U.S. patent application number 10/502659 was filed with the patent office on 2005-02-10 for al-si-mg alloy sheet metal for motor car body outer panel.
Invention is credited to Hoffmann, Jean-Luc, Rebuffet, Olivier, Shanani, Ravi.
Application Number | 20050028894 10/502659 |
Document ID | / |
Family ID | 27619894 |
Filed Date | 2005-02-10 |
United States Patent
Application |
20050028894 |
Kind Code |
A1 |
Hoffmann, Jean-Luc ; et
al. |
February 10, 2005 |
Al-si-mg alloy sheet metal for motor car body outer panel
Abstract
A metal sheet for a motor vehicle body outer panel, having a
thickness ranging between 0.8 and 1.2 mm, containing, in wt. %, Fe
0.25-0.40 and preferably 0.25-0.35; Si 0.90-1.20 and preferably
0.95-1.10; Cu 0.10-0.25 and preferably 0.15-0.20; Mg 0.35-0.50 and
preferably 0.40-0.50; Mn 0.05-0.20 and preferably 0.08-0.15; other
elements<0.05 each and <0.15 in total, the rest being
aluminum. The sheet has, after solution heat treatment, quenching,
pre-tempering or reversion, and maturation at room temperature for
3 weeks to 6 months, an L R.sub.0.2 direction yield strength less
than 160 MPa, and preferably less than 150 MPa. A yield
strength>180 MPa can be obtained on the body stamping part after
the paint has been cured. The sheet of the invention enables a
reduction in the thickness of parts while satisfying all the other
required properties.
Inventors: |
Hoffmann, Jean-Luc;
(Limoges, FR) ; Shanani, Ravi; (Ando sheim,
FR) ; Rebuffet, Olivier; (Grenoble, FR) |
Correspondence
Address: |
DENNISON, SCHULTZ, DOUGHERTY & MACDONALD
1727 KING STREET
SUITE 105
ALEXANDRIA
VA
22314
US
|
Family ID: |
27619894 |
Appl. No.: |
10/502659 |
Filed: |
August 18, 2004 |
PCT Filed: |
February 3, 2003 |
PCT NO: |
PCT/FR03/00318 |
Current U.S.
Class: |
148/417 |
Current CPC
Class: |
C22C 21/02 20130101;
C22F 1/043 20130101 |
Class at
Publication: |
148/417 |
International
Class: |
C22C 021/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 5, 2002 |
FR |
02 01346 |
Claims
1. Sheet for an automobile body outer panel part between 0.8 and
1.2 mm thick, havinq a composition consisting essentially of (% by
weight):
9 Fe: 0.25-0.40 and preferably: 0.25-0.35 Si: 0.90-1.20 and
preferably: 0.95-1.10 Cu: 0.10-0.25 and preferably: 0.15-0.20 Mg:
0.35-0.50 and preferably: 0.40-0.50 Mn: 0.05-0.20 and preferably:
0.08-0.15
other elements<0.05 each and <0.15 total, the remainder being
aluminum, presenting a yield strength R.sub.0.2 in the L direction
less than 160 MPa after solution heat treatment, quenching,
pre-ageing or reversion and ageing at ambient temperature for
between 3 weeks and 6 months.
2. Sheet according to claim 1, presenting a yield strength
R.sub.0.2 in the L direction less than 150 MPa after solution heat
treatment, quenching, pre-ageing or reversion and ageing at ambient
temperature for between 3 weeks and 6 months.
3. Sheet according to claim 1, characterised in that solution heat
treatment is made such that the peak area is less than 1 J/g in the
565-580.degree. C. range in a differential enthalpy analysis
diagram, the test being carried out at a temperature rise rate of
20.degree. C./min.
4. Sheet according to claim 1, characterised in that pre-ageing is
made at a temperature and time such that the equivalent time teq
defined by the formula: 2 T ( eq ) = exp ( - 6000 / T ) t exp ( -
6000 / T ref ) t where T (in .degree.K) is the temperature and t is
the preageing time, and T.sub.ref=373.degree. K, is between 1 and
10 h.
5. Sheet according to claim 4, characterised in that t.sub.eq is
between 3 h and 6 h.
