Thermal analysis of an Aluminium and Near-beta Titanium alloy billet during forging operation

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OVERVIEW OF FORGING SIMULATION TECHNOLOGIES –DEFORM

Dr. James Farar

DEFORM is a simulation software used for the design and optimization of bulk

metals, glass/non-metallic forming, heat treatment and machining. The major industrial

operations such as rolling, extrusion, forging, fastener installation, turning and drilling can be

done using DEFORM, reducing manufacturing costs and lead time.

For example, in the manufacturing of turbine disks of aircraft engines, the parts are

joined together by inertial welding which builds up residual stresses in the material thereby

changing the microstructure. Hence, understanding the entire processing route of a

component is vital for product design and development. Forming simulations are very

mature, robust, reliable and accurate. In thread rolling, it is important to determine the failure

characteristics of tools like carbide (very expensive).

General Overview

Due to international/commercial pressure on highly efficient design and

manufacturing of products, it has become a primary objective for the industries to do it right

first time. The amount of time allowed for design, production and testing of parts/components

is becoming lesser and lesser. The trial and error method using a finite element analysis

software like Ansys, DEFORM etc. not only saves time but also reduces significant amount

of scrap during manufacturing process. It becomes even more challenging when new

materials are employed for better performance. The basic idea of DEFORM in terms of

design and simulation is to develop new tooling and processing routes with quick, reliable

feedback in a computer system at lower cost.

Brief history of simulation processes

1970’s - mathematical model developments for finite element analysis.

1980’s – turbine disc manufacturing project (one specific function analysis)

1990’s, quenching became a vital heat treatment method. Possibility of heat treatment,

machining and forging became a reality and a critical engineering problem to be solved.

Over the past decade, focus is on vertical integration than generic purpose for the whole

range of applications such as cogging, rolling, extrusion, hot/cold working, sheet forming,

heat treatment, joining and machining.

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Why simulation?

Reduces cost of production,

Optimum processing – fewer shop trials and redesign of tooling and processes,

Life cycle estimation shortens lead time in developing a new product.

Open Die Forging

Open die forging is a mix of art and science. The most important design criteria are

grain size and consolidation of porosity. The forgings that are produced at high temperatures

often involve convective and radiative heat transfer. It is also vital to know the thermal

interaction between the tools and dies. Marking operations are used to partition a billet into

different regions for subsequent forging operations. Hence, simulation helps to predict the

behaviour of the target material across a wide range of operating environment. During

forging operations, the ingot primarily breakdown with large bites and excess material on it is

trimmed off after forging.

Ring Rolling

The initial upsetting of stock during the ring rolling process involves punching or

piercing, which concludes with a nasty explosion. The heat distribution in the material can be

simulated in DEFORM (multiple operations) in a single step, resulting in contours of

temperature with respect to load.

Applications of DEFORM – Testing billet size, length and position.

Table 1: Different types of billets and their associated problems

Large billet Estimated even before ordering the material – overfills (flash)

Small billet Did not fill the die

Nominal billet Ends up in a lap

Optimum billet Fills dies without defects or excessive flash

Gear (Caterpillar)

The detailed understanding of the microstructure of critical components like a gear

that spins with load along the circumference can be simulated using DEFORM. So, the

material is initially subjected to loading to estimate its behaviour before multiple iterations

are analysed. It reduces production cost by a big factor, especially in the case of mass

production of components. Machine parts that are pre-heated, hardened or heat treated in an

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induction heating undergo a significant transformation. Induction coils are rotated around the

gear to yield uniform heating around the circumference. Contours of plastic deformation

behaviour of these components can be compared in DEFORM for an actual and a virgin

material. Other important advantages of this advanced simulation package includes analysis

of helical gears machined from round stock or axisymmetric extrusions. Final coining and

assembly of 20 peso Mexican coin has been analysed with 2-3 million tetra elements taking

69 hours of total simulation time.

Industrial Applications

The basic idea behind design and analysis of materials and their products follows the

following principle:

Chemistry + Process = Microstructure >> Properties

In an even simplified manner,

Property = f (Microstructural features)

Prediction of geometry, microstructure anomalies, residual stresses, effective strain rate, flow

stress in a material can be accurately done using DEFORM, provided the input boundary

conditions are accurate.

