Computer simulation of the aluminum extrusion

Introduction

Modern computer program for modeling an extrusion of aluminum capable of providing an effective improvement of the quality of extruded aluminum profiles even at the design stage extrusion dies. Computer modeling can improve the quality of manufactured aluminum profiles по следующим показателям:

  • dimensional accuracy and profile shapes
  • strength and appearance of welds
  • The grain microstructure
  • mechanical properties of the material.

For the average user this model, it often remains “black box”, whose internal structure is unknown or unclear. Below we will try to explain some of the properties of these models, and also to show their potential usefulness for practice production of aluminum extruded products.

computer model

Finite element technique

Практически все компьютерные модели имеют в своей основе метод конечных элементов (МКЭ). The Finite Element Method - is a numerical method for solving problems in various branches of physics and engineering, such as, mechanics of materials, Heat Transfer, for liquids and the electromagnetic field. The analytical solution of these problems require complex solutions of partial differential equations, which is possible only in the simplest cases. The formulation of the same tasks in the finite element method reduces to the solution of large systems of algebraic equations, с которыми легко справляются современные компьютеры [1].

To solve the problem of modeling the object is divided into small elements of simple shape – finite elements. Для двумерных задач такими простейшими элементами являются треугольники (рисунок 1) и четырехугольники, for three-dimensional - tetrahedra and parallelepipeds. Finite elements interconnected, but only at the connection points – sites. Using mathematical relationships, которые задают связь между напряжениями и деформациями (в простейшем случаезакон Гука), simulate the behavior of each of the finite elements during loading or moving its components.

Figure 1 - triangular finite elements
c компонентами узловых перемещений [1]

Then all of these finite elements are combined into larger systems of algebraic equations, которые моделируют уже весь объект в целом (рисунок 2). These systems of equations and solves computer.

Figure 2 - Three-dimensional finite element mesh
for simulation of the aluminum extrusion
в компьютерной программе QForm [2].

material model

Течение материала через контейнер и матрицу наиболее сильно зависит от:

  • aluminum temperature and
  • Aluminum strain rate.

Поэтому чаще всего поведение алюминия представляется моделями жестко-вязко-пластического материала [2-4]. Typically, these models do not take into account the elastic deformation of the material, that in general it is justified, as in the process flow of aluminum through the container and the elastic deformation of the matrix is ​​negligibly small fraction of the total deformation. However, in the channel operating belts elastic behavior of the material can play an important role and have a significant impact on the simulation results. Therefore, to more accurately simulate aluminum velocity at the exit of the matrix is ​​often used visco-elastic-plastic material model.

Strain hardening of the material is usually neglected, so says, in the temperature range of aluminum extrusion is compensated by dynamic recrystallization.

In the end, model links the stress in the material and its rate of deformation of certain mathematical relations, parameters which are specific for each aluminum alloy.

friction Model

An important condition for adequate modeling is also a model of friction between the aluminum and pressing tool, which reflects the true interaction with the aluminum walls of the container and the elements of matrix, including, with its running web.

aluminum matrix during and deflections

It is known, that the elastic deflection elements of the matrix can have a significant impact on the course material. In turn, These deformations of the matrix depend on the pressure, which renders it passing therethrough material. Therefore modern simulation methods are based on an integrated approach, который объединяет в одной модели на базе метода конечных элементов (МКЭ):

  • расчет пластического течения алюминия через матрицу и его температуры;
  • calculating the elastic strain and temperature steel matrix.

calculation procedure involves several iterations, in which there is an automatic realignment calculated finite element mesh.

Такой подход обеспечивает наиболее точные результаты по:

  • течению алюминия через матрицу;
  • размерам и форме прессуемого алюминиевого профиля;
  • температуре алюминия в заготовке и профиле;
  • temperature matrix.

Optimization of the properties of aluminum profile

With the help of modern computer simulation of aluminum extrusion is already at the stage of designing the matrix measures are taken – design and technological – по повышению качества профиля по следующим показателям [2, 3]:

  • точность размеров и формы профиля;
  • location, прочность и внешний вид сварных швов;
  • оптимизация зеренной микроструктуры материала профиля;
  • efficiency thermal hardening profile material while minimizing its buckling under cooling at the outlet from the press.

The size and shape of the profile

Simulation shows a possible distortion of the shape of the profile and possible deviations thickness of its walls and shelves at the output of matrix. These phenomena can largely be exacerbated by the elastic deformation of the elements of the matrix. Examples of such troughs are troughs so-called "tongue" in the planar array, в результате которых происходит значительное изменение угла наклона плоскостей рабочих поясков относительно оси прессования (рисунок 3).