6. Sheet according to claim 1, with a formability anisotropy
LDH.sub.0 between the rolling direction and the perpendicular
direction less than 1 mm.
7. Sheet according to claim 6, with a formability anisotropy
LDH.sub.0 between the rolling direction and the perpendicular
direction less than 0.6 mm.
8. Sheet according to claim 1, characterised in that it has a yield
strength measured at 160.degree. C. more than 140 MPa.
9. Sheet according to claim 1, with a grain size smaller than 50
.mu.m.
10. Sheet according to claim 1, with a textured surface.
11. Sheet according to claim 1, coated with a dry lubricant.
12. Body outer panel part made from a sheet according to claim 1,
presenting a yield strength R.sub.0.2 (L or TL direction) in the
solution heat treated temper and quenched, naturally aged, drawn
and artificially aged by paint baking, greater than 180 MPa.
13. Body outer panel part according to claim 12, presenting a yield
strength R.sub.0.2 (L or TL direction) in the solution heat treated
temper and quenched, naturally aged and artificially aged by paint
baking, greater than 200 MPa.
14. Body outer panel part made from a sheet according to claim 1,
characterised in that it is assembled on a steel structure before
the paint baking treatment.
Description
DOMAIN OF THE INVENTION
[0001] The invention relates to the domain of Al-Si-Mg alloy
sheets, more particularly made of a 6016 type alloy according to
the Aluminum Association, to be used for manufacturing by drawing
of car body outer panel parts such as wings, doors, tailgates,
bonnets and roofs.
STATE OF THE ART
[0002] Aluminium is increasingly used in automobile construction to
reduce the weight of vehicles and thus reduce fuel consumption and
the release of pollutants and greenhouse gases. Sheets are used
particularly for making body outer panel parts, particularly doors.
This type of application requires a number of properties that are
sometimes contradictory, for example:
[0003] good formability for drawing and hemming operations,
[0004] controlled yield strength in the delivery state of the
sheet, to control springback,
[0005] high mechanical strength after baking the paints to obtain
good dent resistance while minimizing the weight of the part,
[0006] good resistance to corrosion of the painted part,
particularly filiform corrosion,
[0007] good surface quality after shaping and painting,
[0008] good behaviour in miscellaneous assembly processes used in
automobile bodywork such as spot welding, laser welding, gluing,
clinching and riveting,
[0009] compatibility with requirements for recycling of
manufacturing waste or recycled vehicles,
[0010] an acceptable cost for large series production.
[0011] Al-Mg-Si alloys in the 6000 series have been chosen to
satisfy these requirements. In Europe, the 6016 and 6016A alloys
with thicknesses of the order of 1 to 1.2 mm are most frequently
used for this application since they give a better compromise
between the various required properties, particularly enabling
better formability, particularly for hemming, and better resistance
to filiform corrosion than alloys with a higher copper content such
as the 6111 alloy that is widely used in the United States. 6016
type alloys are described particularly in Alusuisse's patent FR
2360684 and the applicant's patent EP 0259232, while 6111 type
alloys are described in Alcan International Limited's patent U.S.
Pat. No. 4,614,552. Alloys with low iron content (<0.2%) like
those described in Alcoa's patents U.S. Pat. No. 5,525,169 and U.S.
Pat. No. 5,919,323 are also known, and an alloy of this type was
registered as 6022. The compositions (% by weight of the main
elements) of the 6016, 6016A, 6022 and 6111 alloys registered at
the Aluminum Association are indicated in table 1:
1TABLE 1 Alloy Si Fe Mg Cu Mn 6016 1.0-1.5 <0.5 0.25-0.6 <0.2
<0.2 6016A 0.9-1.5 <0.5 0.2-0.6 <0.25 <0.2 6022 0.8-1.5
0.05-0.2 0.45-0.7 0.01-0.11 0.02-0.10 6111 0.5-1.0 <0.4 0.5-1.0
0.5-0.9 <0.4
[0012] However, the mechanical strength of the 6016 alloy after the
paint has been baked, and therefore the dent resistance, remains
significantly lower than the corresponding values for the 6111
alloy, particularly since the baking temperature is tending to go
down such that hardening during ageing is less effective. This is
why automobile manufacturers are asking for a higher mechanical
strength after painting.
