rapidly from the product. Air can also be used as a

substitute for liquid. The purpose of quenching is to suppress a phase transformation so

as to obtain increased hardness, or other desirable properties. The severity of the quench

depends upon the capacity of the liquid or air to withdraw heat rapidly from the metal,

this in turn depending upon other factors, such as the latent heat of vaporization, thermal

conductivity, specific heat, and viscosity of the liquid or air.

[0004] Attempts to extend 6020 to a press quenched product were met with several

problems. One problem was that magnesium (Mg) combined with tin (Sn) during billet

reheat, which resulted in low strength, such as tensile strength, and poor machinability.

Tensile strength is the resistance of a product to a force tending to tear it apart, measured

as the maximum tension the product can withstand without tearing. When an aluminum

alloy product, such as a billet or ingot, is extruded, it is first reheated to and held at a

temperature in the alloy above the solubility temperature in the precipitated phases in the

aluminum matrix, for instance the solubility temperature for the magnesium (Mg)-silicon

(Si) phases in a billet made of an Al-Mg-S i-alloy, until the phases are dissolved. The

product is then quickly cooled or quenched to the desired extrusion temperature to prevent new precipitation of these phases in the alloy structure. Between the

temperatures of 800° F and 920° F₅ magnesium combines with tin at a rapid rate to form

magnesium tin. Above 920° F, the magnesium and tin do not combine and will actually

dissociate from each other. Below 800° F, the reaction is sluggish and there is typically

not enough time during billet reheat for these two elements to substantially combine. The

product forms for which a press quenched 6020 alloy is desired are rod, bar, and wire

applications. For press quench products of this nature, billet temperatures of 825 to 900°

F are typically utilized. As described above, this temperature range will not allow alloy

6020 to achieve acceptable machinability in the press quenched product.

[0005] In addition, other problems encountered were that there was a low

producability compared to 6262 when overcoming this magnesium-tin combination issue

and there was a lack of an optimized composition within the sales limits. In extrusion, the

higher the billet temperature, the slower the extrusion speed that can be attained. As

described, previously the preferred billet temperature range for alloys such as 6262 is 800

to 920° F. As aforementioned, these temperatures resulted in unacceptable machinability

for the 6020 alloy. Going to higher billet temperatures resulted in a significant loss of

extrusion productivity. Additionally, the composition was not optimized for press

quenched products. It was discovered that higher magnesium levels resulted in a greater

degradation to machinability. The higher Mg levels provide a higher driving force to

promote the formation of Mg2Sn below approximately 920 ° F. To counter this effect,

the magnesium level is optimized towards the lower side of the sales limits. Additionally, the tin level was maximized to maintain a higher volume fraction of the desirable Sn

phase that provides the favorable machining characteristics of 6020. However, with

lower magnesium levels, the strength in the final product is compromised. To offset this

Si levels are optimized towards the higher side of sales limits.

[0006] The primary object of the present invention is to provide a substantially lead

free alloy that is press quenchable.

[0007] Another object of the present invention is to provide a press quench alloy

with enhanced extrusion productivity and good mechanical properties and machinability.

[0008] A further object of the invention is to provide a press quench alloy that can

be used as a direct replacement for lead containing alloy 6262-T6.

Summary of the Invention

[0009] The present invention relates to a method of making a press-quenched 6020

aluminum alloy product. The method comprises the steps of: (a) providing an ingot or

billet of a 6020 aluminum alloy consisting essentially of about 0.5 to about 0.6 % silicon,

about 0.7 to about 0.8 % magnesium, about 0.55 to about 0.65 % copper, about 0.35 to

about 0.45 % iron, about 0.01 to about 0.04 % manganese, about 1.05 to about 1.15 % tin,

about 0.04 to about 0.06 % chromium, not more than 0.034 % lead, the balance being

essentially aluminum and incidental elements and impurities; (b) homogenizing the billet

to a temperature of preferably 1025° F to 1050° F for a four hour period; (c) cooling the

homogenized billet at a cooling rate of about 400° F for about an hour; (d) reheating the billet to a temperature preferably from about 775° F to about 800° F for preferably less

than about five minutes; (e) extruding the billet at a speed in the range of preferably about

150 fpm to about 175 fpm and to an exit temperature of preferably 1000 to 1015° F; (f)

quenching the extrusion to an exit temperature of about 200° F to about 350° F; (g)

stretching the extrusion at least about 1%; and (h) artificially aging the extrusion to a

temperature in the range of 340° F to 355° F for a time period of about 8 hours.