Grain size has a direct relation with strength of a material. For example, IN 718 alloy

of 8” diameter billet of 20 microns had 10% more strength than 30 microns billet. So,

DEFORM essentially helps to understand the source of and evolution of stresses through

manufacturing/processing routes.

Conclusion

The role of DEFORM in the various operations at Vulcan Forge is exceedingly higher

than anything else for the business of the industry. It can been established that the simulation

package forms the backbone of the industry. Even though, the accuracy of the results depend

a lot on the accuracy of the input data, they are a reliable, quick and accurate methods of

understanding the DNA of the material. Since, Vulcan Forge is linked to a lot of work on gas

turbine engines, the swift and effective way of designing and modelling makes DEFORM an

instrumental asset to the organisation.

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USE OF DEFORM AT SHEFFIELD FORGEMASTERS

Dr. Sinan Al-Bermani

Sheffield Forgemasters – Brief History

Sheffield Forgemasters International Ltd. is the parent organisation of seven

subsidiaries – Sheffield Forgemasters Steel, Sheffield Forgemasters Engineering, Steel

Propeller, Vulcan SFM, Euro SFM, Sheffield Forgemasters Inc. It is one the largest

casting/forging industries in UK and possesses an incredible capacity to cast up to 650 tonnes

of steel at one time.

The forging capabilities of Sheffield Forgemasters include 10,000 tonne press (No. 5

press), 4500 tonne press (No. 3 press) and 2500 tonne press (No. 1 press). The major

processes involved are steel making, metal working, casting, heat treatment, machining,

CAD/CAE. The major business partners are Siemens, General Dynamics, BAE, BHEL &

ALSTOM. The principle application of DEFORM at Sheffield Forgemasters involve

determining porosity in ingots and analysing temperature distribution in the melt shop.

Modelling strategy

Figure 1: Essential elements of modelling and simulation in DEFORM

The finite element analysis of materials and processes begin with a two dimensional

or three dimensional computer aided design (CAD) followed by addition of material

properties and boundary conditions. It is really important for designers to do it right for the

first time because some products/assemblies takes a long time and it’s a big loss of

investment if the process needs to resume again even due to fractional failures. Material

properties are usually calculated by thermodynamic software.

2D/3D CAD

Boundary Conditions

DEFORM Model

Material Data

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Case 1 – Nuclear head forging

Nuclear head is basically a closure head that goes on the head of pressure vessels.

They are subjected to enormous pressure. The initial method of manufacturing nuclear head

is called generation 1 manufacturing. An ingot is forged into a cylinder, cut from top and

bottom (because they are discard materials) because they form segregations and inclusions.

The ingot is then turned to 90 degrees and set to vertical forging. The ‘cheese’ is machined to

be left with a clean head. It sacrifices nearly 50% of the material, takes a long time but it is

cost effective. In generation 2, more work is done on the material, thereby requiring less raw

materials and has better properties. This methods yields a head that is combination of cheap

manufacturing and good properties. The material is analysed in DEFORM from cheese to

generate a quality end product. Generation 3 comprises of punch and rings and the material is

formed around the punch. DEFORM aids in reduced defects by predicting the alignment of

defects with the pressure boundary. The products have no scales and are normally thick. The

point tracking method helps to simulate inclusions that can be machined away.

Case 2 – Nozzle extrusion

If a nozzle is required to be designed inside a cylinder, a hole is cut and the nozzle is

weld onto it. The problem with welding is heat defects and varied microstructure that has to

be inspected at regular intervals. It is very expensive and difficult to inspect, if it is meant for

nuclear applications. An integral nozzle without weld can be designed using DEFORM. The

strain distribution in actual piece and trial piece revealed the same information. A disk bent

on a dome was designed without welds for a civil/nuclear application.

Conclusion

Finite element analysis is a critical tool for process design and optimization. It enables

innovative forging techniques that improve product performance and efficiency of

manufacturing processes. Accurate FEM modelling requires accurate material data and

boundary conditions. Some simulation processes take a lot of time to process but saves a lot

of time and money. Unlike general purpose FEM codes, DEFORM is tailored for

deformation modelling. It has a user friendly graphical user interface that provides easy data

interpretation on metal forming and tool design.