[0013] In order to achieve this, the applicant has developed new
variants of the 6016 alloy, and particularly a "DR120" variant
leading to a yield strength in the T4 temper of the order of 120
MPa. These developments were covered in publications, particularly
the articles by R. Shahani et al. "Optimised 6xxx aluminium alloy
sheet for autobody outer panels" Automotive Alloys 1999,
Proceedings of the TMS Annual Meeting Symposium, 2000, pp. 193-203,
and D. Daniel et al. "Development of 6xxx Alloy Aluminium Sheet for
Autobody Outer Panels: Bake Hardening, Formability and Trimming
Performance" IBEC'99--International Body Engineering Conference,
Detroit, 1999, SAE Technical Paper No. 1999-01-3195.
[0014] Alcan has proposed a new variant of the 6111 alloy called
6111-T4P, which has an improved yield strength after baking the
paint (typically 270 to 280 MPa) without reducing formability in
the T4 temper. In particular, this product has been described in
the article by A. K. Gupta et al. "The Properties and
Characteristics of Two New Aluminium Automotive Outer Panel
Materials", SAE Technical Paper 960164, 1996. The article also
mentions a new alloy temporarily called 61XX-T4P, for which the
composition has not been divulged, which has a lower yield strength
in the T4 state than the conventional 6111-T4, with a similar
response to paint baking.
[0015] These new developments all include optimised pre-ageing type
heat treatment, carried out after quenching to improve hardening
while baking paint. If this type of treatment is not performed, the
hardening rate while baking reduces as the waiting time at ambient
temperature between quenching and baking increases, and a waiting
time of several weeks is almost inevitable in industrial
production. This phenomenon has been known for a long time and has
been described for example in the article by M. Renouard and R.
Meillat "Le prrevenu des alliages aluminium-magnsium-silicium
(Preliminary annealing of aluminium-magnesium-silicon alloys)"
Mmoires Scientifiques de la Revue de Mtallurgie (Scientific
Memories of the Metallurgy Review), December 1960, pp. 930-942.
[0016] To avoid the unfavourable effect of waiting, it is necessary
to either perform pre-ageing by staged quenching or a heat
treatment immediately after quenching, or to store the metal in a
freezer which is not very convenient for automobile body parts, or
to perform a reversion treatment.
[0017] The pre-ageing temperature and duration for 6000 alloys are
described for example in the article by R. Develay "Traitements
thermiques des alliages d'aluminium (Heat treatments for aluminium
alloys)", Techniques de l'Ingnieur (Engineering techniques),
section M 1290, 1986, in the article by D. W. Pashley et al.
"Delayed ageing in aluminium-magnesium silicon alloys: effect on
structure and mechanical properties", Journal of the Institute of
Metals, No. 94, 1966, pp. 41-49, and in patent EP 0480402
(Surnitomo Light Metal). Patent FR 1243877 (Cegedur) also describes
a continuous furnace that can be used for pre-ageing.
[0018] Considering the increased use of aluminium alloy sheets for
mass-produced automobile body outer panels, there is still a demand
for even more improved grades to reduce thicknesses without
adversely affecting other properties. The reduction in thickness is
usually limited by the lack of stiffness of the formed part, this
limit being equal to the thickness of the equivalent part made of
steel multiplied by 1.4. Therefore, sheets must be capable of
achieving a dent resistance on a formed part after the paint has
been baked equal to at least the resistance for steel parts, for an
aluminium/steel thickness ratio of 1.4, while maintaining good
drawability and hemming ability.