[0010] Following the above method produces a press quenched 6020 aluminum

alloy that is preferably suited for rod, bar, and wire applications. The alloy has enhanced

productivity, strength, and machinability and can be used as a direct replacement for lead

containing alloy 6262 T-6.

Brief Description of the Drawings

[0011] Figure 1 shows the influence of billet reheat time and temperature on

ultimate tensile strength.

[0012] Figure 2 shows the influence of billet reheat time and temperature on tensile

yield strength.

[0013] Figure 3 shows the effect of billet reheat temperature and time on

machinability.

[0014] Figure 4 shows the DSC peak area for the Sn phase versus machinability.

[0015] Figure 5 shows the average yield strength as a function of extrusion speed

and location. [0016] Figure 6 shows a set of curves for exit temperature as a function of billet

location and extrusion speed.

Detailed Description of Preferred Embodiments

[0017] The press quench 6020 alloy of the present invention contains silicon,

magnesium, copper, iron, manganese, chromium, and tin. The silicon content ranges

preferably from about 0.5 % to about 0.6 %, all percentages herein being by weight.

Magnesium is preferably present in amounts of about 0.7 % to about 0.8 %. It is believed

that maintaining the magnesium in this range yields a billet with improved machinability.

In addition to the respective percentages for silicon and magnesium, it is preferred in

practicing the invention that silicon be present in excess over that amount theoretically

consumed as Mg₂Si. However, it is also important that the extent of the excess be

relatively slight. This is largely affected by controlling the amount of magnesium to

exceed the amount of silicon by about 0.1 % to about 0.3 %, although at the highest

magnesium (Mg)-lowest silicon (Si) corner of the composition window a slight excess of

magnesium is tolerated. The significance of this relationship is providing for high yield

and tensile strengths. Limiting the silicon excess to a small excess provides for

combining such strength with improved toughness and impact resistance. Copper is

present preferably from about 0.55 % to about 0.65 %. Iron is present in a preferable

range of about 0.35 % to about 0.45 %. The amount of manganese ranges from about

0.01% to about 0.04 %, with the preferable amount being about 0.02 %. Tin is present at

a range of about 1.05 % to about 1.15 % with the preferable amount being about 1.10%. Chromium is present at a preferable range of about 0.04 % to about 0.06 %. Running

near zero levels for chromium and manganese is believed to be the most desirable for

getting a fine grain size.

[0018] In practicing the invention, it is important that the billets be subjected to a

very high preheat or homogenizing temperature of about 1020° F to about 1070° F,

preferably about 1025° F to about 1050° F for about a four hour period. The billet is

preheated by any method used to heat the billet, but for the purposes of this invention an

electric furnace was used. At this range of temperature, the potential for coarsening of

the tin (Sn) phase is minimized. Coarsening is the growth of the Sn phase to an

undesirable size that results in a distribution (particles per unit volume) that can

negatively influence machinability. Minimizing the coarsening of the tin phase results in

the extrusions having higher tensile properties, such as tensile strength (TS), tensile yield

strength (TYS) and ultimate tensile strength (UTS), and a more desirable machining

performance. For the purposes of this invention, tensile strength, as previously

mentioned, can be defined as the maximum amount of stress that a material can be

subjected to before it will tears. In addition, tensile yield strength can be defined as the

point where deformation of the material is unrecovered, and the work produced by

external forces, such as stress, is not stored as elastic energy but will lead to contraction,

cracks, and ultimately failure of the construction, and ultimate tensile strength is the limit

stress at which the material actually tears. [0019] The billet is then cooled at a cooling rate of about 400° F for about an hour.

