Papers by Keyword: Multiple Forging

Abstract: The AlMgSi1 alloy is generally used in automotive industry owing to its excellent mechanical properties, which can be further improved by applying severe plastic deformation and heat treatment. The dislocation density in the material increases significantly during severe plastic deformation due to the characteristic intensive shear strain. Therefore the motion of dislocations becomes more and more retarded, consequently the strength improves. In addition, the motion of dislocations can be prevented by aging due to formation of coherent precipitations in the metal matrix in order to realize further increasing in strength. In this paper the combined effect of severe plastic deformation and artificial aging treatment on the evolution of mechanical properties was investigated. The samples were subjected to multiple forging (MF) process at room and enhanced temperature. One part of the deformed samples were heat treated at 150°C for different times. The deformed as well as deformed and heat treated samples were investigated by micro hardness testing and X-ray profile analysis.

Abstract: This work studies the effect of multiple forging (MF), followed by cold rolling, on the tensile and bending behavior of Al 7075 alloy. The raw material was received as a rod shape. The specimens were subsequently cut and annealed at 450 °C for 30 minutes. The cylindrical specimens were subjected to MF at 250°C for three passes, then the bulk of multiple forging specimens were faced rolling at room temperature. Three different base materials were tested, raw material (as received then annealed), MF material, and rolled sheet. Hardness test, tensile test, and bending test were carried out in order to measure the mechanical properties, which are affected by the MF and cold rolling. It has been proved that the MF base specimens have higher tensile strength and better maximum elongation comparing with the raw material specimens. Rolled sheets specimens have got the highest tensile strength and lowest ductility. The result of bending test was similar to the tensile test results, beside these mechanical tests some metallographic samples were performed to help finding an explanation of this properties change.

Abstract: The dynamic process of grain evolution in a Super304H austenitic stainless steel was studied in compression tests. The tests were carried out to a strain of 0.7 at temperatures ranging from 700 to 1000°C and strain rate of 10^-3s^-1. In addition to single pass compression the multiple compressions with changing the loading direction in 90^o and decreasing the temperature with step of 100°C from 1000 to 700°C in each pass were utilized to achieve large cumulative strains. Under multiple compression the values of flow stresses were lower than those at single-pass compressions under the same temperatures. The fraction of dynamically recrystallized grains decreased from 1.0 to almost zero with decreasing temperature in single-pass compressions. On the other hand, almost fully recrystallized structure developed under conditions of multiple compressions. The size of dynamically recrystallized grains decreased with decreasing the deformation temperature, approaching a submicrometer scale level at 700°C. The relationship between the deformation conditions and operating mechanisms of dynamic recrystallization is discussed in some details.

Abstract: The dynamic process of grain evolution in an S304H-type austenitic stainless steel was studied in multiple forging tests at temperatures of 500°C, 600°C and 700°C. The deformation microstructure with a grain size of about 100 to 400 nm resulted from continuous dynamic recrystallization. The size of new grains and the recrystallization kinetics decreased with decreasing the deformation temperature. The dynamically equilibrium grain size evolved at large strains followed a power law function of the flow stress with a grain size exponent of about-0.2. The formation of new fine grains was assisted by dynamic recovery, which leads to an apparent steady state flow at large total strains.

Abstract: The deformation behavior and the microstructure evolution in a 304-type austenitic stainless steel were studied in multiple forging tests at temperature of 700°C. The flow stresses increased to its maximum value with straining to about 1 and, then, slightly decreased resulting in a steady state deformation behavior at strains above 3. The structural changes were characterized by the development of a spatial net of deformation subboundaries, the misorientations of which increased to the values typical of conventional grain boundaries. The number of ultrafine grains increased with straining, leading to development of submicrocrystalline structure. The fraction of submicrocrystalline structure composed of ultrafine grains with an average size of about 300 nm exceeded 0.7 after straining to 2.

Abstract: Deformation behavior and structural changes were studied in a 304-type austenitic stainless steel subjected to large strain multiple forging at an ambient temperature. The number of forging passes was 10, leading to the total cumulative strain of 4.0. The yield stress rapidly increased to about 1000 MPa after the first forging pass and then gradually approached a saturation level of about 2000 MPa in large strains. The grain/subgrain size decreased to about 50 nm at total strain of about 2. This grain/subgrain size reduced a little upon further processing; and comprised 35 nm after a total strain of 4.0. The fast kinetics for grain refinement was associated with deformation twinning and strain-induced martensitic transformation. The both of them resulted in fast grain subdivision at relatively small strains.

Abstract: Ultrafine-grained treatment (multiple forging and heat treatment) were carried out on FGH4096 alloy to get ultrafine grain with grain being of 1-5μm. OM, SEM, TEM and tensile test were used to study the action of ultrafine-grained treatment on improving structure and properties. The tensile properties of the alloy processed under ultrafine-grained treated have shown quite satisfactory levels, which ultimate strength (UTS) reach 1730Mpa and yield strength (YS) values reach 1470Mpa at room temperature. The apperance of advanced strength has originated chiefly form not only the persence of fine grains, but also structure of grain boundaries and diaperce pricipitations of secondary phase. A better combination of mechanical properties achieved by use of ultrafine-grained treatment makes it possible to explore the dual property turbine disc to its maximum potential for FGH 4096 alloy.

Abstract: The influence of severe thermomechanical treatment via multiple forging on the formation of a nanocrystalline (NC) structure in bulk samples of Alloy 718 and ATI 718Plus has been investigated. It was observed that a step-wise decrease of processing temperature from 950 down to 575°C allowed the refinement of the initial coarse grain structure to a NC state. Investigations of structural changes in the deformed samples have shown that extending the temperature interval of dynamic recrystallization to low homologous temperatures resulted in the formation of a fine-grained recrystallized structure. The temperature of NC structure formation in ATI Alloy 718Plus was 50-100°С higher than that required for alloy 718. This was due to the presence of the additional γ′-phase, which increased the recrystallization temperature. This decreased the total strain required to produce NC structure, as compared with Alloy 718. It was observed that increasing the total strain and decreasing temperatures step-wise during deformation via multiple forging resulted in a uniform structure across the cross-section of the samples. The room temperature mechanical properties of the investigated alloys with various grain sizes are also compared.

Abstract: Two relatively simple schemes are described for the interactions of grain deformations during large plastic deformations with the aim of evaluating their influence on texture development. The stress transfer model basically assumes that there is some degree of stress transfer across the boundaries proportional to the boundary area. The reduced stress incompatibility model minimizes the stress incompatibilities between each grain and their surrounding grains These models assume 3D topological schemes using evolving truncated octahedra for the spatial distributions of the grains. They are applied to the cases of hot rolled and cross forged Al alloys. Both give quite similar predictions for texture development which are moderate improvements on the Taylor models, confirming that the incorporation of grain interaction effects can be useful for texture modeling without major modifications. Moreover, they can yield interesting results for local orientation effects and their influence on orientation stability; an example of cube grains hot rolled in different crystallographic surroundings is also treated.

Abstract: Multiple forging (MF) can be used to attain large plastic strains in bulk alloys by successive forging along three orthogonal directions to retain the initial sample shape. An original multiple forging technique enabling 3-D cross forging at constant temperature up to 500°C has been applied to two Al alloys (Al-1%Mn and Al-3%Mg-Sc,Zr). Their rheology, texture and microstructure evolution are compared with those obtained in plane strain compression (PSC). The results are interpreted in terms of slip activity behaviour during both deformation modes. They can also be correlated with the contributions of free dislocations and sub-boundaries.