SUBJECT OF THE INVENTION
[0019] The purpose of this invention is to provide 6016 type alloy
sheets for automobile body outer panels with a composition adapted
to recycling, sufficient formability for deep drawing and hemming
under severe conditions, improved resistance to indentation
compared with sheets of the 6016 type according to the prior art
while controlling springback, with good gluing properties, cutting
without the formation of slivers, and good resistance to filiform
corrosion. The subject of the invention is a sheet for an
automobile body outer panel part between 0.8 and 1.2 mm thick, and
the following composition (% by weight):
2 Fe: 0.25-0.40 and preferably: 0.25-0.35 Si: 0.90-1.20 and
preferably: 0.95-1.10 Cu: 0.10-0.25 and preferably: 0.15-0.20 Mg:
0.35-0.50 and preferably: 0.40-0.50 Mn: 0.05-0.20 and preferably:
0.08-0.15
[0020] other elements<0.05 each and <0.15 total, the
remainder being aluminium,
[0021] presenting a yield strength R.sub.0.2 in the L direction
less than 160 MPa and preferably less than 150 MPa after solution
heat treatment, quenching, pre-ageing or reversion and ageing at
ambient temperature for between 3 weeks and 6 months. The yield
strength of the drawn part after a heat treatment corresponding to
baking the paint is greater than 180 MPa and preferably greater
than 200 MPa.
DESCRIPTION OF THE INVENTION
[0022] The invention is based on a narrow composition range within
the definition of the 6016A composition registered at the Aluminium
Association, in order to obtain all the required properties.
[0023] The silicon content is near the bottom part of the content
range of 6016A, while the magnesium content is at the centre of the
range. This drop in the silicon content contributes to more
complete solution heat treatment of the alloy, favourable for
formability. The iron content remains above 0.25% which, unlike low
iron grades such as 6022, enables the use of recycled metal and
results in a better surface appearance after drawing.
[0024] The copper content is controlled within very narrow limits:
a content of at least 0.1%, slightly more than the content for
existing 6016 or 6022 grades, contributes to the mechanical
strength, but above 0.25% there is a risk of filiform corrosion of
the alloy. The alloy must contain at least 0.05% of manganese,
chromium, vanadium or zirconium to control the grain size and
prevent the appearance of orange peel during severe deformations,
for example such as hemming used for bonnets. Conversely, if the
total content of these elements exceeds 0.20%, it is bad for
formability.
[0025] The sheet manufacturing process according to the invention
typically includes casting of a plate, possibly scalping of this
plate and homogenisation by simply heating it to a temperature of
between 400 and 570.degree. C. for between 6 and 24 h. Hot rolling
preferably takes place at an input temperature of more than
510.degree. C., which contributes to making the mechanical strength
better than what would be obtained at a lower input temperature.
The winding temperature of the hot rolled strip must be less than
350.degree. C., and preferably 300.degree. C., to guarantee
mechanical characteristics and to avoid any ridging defects. The
hot rolled strip is then cold rolled down to the final thickness,
possibly with intermediate annealing at a temperature of between
300 and 450.degree. C. if it is done in a batch furnace, or between
350 and 570.degree. C. if is done continuously. The last cold
rolling pass may be made with a textured cylinder, for example by
electron beam treatment (EBT), electro-erosion (EDT), or by laser
beam which improves the formability and surface appearance of the
part formed after painting.
[0026] It is also possible to use strips obtained directly by
continuous casting, either by twin-roll casting or twin-belt
casting, and to perform cold rolling and subsequent operations
under the same conditions.
[0027] The solution heat treatment takes place at a temperature
above the alloy solvus temperature while avoiding overheating. The
composition according to the invention is capable of very complete
solution heat treatment, resulting in an almost complete absence of
silicon type phases in the microstructure and by a very small peak
area, less than 1 J/g in the 565-580.degree. C. range in a
differential enthalpy analysis diagram, the test being carried out
at a temperature rise rate of 20.degree. C./min.
[0028] After the solution heat treatment, the sheet is quenched,
usually with cold water or air. Quenching may be followed
immediately by a pre-ageing type heat treatment like that described
in the prior art mentioned above, in order to improve hardening
performances of the paint during baking.
[0029] Pre-ageing is not necessary isothermal and its duration
depends on the temperature. This can be taken into account by
defining an equivalent time t.sub.eq using the formula: 1 T ( eq )
= exp ( - 6000 / T ) t exp ( - 6000 / T ref )
[0030] where T (in .degree. K) is the temperature and t is the
pre-ageing time, T.sub.ref is a reference temperature of
373.degree. K, namely 100.degree. C. It is known that if pre-ageing
is to be efficient, it must be done at a temperature of more than
50.degree. C. for a time of between 0.3 and 20 h. If the equivalent
time is insufficient, the hardening rate of paint while baking
reduces with the waiting time at ambient temperature. On the other
hand, if the equivalent time is too long, the mechanical
characteristics increase too much during pre-ageing and the
formability of the sheet is degraded. For 6016 type alloys, an
equivalent time of 1 to 10 h, and preferably 3 to 6 h, is quite
suitable.