Figure 3 – Прогиб языка в плоской матрице [4]

Joint modeling as the metal flows through a die, and deformation of the matrix itself under the influence of this trend, to compensate for the elastic deformation of the matrix and to achieve compression profile size and shape, which is securely located within the tolerance. This is achieved, for example, за счет правильного выбора типа матрицы и оптимизации ее конструкции [3].

Welds

Сварные швы на прессованных алюминиевых профилях бывают двух типов (рисунок 4):

  • longitudinal and
  • cross.

Longitudinal welds

Longitudinal seams occur only in the profiles, are compressed on a matrix of type "porthol", а поперечные швы – на всех профилях (рисунок 4).

Figure 4 - Scheme of the longitudinal and transverse welds
при непрерывном прессовании алюминия [5]

The longitudinal welds formed, when a hot aluminum alloy is separated in matrix ports into separate streams porthol, которые затем металлургически свариваются непосредственно перед выходом из матрицы (рисунок 5).

Figure 5 – Формирование продольных сварных швов в матрице-портхол [6]

Transverse welds

Transverse welds occur, when the leading end of a new workpiece comes into contact with the remaining in the metal matrix from the previous workpiece and metallurgically welded to them under high pressure and high temperature. Depending on the size and shape of the cross-sectional profile and the design matrix transverse the seam can have a considerable length, which may be about one meter or more.

Design of welds

These welds, as longitudinal, and transverse, can significantly spoil the appearance of facial surfaces anodized profiles, поскольку часто они значительно отличаются от остальной поверхности по степени блеска или матовости (рисунок 6). This is due to the different sensitivity of the grains of the material to alkaline pickling, which is made before anodizing operation.

Figure 6 - Stripes on matte anodized surface
по продольным сварным швам [6]

Компьютерное моделирование дает следующие возможности для оптимального проектирования сварных швов:

  • Changing the location of the welds, for example, transferring them with faces of the profile at the corner portions.
  • Достижение заданной прочности сварного шва за счет создания необходимого давления и температуры в сварочной камере (выбор ее размеров и определение заданной скорости прессования).
  • Optimization of the parameters of plastic deformation in the welding chambers to reduce the degree of nonuniformity of microstructure.

The grain microstructure

Recovery and recrystallization

During the plastic deformation the original microstructure of the workpiece material is subjected to considerable rearrangement under the influence of such mechanisms, как возврат и различные типы рекристаллизации (рисунок 7). The dynamic crystallization occurs during continuous energy supply as a result of plastic deformation. Static recrystallization occurs after plastic deformation and the energy controlled, which is already stocked in the material.

Figure 7 – Процессы возврата и рекристаллизации при экструзии алюминиевых сплавов:
а) в стационарном режиме прессования; б) при остановке пресса для смены заготовки [7]

alloys 6060 and 6082

In the 6xxx series alloys nizkoprochnyh – alloys 6060 (6063) – рекристаллизация происходит относительно легко и профили из этих сплавов обычно имеют полностью рекристаллизованную структуру (рисунок 8). In the alloys with a higher level of strength, such as, 6005, 6061 and 6082, нередко наблюдается частично рекристаллизованная структура (рисунок 9). This is due to the presence in these alloys dispersoidnyh particles, которые сдерживают рекристаллизацию [8].

Figure 8 - fully recrystallized alloy microstructure 6060 [6]

Figure 8 - Partially recrystallized alloy microstructure 6082 [6]

Coarse recrystallized grains are almost always located on the surface of aluminum profiles or near its. Sometimes they appear on only a portion of the surface profile. This layer of coarse grains on the surface of the aluminum profile may cause problems, such as marriage increased when machining or bending profiles, defects in appearance “Orange peel”, полосы и неоднородность блеска анодированной поверхности [8].

factors recrystallization

На скорость и полноту рекристаллизации оказывают влияние следующие факторы [8]:

  • температура на выходе из матрицы;
  • температура заготовки;
  • скорость прессования;
  • отношение прессования (вытяжка);
  • скорость охлаждения при закалке;
  • the presence of the chemical composition of items, сдерживающих рекристаллизацию (например, марганец и хром).

The computer model makes it possible to perform optimization as a design matrix, and pressing technological parameters to minimize the possibility of brute recrystallized microstructure profiles.