Cooling is achieved by placing the homogenized load of ingots in a specially designed

cooling chamber that forces air or other cooling media through the billet to achieve the

cooling rate. This cooling rate minimizes the formation of magnesium tin (Mg2Sn),

which can negatively impact machinability. Thereafter, the billet is reheated to a

temperature in the range of from about 600° F to about 900° F, preferably from about

775° F to about 800° F. The billet is reheated for less than about thirty minutes,

preferably for less than about five minutes. Any method could be used to reheat the

billet, but for the purposes of this invention the billet was reheated via the use of both gas

and electric furnaces. Figures 1-4 show that reheating the billet at this preferred

temperature and for this amount of time yields the highest strength and best

machinability. Figures 1 and 2 show the influence of billet temperature and time on

ultimate tensile strength and tensile yield strength. From these figures, it is apparent that

longer hold times result in a lowering of strength. Additionally, 850° F results in lower

strength than either 800° F or 900° F reheat temperatures. For purposes that will be

described later, reheating the billet to a temperature of 800° F or below increases the

chances of obtaining the preferable billet exit temperature of 950 to 975° F from

extrusion. The ultimate tensile strength is preferably at least about 41 kilopounds per

square inch (ksi) and the tensile yield strength is preferably at least about 35 ksi.

[0020] In addition to the tensile properties, machinability was evaluated for the

extrusions. Figure 3 shows the effect of billet reheat temperature and time on machinability. From this graph, it is observed that the longer hold times and the 850° F

reheat temperature are detrimental to machinability. Overall, the 800° F billet reheat for

hold times less than about 5 minutes yielded the best machinability. In order to

understand what was being affected in the microstructure by the various billet reheat

conditions, differential scanning calorimetry (DSC) was performed. DSC is a precise

measure of energy consumption or release per unit mass during the heating or cooling of a

material. Phase transformations, such as the aforementioned Sn to Mg₂Sn, can be

detected with this technique and the amount of energy change is a function of the volume

fraction of the phase present. Figure 4 shows DSC peak area for the Tin (Sn) phase

versus machinabilty results. Here it can be observed that the larger peak area, which

occurs when the billet is reheated at about 800° F for less than about 5 minutes, results in

improved machinability. However, the difference in peak area between a C+ rating and

an A rating is small, again suggesting that the microstructural difference is subtle.

[0021] Prior to extruding, the billet is placed in a container with the container

having a temperature of about 750° F. For the purposes of this invention, an extrusion

press container was used. The billet is then extruded via direct or indirect extrusion.

Direct extrusion is a process in which a die is held stationary and a moving arm or ram

forces the billet through it. Indirect extrusion is a process in which the billet remains

stationary while the die moves against the billet creating pressure needed for metal to

flow through the die. For purposes of this invention, direct extrusion is preferred. The

die can be any type of die used to extrude an alloy. For the purposes of this invention, a single hole flat faced die was used. A higher extrusion ratio is realized with the single

hole die because it has a better opportunity to "break-up" and redistribute the coarsened

tin phase from the billet. Extrusion ratio is the ratio of billet cross section area to the

extrusion cross section. Using a flat-faced or shallow pocket die prevents significant heat-

up and avoids compromising speed. Flat face dies and shallow pocket dies do not have a

weld pocket that allows for the welding together of two extrusions as metal flows through

the die opening. This results in less work and less heat build-up as the metal flows

through the die opening. The extrusions are run at speeds which achieve exit temperatures

of 950° F to 1015° F, preferably 1000 to 1015° F. Based on the tin (Sn) to magnesium tin

(Mg₂Sn) transformation starting at around 930° F, it is preferable that the exit temperature

be above 950° F. However, temperatures around 1000° F are even more desirable from

the standpoint of reverting any of the transformation of Sn to Mg₂Sn that has taken place

either during the cooling from ingot homogenization or during the billet reheat.

[0022] The speeds are measured in feet per minute (fpm) and range from about 150

fpm to about 175 fpm with a preferable speed of about 175 fpm. Figure 5 plots the yield

strength as a function of extrusion speed and location. This demonstrates that the

properties increase from front to rear. Since exit temperature increases from front to rear

for a given extrusion speed and set of temperature conditions, the low front-end

properties are a result of low extrusion exit temperatures. The graph in figure 6 shows a

predictive set of curves for exit temperature as a function of billet location and extrusion

speed. Product speed varied from 100 fpm to 200 fpm by 25 fpm increments. Based on this plot, speeds of about 150 fpm to about 175 fpm would generate marginal properties

on the front end of the extrusion due to the front end exit temperatures being above the

preferable 950° F temperature. For reasons previously discussed, exit temperatures above

950° F and preferably around 1000° F are desired to achieve maximum properties.

