Laser Annealing of Cold Rolled Sheets: A Literature Review - IVF

39 Pages·2002·1.38 MB·English

Delrapport 1

Laser Annealing of Cold
Rolled Sheets:
A Literature Review

Jozefa Zajac, IM

2000-02-28

ACCRA Teknik AB IM – Institutet för Metallforskning AB
AK-Konsult IVF Industriforskning och utveckling AB
Amada/Promecam AB ORTIC AB
AvestaPolarit AB PRESS & PLÅTINDUSTRI AB
Bendiro AB Scandinavian CAD AB
Chalmers Tekniska Högskola SSAB Tunnplåt AB
-Institutionen för byggnadsmekanik Volvo Personvagnar AB
Ferruform AB
Kanthal AB VINNOVA
LASER ANNEALING OF COLD ROLLED SHEETS
A Literature Review

Jozefa Zajac
Swedish Institute for Metals Research
Drottning Kristinas väg 48
S-114 28 STOCKHOLM
SWEDEN

Keywords: Nd:YAG laser, surface treatment, laser annealing, laser forming,
hardness, microstructure,

Abstract
Localised heat treatment by laser may be utilised to soften certain areas of cold rolled
sheets prior to forming operations. Softening can be achieved by recovery,
recrystalisation or soft annealing. Laser enables not only a separate local softening
but also allows to perform forming operation at elevated temperatures,
simultaneously with laser heating.
Localised annealing combined with bending or drawing offers a variety of both
applications and advantages, which have not yet been evaluated. Therefore, intensive
investigations have been in progress. In most cases their results are covered by patents
in which the main role is attributed to lasers which generate a suitable temperature field.
The present review outlines examples of laser annealing, “hot bending and drawing”,
non-contact bending, laser bending, welding and annealing as well as complex
treatment comprising laser cutting + bending + welding + final heat treatment. They
show the possibilities of laser sheet forming and challenge to innovative solution in
plastic deformation.
Some annealing trials of cold rolled sheets were performed using the IM's Nd:YAG
laser. They confirmed that pulsed Nd:YAG laser can be successfully used for local
recrystallisation and softening.
Mathematical modelling of heat flow was performed in order to calculate the
temperature profile during annealing trials. Both simple models and complex FEM
models were tested and compared with experimental results.
1

CONTENTS

1. INTRODUCTION..........................................................................................................................................2
2. SOFT ANNEALING BY LASER TREATMENT....................................................................................3
2.1. SHEET METAL ANNEALING.......................................................................................................................3
2.2. LASER SURFACE ANNEALING OF STEEL WIRES FOR AUTOMOTIVE TIRES................................................7
2.3. LASER ANNEALING OF NON-FERROUS SHEETS.........................................................................................9
3. LASER FORMING........................................................................................................................................9
3.1. HOT FORMING OF SHEET MATERIALS.......................................................................................................9
3.2. IMPROVING COLD ROLLABILITY AND/OR STRIP ANNEALABILITY.........................................................13
3.3. LASER BENDING......................................................................................................................................15
4. OTHER APPLICATIONS..........................................................................................................................16
4.1. SOLUTION ANNEALING HEAT TREATMENT.............................................................................................16
4.2. INCREASING THE DRAWABILITY OF AL-CU ALLOYS BY DIFFERENTIAL HEAT TREATMENT.................16
4.3. LASER WELDING AND ANNEALING.........................................................................................................18
5. HEAT FLOW THROUGH THE LASER TREATED SHEET............................................................19
5.1. DETERMINATION OF PROCESS PARAMETERS.........................................................................................19
5.2. MATHEMATICAL MODEL OF HEAT FLOW...............................................................................................19
6. PRELIMINARY INVESTIGATION OF LASER ANNEALING OF COLD ROLLED SHEETS33
7. SUMMARY...................................................................................................................................................36
8. ACKNOWLEDGEMENTS........................................................................................................................37
9. REFERENCES.............................................................................................................................................37
2