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1. Finite element simulation of high strength aluminium forged wheel

A two dimensional model of the cylindrical billet and top/bottom dies of given

dimensions are drafted and simulated in a pre-determined environment using DEFORM.

Forging is a mechanical process of transforming a metal into a useful shape/product by

hammering or applying pressure. It is a non-isothermal die forging process where heat

dissipation takes place between its surroundings.

Objectives

Perform 2D simulation of the cylindrical billet in DEFORM,

Obtain load time plot and temperature distribution at the end of the forging,

Viability of the forging characteristics of the aluminium wheel under given processing

conditions.

Initial boundary conditions

Temperature of the billet and dies are 350ºC and 250ºC respectively,

The dies are coated with a lubricant having low friction co-efficient, α = 0.05,

The air around billet is hot (50ºC) with a thermal transfer coefficient of 20Wm-2K-1

Heat transfer coefficient between the billet and the dies = 1000 W m-2K-1(=1N/s/m/C)

Table 2: Boundary conditions for FE simulation of high strength aluminium forged wheel.

Maximum capacity of forging press 70 tonnes

Material Aluminium 2024

Environment temperature (Air temperature around the billet) 50ºC

Thermal heat transfer coefficient 20 Wm-2K-1

Number of objects 1 work piece + 2 dies

Speed (movement in y-direction downwards) 50mm/s

Process type Hot forging

Primary die travel 17mm

Process - Finite Element Simulation

The modelling is carried out in DEFORM by a sequence of operations that define the

geometry of the dies and the work piece, material properties, meshing – contact nodes,

motion speed, position before running simulation. Since, the effect of friction on the

microstructure of the material is of high importance, it is important to understand that

lubrication with larger values tend to push the material towards the transverse direction in

such a way that the metal fills the die cavity.

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Figure 2: Primary sequence of operations carried out in FE simulation of cylindrical billet.

The modelled dies (top/bottom) and workpiece of the alloy wheel is depicted in figure 2. The

billet is automatically positioned by the software to minimize the contact gap between them.

Figure 3: Automatically aligned cylindrical billet & die (Al alloy wheel) before compression.

Since, the given product is meant to be forged in a press having a maximum capacity of 70

tonnes, the probability of the material getting hammered is lesser than being squeezed

laterally. Because of this, the size of the dies can be less massive and has better life than

hammers. However, the longer contact time results in heat loss from the forging environment

Define Geometry, Environment

& Process.

Primitive Geometry

Material Properties

Top/Bottom Die Design

Run Simulation

Meshing, Nodes, Die travel

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and cause die deterioration. But, the advantage lies in the close tolerances of the finished

products.

Figure 4: Load time plot and temperature distribution in a hot forged billet.

The temperature distribution plot divulges the fact that the edge of the aluminium

alloy wheel has a larger temperature gradient that has a poor microstructure than the rest of

the product.

Figure 5: Load vs. time plot of the squeezed aluminium billet.

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The residual stresses must ne normally smaller during the upsetting of a cylindrical

billet but produce a poor edge that cannot be neglected before processing and manufacturing.

The load time plot tells the requirement of a press have a capacity of a little above 87 tonnes.

It can be seen that the deformation is confined to a small portion of the workpiece.

Conclusion

The load time plot of the aluminium alloy wheel (cylindrical billet) illustrates the

relative variation of applied load with respect to time. It is evident that the load applied by the

forging press towards y direction allows the material to undergo sliding deformation and it

will flow laterally to take its designed shape. The effective strain within the material

multiplies as the top die approaches near the bottom die. The temperature distribution

indicates that the mean temperature of the material certainly lies below 390ºC, above which

the material experience undesired change in the microstructure. But, a vigilant observation of

the temperature distribution plot reveals the critical area of the alloy wheel which may

experience temperatures above 390ºC. The friction coefficient of 0.05 is ideal for proper

forming of the material but the ideal product depends upon applied load as well. Since, the

required load to create uniform temperature distribution in most part of the designed

aluminium wheel alloy is larger than the maximum capacity (70 tonnes) of the forging press,

it is not viable to use the given machine to manufacture the cylindrical billet.