[0031] The sheet is usually stored for a variable time period at
this stage, which leads to natural ageing that increases the yield
strength with time. After three weeks of ageing, the thickness of
sheets according to the invention is of the order of 0.9 to 1 mm,
the yield strength in the L direction is of the order of 130 MPa
which is higher than all 6016 variants, including high strength
grades DR100 and DR120 described in the article by R. Shahani et
al. mentioned above, and only slightly lower than the value for
6022. After six months ageing, this yield strength remains below
160 MPa or even 150 MPa, unlike 6022 and 6111 alloys. This special
feature enables control over springback during forming which
becomes increasing difficult to control when thicknesses are
reduced and when the yield strength is increased, so that many
iterations are necessary when developing stamping tools. Before
forming, the sheet may be coated with a lubricant (oil or dry
lubricant) adapted to drawing, assembly and surface treatment of
the part to be made.
[0032] Sheets according to the invention have formability as
measured by the LDH.sub.0 ("Limiting Dome Height" in plane
deformation) parameter that is better than the 6111 and 6022 alloys
and as good as high strength 6016 grades.
[0033] The LDH parameter is broadly used for evaluating the
drawability of 0.5 to 2 mm thick sheets. Many publications have
been made, particularly the publication by R. Thomson, "The LDH
test to evaluate sheet metal formability--Final Report of the LDH
Committee of the North American Deep Drawing Research Group", SAE
Conference, Detroit, 1993, SAE Paper No. 930815.
[0034] The LDH test is a drawing test on a blank blocked by a
retaining ring around its periphery. The pressure of the blank
clamp is controlled to prevent sliding in the retaining ring. The
120.times.160 mm blank is strengthed in a mode similar to plane
deformation. Lubrication between the stamp and the sheet is
achieved using a plastic film and grease (Shell HDM2 grease). The
punch lowering speed is 50 mm/min. The LDH value is the
displacement of the punch at failure, namely the drawing limiting
depth. The average of three tests is determined, giving a
confidence range of 95% on a measurement of .+-.0.2 mm.
[0035] Sheets according to the invention have better crimpability
than 6111 or 6022 alloy sheets, and the crimpability is just as
good as high strength 6016 alloy sheets according to the prior art.
This hemming ability is evaluated by a laboratory test including
flanging at 90.degree., pre-hemming at 45.degree. and final flat
hemming.
[0036] Sheets according to the invention also have a very small
deformation anisotropy that can be measured by the difference
between the LDH for a principal deformation parallel to the rolling
direction, and a principal deformation perpendicular to the rolling
direction. This difference is less than 1 mm and preferably less
than 0.6 mm.
[0037] The body outer panel part is usually made by cutting out a
blank in the sheet, drawing this blank and trimming it with the
press. During stamping, it is essential to avoid the occurrence of
roping or ridging, which deteriorates the final paint appearance
and can reduce formability, particularly in the case of severe
deformation in the direction perpendicular to the rolling
direction. Different means have been provided for this purpose, for
example controlling the hot rolling outlet temperature between 270
and 340.degree. C., as indicated in the applicant's patent EP
0259232. It is also important to avoid the occurrence of "orange
peel" stamping which contributes to a visible appearance defect
after painting. This is done preferably by keeping the grain size
smaller than 50 .mu.m, which can be achieved by the presence of a
sufficient quantity of manganese in the alloy, or other elements
playing a similar role such as chromium, vanadium or zirconium, by
temperature control and by the time of the solution heat treatment
and by a sufficient reduction, typically at least 30% by cold
rolling. For some parts such as bonnets, the edges of the drawn
blank are flanged at 90.degree. and a lining stamping is inserted
on which pre-hemming is done followed by final flat hemming.
[0038] It is also necessary to avoid the formation of slivers
during blank cutting and turning operations after drawing, these
slivers possibly causing the appearance of defects requiring manual
touching up. The design of the cutting tool is important in this
respect, and recommendations were made in the article by D. Daniel
et al. mentioned above.