1. INTRODUCTION
Laser radiation can be used in several ways to modify surface properties of metals and
alloys. Laser transformation hardening, remelting or alloying are established methods for
producing hard, wear resistant cases on selected surface areas of engineering components.
Laser radiation can also be used to perform a simple heating for solution annealing, soft
annealing or ageing. This review is focused on soft annealing of cold rolled sheets or
final bending or drawing.
When soft annealing or annealing of hardened components are used as bulk treatments
conventional heat treatments are generally considered as being cheaper1. Laser, on the
other hand, is very efficient in heating a localised area. It is known that localised heat
treatment may be utilised to soften portions of components for a variety of purposes such
as stress relief or creation of components with different physical properties in different
areas. This is especially true for processing of sheet metals. Consider a cold rolled sheet,
which should be deformed by bending. Laser enables not only a local softening but also
application of non-contact bending without forming dies. The sheet can be bent into a
“V” or “C” or other shape by repetitively scanning the beam over the sheet at particular
zones. In conventional sheet metal processing, bending or drawing is performed by a
sheet metal bending or drawing machine using various special dies.
The forming operation can also be performed simultaneously with laser heating. Such a
forming process is often called as superplastic forming. The yield strength of a metal
material is significantly reduced at elevated temperatures and even a hard metal sheet can
be easily “hot” formed into complex shapes.
The technique of local softening, recrystalisation and forming of sheet materials has
obviously a number of possible applications and advantages, which have not yet been
evaluated. Therefore, intensive research activities have been in progress. Most of them
have resulted in specific patents in which the main role is played by the lasers, which
generate a suitable temperature field. Considering claims from these patents one may
have the impression that even normal laser heating is restricted. Obviously they pertain to
a specific part or process, which leaves a broad area for new solutions and inventions.
If a new laser treatment or forming has to be applied, a careful experimental optimisation
of the process parameters must be carried out in order to obtain maximum processing
speed and optimum quality. In most cases, there are no systematic investigations, which
might help design and simplify the step from experimental state into a manufacturing
line, despite a growing area of mathematical modelling of the interaction between laser
radiation and heated surface, heat transfer and the thermal field in the treated materials.
The present review is aimed at summarising the possibilities for laser processing of sheet
materials. Especially, a soft annealing for bending and drawing is highlighted.
Mathematical modelling of heat flow through the sheet based on heat flow by conduction
is also described.
3

2. SOFT ANNEALING BY LASER TREATMENT
A complete soft annealing of the hot rolled sheet before final forming is often undesirable
as it reduces the strength and may cause distortion problems. The optimum solution is
local, soft annealing only in these regions where the sheet has to be deformed. More
generally, if a hardened part has to undergo a final shaping, localised soft annealing can
be performed with laser heating systems.

2.1. Sheet metal annealing
The possibilities of local soft annealing of different sheet materials using CO -lasers were
2
analysed by Bergman1. The experiments were carried out with CO lasers of 0,6 and 4
2-
kW output power. Figure 1 gives a schematic drawing of the experimental set up, for
experiments carried out with 0,6 kW laser. The absorption was simply increased by
drawing a black line where the laser treatments were applied. The optimum treatment
conditions were found by varying the power density and feed rate, ensuring that ripple
formation did not occur and the sheets were soft annealed throughout the whole
thickness.
The materials investigated and the treatment conditions for laser power of 600W are
listed in Table 1 and 2. Sheets of austenitic stainless steel and of a typical deep drawing
grade were used as reference materials.

Table 1. Investigated materials and obtained microstructure after treatment with 600W
laser power 1.

No. Material Degree of Sheet Microstructure after laser
deformation thickness annealing
%
1. CuNi4,5Al4,5Mn2 Cr0,8 75 0,5 50% of the cross section molten,
showing dendritic structure, rest
recrystallised with equated grains.
2. CuNi78Zn20 33 0,25 As above, 30% molten
3. AlMgSi2 (0) 1,5 Differences in microstructures
with optical microscopy not
age hardened
detectable.
4. X12Cr18Ni8 33 1,0 Partially recrystallised
microstructure, no melting.
5. C10 75 1,0 Partially recrystallised micro-
structure in the heat affected zone.
4

Fig. 1. Schematic drawing of the experimental set up1.

B
A

a) b) c)
Fig. 2 Microstructures of soft annealed C10 steel
a) general view
b) transition zone
c) recrystallized region

A

a) b)
Fig. 3 Microstructure of partially melted CuNi Al Mn Cr sheet. a) General view and
4,5 4,5 2,5 0,8
b) enlargement.
5

Table 2. Laser parameters and hardness values

Power Feed
No. Power Focus density rate Vickers Hardness (HV0,5)

W cm W/cm2 m/min Starting Laser After
material annealed bending
1. 600 0,0 4,8*105 5,0 305 145 190
2. 400 2,5 2,3*104 9,0 220 150 175
3. 400 2,5 2,3*104 0,5 75 53 64
4. 600 2,5 3,4*104 0,2 285 220 255
5. 400 2,5 2,3*104 0,5 240 170 195

The final optimisation of processing parameters for austenitic stainless steel and a typical
deep drawing steel was performed using a 4 kW laser, Table 3. In this case the intensity
distribution of the output energy was modified using a BIAS optic in order to reach a
constant energy distribution throughout a rectangular beam of 5mm x 14mm in cross-
section.