[0023] Once the billet has been extruded, the extrusion is then quenched. For the

purposes of this invention, the extrusion was quenched by use of a standing wave water

quench. A standing wave is a wall of water several feet in length and a height sufficient

to completely immerse the extrusion. Pumps and piping are used to create the wave and

to provide a continuous replenishment of cool water. However, any method of quenching

the extrusion, such as air quenching, could be used. The speed at which the extrusion is

quenched can be at speeds of up to about 200 fpm, but a speed of around about 150 fpm

is preferred. Upon exiting quench, the extrusion is preferably at a temperature of below

about 400° F. It is necessary to get below about 400 ⁰F in order to achieve the required

strength levels. After the extrusion is quenched, it is then stretched by at least about 1%.

For the purposes of this invention, an extrusion stretcher was used. However, other

means could be used to stretch the extrusion. Stretching the extrusion by this percentage

increases the producability of the extrusion. Finally, the extrusion is artificially aged,

preferably from between about 340 ⁰F to about 355 ⁰F for about 8 hours. Artificially

aging is typically performed in, but not restricted to, a batch age oven. The extrusions are

heated in the batch oven to the temperatures listed above. This process is the final processing step that is required to achieve the required strength. This process is

dependent on all prior processing steps being performed correctly.

[0024] Following the method outlined above will produce a fine-grained, fully

recrystallized, press quenched product that demonstrates good strength and elongation. It

is clearly capable of meeting the 6262-T6 property minimums with good press

productivity.

[0025] Having described the presently preferred embodiments, it is to be

understood that the invention may be otherwise embodied within the scope of the

appended claims.

Claims

What is claimed is:

1. A method of extruding a press quenched 6020 aluminum alloy

product comprising the steps of:

(a) providing an ingot or billet of a 6020 aluminum alloy;

(b) homogenizing said billet;

(c) cooling said homogenized billet;

(d) reheating said billet;

(e) extruding said billet;

(f) quenching said extrusion;

(g) stretching said extrusion; and

(h) artificially aging said extrusion.

2. The method according to claim 1 wherein said alloy is of a composition

consisting essentially of about 0.5 to about 0.6 % silicon, about 0.7 to about 0.8 %

magnesium, about 0.55 to about 0.65 % copper, about 0.35 to about 0.45 % iron, about

0.01 to about 0.04 % manganese, about 1.05 to about 1.15 % tin, about 0.04 to about 0.06

% chromium, not more than about 0.034 % lead, the balance being essentially aluminum

and incidental elements and impurities.

3. The method according to claim 1 wherein said reheating is to a temperature

in the range of from about 750° F to about 800° F.

4. The method according to claim 3 wherein said temperature is in the range

of from about 775° F to about 800° F.

5. The method according to claim 1 wherein said billet is heated to a time of

less than about thirty minutes.

6. The method according to claim 5 wherein said time is of less than about

five minutes.

7. The method according to claim 1 wherein said billet is placed in a container

prior to extruding, said container having a temperature of about 750° F.

8. The method according to claim 1 wherein said extruding occurs at a speed

in the range of about 150 fpm to about 175 fpm.

9. The method according to claim 8 wherein said speed is about 175 fpm.

10. The method according to claim 1 wherein said extrusion is at a temperature

range of about 950° F to about 1000° F after said extruding.

11. The method according to claim 10 wherein said temperature is above about

1000⁰ F.

12. The method according to claim 1 wherein said temperature of said extrusion

is in a range comprising about 200° F to about 350° F after said quenching.

13. The method according to claim 1 wherein said quenching comprises air or

water quench.

14. The method according to claim 1 wherein said extrusion is stretched about

1%.

15. The method according to claim 1 wherein said artificial aging occurs at a

temperature in the range of about 340° F to about 355° F for a time period of about 8

hours.

16. The method of claim 1 wherein said extrusion comprises a bar, rod, or wire.

17. The method of claim 1 wherein said extrusion has an ultimate tensile

strength of at least about 41 ksi.

18. The method of claim 1 wherein said extrusion has a tensile yield strength of

at least about 35 ksi.

19. The method of claim 1 wherein said extrusion has an ultimate tensile

strength of at least about 41 ksi and a tensile yield strength of at least about 35 ksi.