[0039] After stamping and possibly hemming, the part is covered by
one or more coats of paint, with a baking step following each coat.
The critical step is baking of the cataphoresis layer, which is
usually done at a temperature of between 150 and 200.degree. C. for
15 to 30 minutes. The baking temperature rarely exceeds 170.degree.
C. if there is no cataphoresis. Paint baking contributes towards an
ageing treatment of the part. The yield strength of the part made
with a sheet according to the invention, with baking for 20 minutes
at 165.degree. C., is higher than 180 MPa and often higher than 200
MPa. Thus, with a part made from a 0.9 mm thick sheet, the
resistance to dynamic indentation is similar to the value for a
part made from a typical body steel sheet with a yield strength of
the order of 250 to 300 MPa and 0.7 mm thick, which is not the case
for other 6016 grades.
[0040] Sheets according to the invention can be used to perform
different operations routinely performed for making car body outer
panel parts, such as hemming, clinching, riveting, spot welding,
laser welding and gluing. In particular, it is possible to glue
hemmed joints, used particularly in making bonnets, without firstly
performing a chemical surface treatment such as chemical conversion
or passivation, for example using phospho-chromic compounds, or
products based on titanium, zirconium or silanes.
[0041] Parts made from sheets according to the invention also have
good resistance to filiform corrosion after painting, better than
alloys with a high copper content like 6111 alloy.
[0042] For economic reasons, it may be useful to combine steel
structures and aluminium body outer panel parts on the same
vehicle, for example for wings, roofs and doors. In this type of
assembly, the major difficulty is with the management of
differences in thermal expansion between two materials when the
paint is being baked, particularly during the cataphoresis baking
which is usually done at between 160 and 200.degree. C. It is
essential to limit residual deformations after baking to a level
acceptable for the appearance of the vehicle.
[0043] Sheets according to the invention can limit these
deformations, independently of the geometry of the parts and the
assembly mode chosen. The applicant thus demonstrated that a high
yield strength at the baking temperature, for example more than 140
MPa at a temperature of 160.degree. C. for the alloy according to
the invention, had a favourable effect on the deformation level, if
the assembly is made after baking, and it is thus preferable to
limit the baking temperature.
[0044] Other factors may also limit deformations, for example the
presence of ribs designed to stiffen the aluminium panel, or the
spacing between assembly points. An assembly with a continuous link
such as gluing could also be used, with at least a partial
polymerisation of the glue before baking, or a transparent laser
welding.
EXAMPLES
Example 1
[0045] 500 mm thick plates were cast of 8 alloys A to I with the
composition (% by weight) as indicated in table 1: Table 1
3 TABLE 1 Alloy Si Fe Cu Mn Mg A 1.15 0.31 0.07 0.10 0.41 B 1.0
0.29 0.09 0.11 0.33 C 0.58 0.26 0.79 0.10 0.73 D 0.58 0.26 0.79
0.10 0.73 E 1.22 0.13 0.07 0.08 0.56 F 1.0 0.30 0.35 0.15 0.45 G
1.0 0.30 0.18 0.30 0.45 H 1.0 0.30 0.18 0.05 0.45 I 1.0 0.30 0.18
0.15 0.45
[0046] Composition A represents a conventional 6016 alloy, B is the
applicant's grade DR100 described in the articles mentioned above,
C and D are a 6111 alloy, E is a 6022 alloy, F, G, H and I are
alloys with similar compositions, differing either by Cu (F), or by
Mn (G and H) from the composition I according to the invention.
[0047] The plates were scalped and homogenised for 10 h at
570.degree. C., and then hot rolled directly on homogenisation
heat, firstly on a reversible mill, then on a tandem mill. The
lamination start temperature was of the order of 540.degree. C.,
and the hot strip winding temperature was of the order of
310.degree. C.