Table 3. Optimum processing parameters for obtaining full recrystallisation of cold rolled
stainless and C-Mn sheets using 4kW laser.

Material Thickness Power Feed rate
mm kW m/min
0,7 2,5 6,5-7
X12Cr18Ni8 1 2,5 3,5-4
1,5 2,5 1,5
1,5 1,7 2-3

1,0 2,5 5-6
C10 1,5 2,5 3
1,5 1,7 2-2,5

The experimental results confirmed that it is possible to obtain fully recrystallised soft
microstructure, Fig. 2. The recrystallised region contains eqiaxed grains and is clearly
distinguisable from the cold rolled structure. However, if the laser parameters are not
optimised a partial melting can occur, Fig. 3. Hardness profiles across the laser annealed
zones on steel sheet and aluminium sheet are shown in Fig. 4(a) and (b) respectively. It
was noticed that the width of the softened zone on the steel sheet was similar to the
diameter of the laser beam with a narrow heat affected zone. However, for aluminium
sheet the width of the softened zone was much wider due to a higher thermal
conductivity.
After laser softening treatments, all sheets could easily be bent to a rectangular edge. The
bending followed exactly the treated track and no cracks were found.
6

a) cold rolled steel C10 b) aged hardened AlMgSi2

Fig.4. Hardness profiles of soft annealed materials.

Fig. 5 Laser surface annealing of steel wires for automotive tires- the schematic diagram
of the method.
7

The following conclusions can be consider as general findings from the above
investigation:
• It was demonstrated that local soft annealing of cold rolled materials is possible.
• The required depth in which a surface layer has to be modified depends on power
density and interaction time. The width of the softened track is not only determined
by the laser parameters but also by thermal conductivity of the material and the
thickness of the sheet.
• Laser power and feed rate should be adjusted in order to ensure complete soft
annealing throughout the sheet thickness and avoid local melting of the surface. For a
low thermal conductivity, the feed rate has to be reduced to achieve soft annealing
over the whole sheet thickness and avoid local melting.
• The reduction in hardness can be due to the recrystalisation or overaging, or possibly,
homogenisation and subsequent ageing might have occurred.
• The result of the treatment with a rectangular beam confirm that it is possible to
obtain fully recrystallised traces even for material with low thermal conductivity
(austenitic stainless steel). Treatments of thicker sheets are also possible without local
melting.

2.2. Laser surface annealing of steel wires for automotive tires
The application of tire rod wire with diameter <300µm and tensile strength above 3000
MPa is limited by their insufficient fatigue strength. A new method for improvement of
the fatigue strength based on laser surface annealing of a thin surface layer was
developed at Nippon Steel Corporation2. In this method, the coaxially focused laser beam
of high energy irradiates the wire to anneal uniformly a thin surface layer. Using
optimum parameters, up to 3 µm depth can be uniformly annealed. After laser annealing
the fatigue strength of the test wire was increased four times without any deterioration of
the tensile strength.
The schematic diagram of the method is shown in Fig 5. The laser beam was introduced
by the bending mirror into the cone shaped focusing mirror. When the steel wire passes
through the laser cloud zone, the surface layer of the wire is heated. Heat input into the
wire was controlled by regulating the wire feeding speed and the laser power. The surface
was annealed because the wire is so thin in diameter that the cooling velocity by heat
conduction is slower than that of typical laser transformation hardening. In processing
line, the feeding speed of the wire was in the range of 30-100 m/min.
Microstructural investigation confirmed that the fatigue strength of the wire is improved
when the fine columnar cementite is formed at the surface layer as the result of annealing.
Martensite structure was not observed despite a high carbon content of 0.82%C.
The improvement of fatigue strength after laser annealing was somewhat contradictory to
the traditional view that fatigue strength increases as a result of laser hardening of the
surface layer. It was suggested that the annealed surface of the wire may be less sensitive
for initiation of fatigue cracks as laser annealing produces a smooth surface without
melting or oxidation defects. This seems to be true for bending loading of tire rods.
8

Fig.6 Annealing method of the part of the spring member made of non-ferrous metal by
pulsed Nd-YAG laser

Back Front
250
g
0
0
5200
H
P
D150
s
r
e100
k
c
Vi
50
7 7,2 7,4 7,6 7,8 8 8,2 8,4 8,6 8,8
Distance from reference, mm

Fig.7 The hardness across the irradiated zone on both the irradiated and
reverse sides.