Investigating the effect of semi-solid Al-A356 alloy ingot production process parameters on the properties of the produced piece by thixoforging method

Dissertation for obtaining a doctorate degree

Mechanical engineering field - manufacturing and production trend

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The advantages of using semi-solid processes are reducing the amount of gas porosity and shrinkage and modifying the microstructure, which aims to improve the mechanical properties of the part. In this research, the cooling ramp method has been used to produce the primary shimshals. The main goal of this stage is to produce shimshali with the optimal microstructure in terms of the smallest grain size and the highest shape factor, which was achieved through the creation of controlled and uniform loading conditions with the ability to control the atmosphere. In the next step, by using the obtained ingot and through the forging process with semi-solid ingot, the model 62 helical gearbox cover part was produced due to its smooth and thin walls. The aim of the second part was to produce industrial parts close to the final shape with proper strength. In this research, the influence of ramp process parameters, including temperature, ramp length and angle, temperature and loading rate were comprehensively investigated. In order to analyze the effect of mutual parameters, factorial design method was used. Also, the relationship between the process parameters and the factors affecting the non-dendriticization of the microstructure, which include the cutting rate, the duration of its application and the solid fraction of the slurry, was investigated using finite element simulation and through Flow-3D software. In addition, due to the fact that aluminum is a metal that, especially in the molten state, has the capability of chemical reaction and physical solubility with the components of the ambient air, especially oxygen and hydrogen, which leads to the creation of oxide compounds and porosity in the shamshal, the effect of atmospheric control by argon gas was investigated. In the second stage, after the production of the healthy part, the effect of the effective factors such as the temperature of the part and the mold, the duration of keeping the part at the semi-solid temperature, the pressure applied to the part and the speed of the ram press were investigated. In order to have a more accurate analysis and to establish the governing relationships in the influence of the effective factors using Deform-3D software, simulations were carried out with the prevailing conditions and compared with the results of laboratory tests. The results showed that the most favorable result in terms of the highest percentage of sphericity, the highest amount of hardness and the smallest grain size is obtained at the loading rate of 8 ml/s, surface length of 400 mm, surface angle of 40? and loading temperature of C?625. In this condition, the percentage of sphericity and grain size are about 77% and 76 µm, respectively, and the hardness value is about HB80. Also, the mutual effect of the parameters has a greater effect on the output values ??than the effect of each parameter separately, among which the mutual effect of the slope length parameter and loading rate is about 40% and the mutual effect of the parameters of loading temperature, slope length and loading rate is the most effective at about 17%. At the same time, due to the reduction of impurity and porosity in the microstructure, the malleability and strength of the alloy increases by 17.5% and 28%, respectively, by using a protective atmosphere. Comparing the results of experimental tests with simulation results also shows that in order to have a suitable microstructure, if the solid fraction of the output slurry is between 30 and 35%, the cutting rate and turbulence energy should be increased as much as possible. At the same time, the simulation performed in the semi-solid forging section and the presentation of parameters dependent on the software, well approximates the silane alloy in the semi-solid state and shows a good agreement with the results of the laboratory tests. Also, the results showed that by increasing the mold temperature from 25°C to 450°C, the amount of press tonnage decreases by 21%. Also, a higher mold temperature causes a coarser and more heterogeneous microstructure in the alloy, which has caused an approximate 13% decrease in hardness in the samples. By increasing the temperature of the piece and the duration of storage at that temperature, the growth of seeds happened due to the phenomenon of ripening. The results show that the size of grains increases by 5% for a storage time of 5 min, by increasing the temperature from 570 to 600 °C, and by 74% at a temperature of 570 °C by increasing the storage time from 5 to 30 min..

Key words: semi-solid forming, design of experiments, laboratory test, cooling ramp, semi-solid ingot forging process

1-1-Introduction

Semi processing Solidification is a process of preparing metals and alloys that has developed rapidly in the last few years. In this process, the desired alloy is first melted under controlled conditions, then it enters the shear stress melt in its freezing range. The application of shear stress in the two-phase region leads to the destruction of the horn structure (dendritic [1]) and as a result, a liquid-solid mixture [2] can be transformed into pieces with a non-dendritic structure [1].

As a simple definition, the semi-solid microstructure includes primary solid phases that have a non-dendritic and almost spherical morphology and are surrounded by a eutectic context [2]. One of the important features of metal forming processes in the semi-solid state is less porosity and the ability to produce parts with complex shapes. As shown in Figure 1-1, this process is in the middle of the two processes of casting and forging from the point of view of the working temperature range. In other words, the working temperature in this process is lower than casting and higher than forging.

= Among the main disadvantages of casting, the following can be mentioned [3]:

1- The presence of gas cavities due to the high solubility of gas in the melt at high temperature

2- The creation of contraction cavities, that is, the formation of branches of solid metal in fields of molten metal known as dendrites. These branches increase the viscosity of the melt and prevent the feeding and filling of the holes. Both of the above factors lower the quality of the produced part. Ironing also has limitations as follows [3]: 1- Inability to produce parts. Complicated

    2-Overlapping[3] of the walls of the parts

    3- The need for high-tonnage presses and as a result the production cost increases

Forming processes in the semi-solid state are actually aimed at removing the limitations of the two mentioned methods. The production of parts with this process has received attention since about 30 years ago due to the useful properties they have shown [4].

1-1-1-Methods for the production of molten and solid mixtures

The raw material input to the process and its preparation method as well as how these materials are formed are the most important key characteristics in order to understand semi-solid methods. In these processes, a mixture consisting of non-dendritic solid particles dispersed in the molten metal phase is used as the starting material.

Generally, the methods of producing semi-solid slurry are divided into two categories: turbulent and non-turbulent (thermal). The methods of mechanical stirring [4], magnetic stirring [5], inclined surface [6], ultrasonic operation [7], cooling roller and refractory balls can be counted among the types of turbulent methods. Spraying methods [8], semi-solid reuxet [9] and new reuxet [10] are types of non-turbulent methods.

Given that the cooling ramp method is used in this dissertation, hence, a more comprehensive explanation of this method will follow. The use of a cooling ramp is one of the simplest and at the same time the most interesting innovative methods for producing a solid-melt mixture and finally producing a spherical microstructure. It is necessary and interesting to explain this method because it precisely incorporates the concept of the speed of angular deformation (shear rate) as well as the concept of Newton's law of viscosity. This method is one of the new methods of producing parts through the semi-solid process and is used for the production of thixoxed ingots and reoxed parts [4]. Incline casting involves melting the alloy in a suitable furnace, such as an induction furnace, and then slowly cooling it to a certain temperature, just above the melt line of the alloy. In order to provide a specific solid fraction at the end of the inclined surface, the discharge temperature is determined. The melt with the minimum super-melt temperature is poured on the ramp, which is usually made of the same metal as the melt. The inclined surface usually has an angle between 30 and 60 degrees with respect to the horizon line. Sometimes the ramp is cooled by circulating water on its underside.