[0048] The hot rolled strip rolled to 3 mm is then cold rolled to
the final thickness of 1 mm. An intermediate annealing is carried
to a thickness of 2.5 mm, consisting of either a "batch" annealing
in a coil with a temperature rise to 350.degree. C. in 10 h, 2 h
waiting time followed by slow cooling, or a "flash" annealing in a
continuous furnace with a temperature rise to 400.degree. C. in
about one minute and immediate cooling. Samples taken from the
strips are subjected to a solution heat treatment at a temperature
of 570.degree. C. for less than one minute, and are then quenched
in cold water. Complementary treatment for 2 h at 100.degree. C. in
the oil bath immediately after quenching to simulate industrial
pre-ageing, is applied to samples made of alloys B, D, F, G, H and
I.
[0049] The yield strength R.sub.0.2 (in MPa) was measured in the L
direction after 3 weeks and 6 months ageing at ambient temperature,
followed by an ageing treatment for 30 minutes at 165.degree. C. or
185.degree. C., simulating the paint baking treatment. The
formability was also measured using the LDH parameter (in mm), the
principal deformations being parallel to and perpendicular to the
rolling direction respectively. The results are given in table
2:
4 TABLE 2 A B C D E F G H I Inter. batch batch batch batch batch
flash flash flash flash an- nealing R.sub.0.2 121 101 140 139 158
129 123 125 124 3 weeks R.sub.0.2 133 112 152 156 173 144 138 142
145 6 months R.sub.0.2 135 157 160 221 163 194 187 189 191
165.degree. C. R.sub.0.2 159 186 190 258 190 228 222 225 225
185.degree. C. LDH // 27.4 29.0 25.4 26.1 25.9 27.2 25.9 27.5 28.3
LDHL 27.4 28.4 25.6 27.0 25.7 25.3 26.3 26.9 28.2 perp.
[0050] It was found that after 3 weeks of ageing, sample I
according to the invention has the same order of yield strength as
the conventional 6016 (sample A) and significantly lower than the
corresponding value of the 6111 alloys (C and D) and the 6022 alloy
(E). The position of the yield strength of sample I with respect to
other alloy samples has not changed after 6 months of ageing.
[0051] Formability, as measured by the LDH parameter, is
practically as good as the formability of the best alloy, which is
the DR100. Moreover, measured values of LDH in the rolling
direction and in the direction perpendicular to the rolling
direction are practically identical, which is not always the case
for other samples, which enables good isotropy in forming.
[0052] Conversely, the yield strength of sample I after the paints
have been baked following a pre-ageing is high, significantly
higher than the value for the 6016 and DR100 alloys, of the same
order as the value for alloy F which has a higher content of
copper, and is intermediate between the values for the two 6111
grades, guaranteeing high dent resistance of the finished part.
[0053] The behaviour during hemming was also measured on 1 mm thick
sheets in the direction parallel to rolling and in the direction
perpendicular to rolling, the resistance to filiform corrosion
after phosphating, cataphoresis and painting and the occurrence or
lack of occurrence of slivers of filaments when cutting out or
trimming after drawing.
[0054] The hemming test is done in three operations: flanging of
edges at 90.degree. C., pre-hemming at 45.degree. and hemming flat
on a 0.7 mm thick lining plate. The hemmed edges are then
classified by visual inspection, as described in the article by D.
Daniel et al. in IBEC 99.
[0055] The resistance to filiform corrosion is evaluated according
to standard EN 3665, with sample size 150.times.60.times.1 mm after
painting and scratching. The test procedure includes corrosion
activation by HCl vapour for 1 h, followed by exposure in a wet
room at 40.degree. C. for 1000 h. The maximum length of corrosion
filaments is measured, taking an average of 3 test pieces per case
with the following classification: <2 mm=good; 2-5 mm=medium;
>5 mm=poor. The cut test is described in the article by D.
Daniel et al in IBEC 99 mentioned above. The clearance was 10% of
the thickness and the cutting angle was 0.degree..
[0056] The results are given in table 3:
5 TABLE 3 A B C D E F G H I hemm good good crack crack crack orange
good good good // skin hemm good good crack crack orange orange
start orange good perp skin skin crack skin Fil. good good bad bad
good medium good good good Corr. Slivers no no filaments filaments
slivers no no no no
[0057] It was found that the behaviour of sample I is satisfactory
for these various criteria, so that body outer panel parts with an
impeccable appearance can be made.