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2. ISOTHERMAL FORGING

2.1 Forging characteristics of double truncated cone (Near β titanium alloy

“Beta-MAX”)

A two dimensional model of the double truncated cone developed to enable controlled

strain distributions is deformed between two platens to a ‘pancake’ such that the magnitude

of strain is maximum at the centre. The results (metallographic data with the strain profiles)

obtained from the finite element analysis test geometry are compared with microstructural

analysis to estimate the viability of the forging press.

Objectives

Design a model of the preform and the die and simulate (double truncated cone) in

DEFORM,

Obtain load and temperature distribution plot at the end of the forging,

Viability of the proposed material under given processing conditions.

Boundary conditions

The glass lubricant created a friction co-efficient of 0.2 during the isothermal forging

of the double truncated cone,

The sample (35mm dia.) is isothermally forged at 760ºC; velocity of the top platen –

0.158mm/s, initial strain rate – 0.01s-1.

Process Simulation – Finite Element Method

The truncated double cone (Near β titanium ‘Beta MAX’ alloy) is modelled precisely

as per the given dimensions. The new material properties are added to the database before

application of load and meshing. The environmental conditions are set to isothermal with a

temperature of 760ºC. The tooling (top and bottom platens) are also designed and

automatically positioned to hold the truncated conic billet within its cavity. The material gets

squeezed to a flat plate due to isothermal compression and spreads laterally under the effects

of thermal strain. The sample is deformed under vertical compressive loading and compared

with best suitable microstructure derived from experimental research at The University of

Sheffield. The point tracking of five different regions of the flattened billet to demonstrate the

right material properties for aerospace applications (for Boeing 787 and Airbus 350 – Vulcan

Forgings plc.) is done using DEFORM.

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Initial Meshing & Tooling

Figure 6: Geometric modelling of the double truncated cone.

The two dimensional simulation process of near β titanium alloy “Beta MAX” begins with

axisymmetric modelling of the double truncated cone of height 15.8mm and width 19.8mm.

The sample is meshed under operating conditions of thermal and mechanical stresses and

tested for strength and possibility of forging using the press available at Vulcan Forge plc.

Figure 7: Initial meshing of the double truncated cone under design boundary conditions.

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Figure 8: Workpiece positioned between top and bottom die before compression.

The done truncated titanium beta max alloy billet is positioned between the top and bottom

platen. The position of the top platen at this point (before isothermal compressive loading is

applied by the forging press) is in contact with the upper surface of the double cone. Figure 9

illustrates the compressed workpiece under the dies in the presence of a glass lubricant with a

friction coefficient of 0.2.

Figure 9: Deformed sample between two platens during isothermal compression.

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Norton-Hoff Equation

The deformation behaviour of the viscoelastic alloy (Near β titanium Beta MAX”) can be

modelled by Norton Hoff equation. This law confirms that the flow stress in a forged material

is a function of effective strain, strain rate and temperature.

Mathematically, Norton Hoff equation is expressed as:

σ = k (εo + ε)n εm exp (β/T)

σ = k.ε

The value of k is an essential part of the simulation process and it can be calculated from the

experimental measurements of strain rates at various flow stress in the material.

Table 2: Steady state flow stress value for different strain rates at 760ºC for Ti alloy β-max.

Figure 10: Graph to determine strain sensitivity parameter and scaling factor.

The value of m (strain rate sensitivity parameter) and k (scaling factor) can be determined

from the graphical plot of ln (flow stress) vs. ln (strain rate). The equation of the line is:

5.14

5.01

4.79

4.65

4.38

y = 0.1615x + 5.5081

4.30

4.40

4.50

4.60

4.70

4.80

4.90

5.00

5.10

5.20

-8 -7 -6 -5 -4 -3 -2 -1 0

ln (

Flo

w S

tres

s) M

Pa

ln (Strain rate)

ln (flow stress) vs. ln (strain rate) Curve

Strain rate (s-1) Flow stress (MPa) ln (strain rate) ln (flow stress)

0.1 170 -2.302585093 5.14

0.05 150 -2.995732274 5.01

0.01 120 -4.605170186 4.79

0.005 105 -5.298317367 4.65

0.001 80 -6.907755279 4.38

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y = 0.1615x + 5.5081 …… Eq. 1

From Eq. 1,

Strain rate sensitivity parameter, m = 0.1615 and

k = e5.5081 ……. k = 246.717

Figure 11: Representation of Norton-Hoff law in DEFORM.