Quenching

Thermal hardening aluminum profiles 6xxx series alloys, включает два этапа:

  • hardening on press
  • artificial aging in an oven.

Hardening in the press is sufficiently rapid cooling of the profile immediately after the release from the matrix to a temperature of about 250 ºС (рисунок .

The object of tempering is to keep the aluminum profile in an aluminum solid solution the maximum amount of alloying elements. For 6xxx series alloys such elements are magnesium and silicon. From this stage on Aging, artificial or natural, зависит уровень достигаемых прочностных свойств (рисунок 10).

Figure 10 – Схема прессовой закалки алюминиевых профилей [9]

Each aluminum alloy has the critical cooling rate until a temperature of about 250 oC. The model is able to determine the cooling rate profile at each point and make recommendations for the cooling intensity of the surface profile, as well as the intensity of the cooling medium - air-cooling fan to the cooling water flow.

Figure 11 – Диаграмма закалки на прессе и искусственного старения алюминиевых профилей [10]

At the same time, the choice of cooling parameters during quenching Profile necessarily take into account the profile of resistance to warping and distortion of the shape. Compare drawings 11 and 12 shows, that in practice the cooling parameters in the hardening are a compromise between achieving high strength profile and minimizing its buckling under the shear and longitudinal profile shape tolerances.

Figure 12 – Ограничения при закалке алюминиевого профиля на прессе [10]

conclusion

Modern computer model aluminum extrusion are effective tools for improving the quality of extruded sections. They give a deeper understanding of the processes, which occur within the die during pressing of a profile. This saves time and money in both the design and implementation of new matrices, and in solving problems in the operation of existing matrices.

Sources:

  1. Application of Finite Element Method / L. Syegyerlind – Per. with English. - World, 1979.
  2. Application of QForm Program for Improvement of the Die Design and Profile Extrusion Technology /N. Biba, S. Stebunov, and A. hair – QuantorForm Ltd., Moscow, Russia – Proc. of ET2008 – 2008.
  3. Quality Prediction and Improvement of Extruded Profiles by means of Simulations /N. Biba, R. Rezvykh, I. Kniazkin – Aluminium Extrusion, 2/2019.
  4. CAD Implementation of Design Rules for Aluminium Extrusion Dies / G. van Ouwerkerk – University of Twente, 2009.
  5. Microstructural Caracterization of Extrusion Welds in 6xxx Aluminium Alloys /X. Ren et al - IC3ME 2015.
  6. Influence of Al Microstructure on Hard Anodising Quality– Profile Material/ Tom Hauge, Hydro Aluminium, Norway IHAA Symposium, 25th of September 2014, New York.
  7. Characterization of Microstructure in Aluminum Alloys Based on Electron Backscatter Diffraction /T. Kayser – PhD Thesis – Technical University of Dortmund - 2011.
  8. Effects of Extrusion Parameters on Coarse Grain Surface Layer in 6xxx Series Extrusions /E.D. Sweet te al – Proc. of ET2004 – 2004.
  9. Distorsion Mechanisms due to the Cooling Process in Aluminum Extrusion /S. Bikass et al - Proc. of ET 2012 – 2012
  10. Thermal Treatments During Processing of Aluminum— AEC Webinar Presentation /R.E. Sanders (Alcoa) – 2010

ATTACHMENT

All, what was stated above, applies, mainly, for production of aluminum profiles of a thermally hardenable alloys 6xxx series, and, partially, thermally neuprochnyaemym alloy series 1xxx, 3xxx and 5xxx. Conditions of production of aluminum profiles of the most thermally hardenable alloys, 2xxx and 7xxx series differ significantly from those, which are characteristic of the 6xxx series alloys. This should be considered when modeling.

alloy 7075, for example, эти особенности заключаются в следующем:

  • только простые поперечные сечения;
  • hollow profiles – только из полой заготовки;
  • более низкая температура заготовки (300-370 ºС);
  • очень малая скорость прессования (1,0-1,5 м/мин);
  • the specific pressing pressure is almost 3 times higher, than an alloy 6063;
  • It does not have the ability to hardening on press – закалка производится с отдельного нагрева:
  • печь нагрева под закалку располагается над закалочным баком: нагретые профили быстро «падают» в ванну с закалочной жидкостью;
  • особенная чувствительность к рекристаллизации: почти всегда образуется крупнокристаллический ободок.

Read more about the features of the production of aluminum profiles from alloys 2xxx and 7xxx series, see. here.