Example 2
[0058] Aluminium alloy panels were made with the composition
indicated in table 4, with a manufacturing procedure similar to
that in example 1, possibly but not necessarily comprising
pre-ageing and heat treatment after forming and before assembly, as
also mentioned in table 4. The panel dimensions are 1.6 m.times.0.9
m.
6TABLE 4 T.sub.eq Si Fe Mg Cu Mn pre- Sample Alloy (%) (%) (%) (%)
(%) ageing Anneal. J Inv. 1.05 0.25 0.45 0.19 0.14 5 h
4-185.degree. K Inv. 1.05 0.25 0.45 0.19 0.14 5 h No L 6111 0.70
0.25 0.60 0.69 0.21 -- No M DR100 1.03 0.26 0.32 0.07 0.11 5 h No N
6016 1.03 0.26 0.32 0.07 0.11 -- No
[0059] For each alloy, three panels were tested with different
geometries each comprising ribs obtained by folding and parallel to
the short side of the rectangle.
[0060] These panels were riveted on rectangular steel frames to
simulate the case of body outer panel sheets made of aluminium
alloy on a steel structure of a vehicle. The assembly is made by
riveting at a pitch of 50 mm on the long sides of the rectangles.
The residual deformations of the panels were observed after 20
minutes heat treatment at 160.degree. C. to simulate cataphoresis
baking. The mechanical properties (ultimate strength R.sub.m and
yield strength R.sub.0.2 (in MPa)) of panels at ambient temperature
and at the baking temperature of 160.degree. C. were also measured,
for a temperature rise rate equal to about 20.degree. C./min. Table
5 contains the results.
7TABLE 5 R.sub.0.2 Panel 1 Panel 2 Panel 3 ambi- R.sub.m R.sub.0.2
R.sub.m Samp. deform. deform. deform. ent ambient 160.degree. C.
160.degree. C. J Very 261 321 239 250 low K Low Very Very 160 282
148 208 low low L High 164 309 137 220 M High Low Very 142 263 127
186 low N Very 122 230 106 161 low
[0061] It is found that the alloy according to the invention
effectively reduces residual thicknesses after baking. The
performance of alloys is clearly correlated with the yield strength
at the baking temperature. Finally, heat treatment before assembly
and the addition of ribs are beneficial in reducing
deformations.
Example 3
[0062] An evaluation was made of the resistance to dynamic
indentation of a 1 mm thick sheet produced according to a
manufacturing procedure of the type shown in example 1, comprising
pre-ageing for an equivalent time of 5 h, a 20-minute heat
treatment at different temperatures simulating baking of the paint,
made of an alloy according to the invention and a 6016 DR100 alloy,
and was compared with the corresponding values of a 0.7 mm-thick
steel sheet with a yield strength equal to 290 MPa after baking the
paint. This value of 290 MPa for the yield strength of a steel
sheet for use in an automobile body after baking corresponds
approximately to the average of yield strengthes of steel sheets
used for body outer panels for the most frequently used recent
European cars. A 1 mm-thick aluminium sheet can resist about 50%
more elongation than a 0.7-mm thick steel sheet.
[0063] The device used for the indentation test comprises a 15
mm-diameter indenter weighing 138 g, released from a height of 1 m
at a speed of about 16 km/h, onto the sample sheet clamped between
two steel plates. The permanent indentation depth is measured (in
mm). The results are given in table 6.
8TABLE 6 R.sub.0.2 Indent. Baking Inv R.sub.0.2 R.sub.0.2 Inv
Indent. Indent. temperature all. DR100 steel all. DR100 steel
170.degree. C. 193 161 290 1.55 1.80 1.45 185.degree. C. 217 189
290 1.45 1.62 1.45 205.degree. C. 230 207 290 1.38 1.46 1.45
[0064] It was found that for a paint baking temperature equal to
185.degree. C., the 1 mm-thick sheet according to the invention has
the same dent resistance as the 0.7-mm thick steel sheet. For the
DR100 alloy, this is only true for a paint baking temperature of
205.degree. C., which is higher than temperatures usually used by
automobile manufacturers. A stronger alloy such as 6111 would
increase the dent resistance to exceed market needs, but this would
be to the detriment of formability, particularly during
hemming.
* * * * *