Figure 12: Effective strain distribution in the double cone truncated titanium alloy beta MAX

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Viability - Forged truncated double cone

The viability of the forged titanium beta alloy is possible due to the availability of a

forging press with required tonnage.

2.2 Isothermal forging of titanium component

The isothermal compression of the near β titanium MAX cylindrical billet under the

operating conditions meant for aerospace application in A350 and B787 is carried out in a

FEM software to identify the best potential match.

Assumptions

The bottom ram is stationary and the top ram moves downwards to displace the metal

along transvers directions,

A microstructure which is at a distance of 8mm from the centre of the deformed

truncated cone sample has optimum high strength properties,

The value of β in the Norton-Hoff equation can be assumed to be zero since the

rheology testing was performed under isothermal conditions (i.e. no work hardening

or flow softening; hence n=0).

Initial boundary conditions

The platens of the hydraulic press are 100mm in diameter,

The top platen moves a speed of 0.5mm per second (initial strain rate of 0.01 s-1),

Maximum travel of the top ram is 45mm.

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Minimum reduction in height for desired microstructure in frictionless upset forging

The minimum reduction in height to get the desired microstructure in a frictionless

upset forging can be determined by the following equation.

ε = ln (ho/hi)

where ε is the effective strain in the material, ho is the initial height and hi final height of the

workpiece. The value of ε is known to 1.24 from the graph, ho of the billet is 50mm.

1.24 = ln (50/hi) hi = 14.49mm

Maximum reduction in height = 50 – hi = 50 -14.49 = 35.51mm

Initial mesh and tooling – Upset forging with friction

Figure 13: Cylindrical billet model after meshing

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Strain distribution – Upset forging with friction at minimum reduction of height

Why a larger reduction in height is required in the presence of friction?

The upsetting of a cylindrical billet causes the metal to flow laterally between the

advancing die surfaces, where there is less deformation due to frictional forces than in the

middle. Friction holds the interfaces of the metallic alloy and reduces sliding effect. The

material cannot undergo uniform strain and hence the applied load (which decreases the

height of the top platen/die) must increase in order to get better microstructure.

Load displacement curve – Upset forging with friction

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Conclusion - Press Capacity

The given sample can produce the microstructure obtained as part of the research at

The University of Sheffield with an applied forging load of roughly 42 tonnes. The maximum

capacity of the hydraulic press available at Vulcan Forge plc. is 50 tonnes. Hence, it is

possible to obtain the desired microstructure.

How can the microstructure be produced more effectively?

The microstructure can produced more effectively by either using a lubricant of lesser

friction coefficient or by increasing the effective strain within the material by applying more

compressive stress from the top. Increased load acting on the billet will further displace the

metal to spread across the horizontal plane of the die. There is an inter-planar dislocation of

the viscoelastic metallic alloy (titanium beta MAX) which manifolds the effective strain

within the material, which in turn, improves the microstructure.

Discussion – Error and limitations in the model`

Most of the products that appear sharp at corners actually have finite radii. When

FEM software creates a mesh, it assumes that the edges are sharp. This is known as stress

singularity. The accuracy of the result also determines on the type and size of the mesh. Finer

the quality of the mesh, precise will be the value of the desired properties. DEFORM has its

own limitations in meshing and plotting output in a proper manner.

References

1. George E Dieter, Mechanical Metallurgy, pg.584-607, McGraw-Hill, 1976.

2. RH Wagoner, JL Chenot, Metal Forming Analysis, Cambridge University Press,

2001.

3. Lecture on ‘Overview of forging simulation technologies’ by Dr.James Farar, October

2013.

4. Lecture on ‘Use of deform at Sheffield Forgemasters’ by Dr.Sinal al Bermani,

October 2013.

Thermal analysis of an Aluminium and Near-beta Titanium alloy billet during forging operation

Engineering

Transcript of Thermal analysis of an Aluminium and Near-beta Titanium alloy billet during forging operation