Mateusz Kopyściański

A Coupled Thermal/Material Flow Model of Friction Stir Welding Applied to Sc‐Modified Aluminum Alloys

A coupled thermal/material flow model of friction stir welding was developed and applied to the joining of Sc-modified aluminum alloy (7042-T6) extrusions. The model reveals that surface material is pulled from the retreating side into the weld zone where it is interleaved with in situ material. Due to frictional contact with the shoulder, the surface material is hotter than the in situ material, so that the final weld microstructure is composed of bands of material with different temperature histories. For this alloy and the associated FSW heating rates, secondary phase dissolution/precipitation temperatures are in proximity to the welding temperatures. Therefore, depending on the surface and in situ material temperatures in relation to these transformation temperatures, disparate precipitate distributions can develop in the bands of material comprising the weld nugget. Based on the numerical simulation and on thermal analysis data from differential scanning calorimetry, a mechanism for the formation of onion rings within the weld zone is presented.

Microstructure and Mechanical Properties of Friction Stir Welded 5083 and 7075 Aluminum Alloys

Through microscopy, mechanical testing, and numerical modeling, the microstructure and mechanical performance of friction stir welded aluminum alloys 7075-T651 and 5083-H111 were characterized. In particular, the influence of the weld configuration, i.e., the locations of the 7075 and 5083 alloys alternately on the advancing and retreating sides, on material flow, microstructure, and mechanical properties was considered. Thermographic data in conjunction with a process simulation demonstrated that the weld configuration significantly impacts heat generation during friction stir welding. The microstructure in the stir zone was a clear visualization of the material flow and was characterized by a vortex-like structure with alternating bands of the alloys being joined. These bands differed in elemental content and grain size. The microstructure became more complex when greater heat generation (higher temperatures) occurred. The weld configuration strongly influenced the material flow, but did not impact the tensile properties (such as yield strength, tensile strength, and elongation). The configuration of 5083 on the advancing side and 7075 on the retreating side produced the most uniform material flow. The joint efficiencies of all tested welds were above 100%.

Friction Stir Welding of Dissimilar Aluminum Alloys

Dissimilar aluminum alloy plates of 2017A-T451 and 7075-T651 with 6 mm thickness were friction stir butt welded. Numerous trials were conducted to determine the conditions that produce the highest weld quality. These parameters were found to be a welding speed of 112 mm/min, a rotation speed of 355 rev/min and a vertical force of 32,8 kN. The weldability and blending of the two materials were evaluated by using macro- and microstructural analysis as well as EDS mapping to show the distribution of main alloying elements within the weld. The effect of material locations, either on the advancing or retreating sides, on the microstructure and mechanical properties was also investigated. Hardness profiles differ substantially for different weld configurations. Regardless of the position of a particular alloy, the weld microstructure was composed of alternating layers of both materials. However, the layers of the 7075 alloy always exhibited smaller grain size and a larger number of secondary phase particles.

Friction Stir Processing Multi-Run Modification of Cast Aluminum Alloy

The effect of multi-run FSP modification of cast aluminum alloy AlSi9Mg are presented. The relationship between the number of trials and microstructures are shown. FSP process was performed on the typical milling machine specifically adopted for the processing trials. The microstructure was examined by light as well as scanning and transmission electron microscopy. The studies have shown that the multi-run FSP process causes decrease of the grain size and increase of the homogeneity of the microstructure. In contrast to the cast condition, the microstructure in the processed material was characterized by a relatively uniform distribution of the second phase particles. The size and aspect ratio of these particles decreased significantly. Application of FSP process resulted in a decrease of the porosity in the modified material. The modified materials achieved at perpendicular runs can be characterized by the higher dislocation density that obtained at parallel ones. The multi-run FSP process caused increase the elongation and ultimate tensile strength of modified material in comparison to properties of the cast aluminum alloy.

Electron Microscopy Characterization of Friction Stir Welded 5083-H111 and 7075-T651 Aluminum Alloys

The microstructural characterization of butt friction stir welds of two different wrought aluminum alloys (work-hardened and heat treated) were studied. The detailed studies on the FSW process of dissimilar Al alloys are limited. In particular, the weld microstructure requires deeper characterization to better understand the phenomena occurring during mixing of dissimilar alloys.The characterization of friction stir welds was performed by scanning electron microscopy (an energy dispersive spectroscopy and an electron backscattered diffraction) and transmission electron microscopy. The dissimilar weld microstructure is complex, resembling a vortex-like structure. The microstructure of weld was highly asymmetrical with regard to the weld centerline. The research revealed a change in grain size in particular areas of the stirred zone. Recrystallization in the stirredzone occurred in particular areas in an irregular manner.

Modeling, microstructure, and mechanical properties of dissimilar 2017A and 5083 aluminum alloys friction stir welds:

Dissimilar aluminum alloy plates of 2017A-T451 and 5083-H111 were friction stir welded in a butt joint configuration along the longitudinal direction. Welding trials demonstrated that placing 5083 on the advancing side enhanced material flow and consequently formed a larger weld nugget. Numerical simulation supported this observation through analysis of volumetric flow rates through reference planes surrounding the stir zone. The analysis also suggests that the weld configuration that results in a decreasing temperature-dependent flow stress in the weldment from the leading edge of the tool to the trailing edge will maximize material flow in dissimilar friction stir welding welds. The decreasing flow stress promotes material flow along the retreating side of the tool as flow conditions necessarily become easier from the front to the back. Regardless of its position during welding, however, 2017A alloy dominated the nugget region. In either weld configuration, alternating bands of 2017A and 5083 with similar grain sizes (approximately 10 µm) comprised the weld microstructure. Within the nugget, numerous second-phase particles as well as dislocations occurring as single dislocations or in the form of dislocation tangles or walls (low angle grain boundaries) were present. The relatively high dislocation density observed in both alloys suggested that recrystallization was incomplete. Hardness mapping revealed an asymmetric variation of hardness across the weld centerline that strictly corresponded to the distribution of particular alloys within the nugget. During tensile testing, the AS 5083-RS 2017A configuration failed under ductile shear rupture occurring in the base 5083 material far from the weld. For the opposite configuration, the tensile samples ruptured perpendicular to the load axis exactly on the border between the nugget and the thermomechanically affected zone on the 2017A alloy side.

Electron Beam Welding of High Strength Quenched and Tempered Steel

The paper deals with the investigation of microstructure and mechanical properties of electron beam welded joints of high strength steel grades S690QL and S960QL in quenched and tempered condition. The microstructure of base metal was composed of bainite and martensite mixture at hardness about 270HV10 and 340HV10 for the S690QL and S960QL steels, respectively. The weldment was composed of several characteristic subzones revealed on the transverse sections. The central region of the weldment consisted mainly of coarse columnar dendritic grains which perpendicular to the fusion zone boundary. The microstructure of the heataffected zone near the fusion line consisted mainly of martensite, however, in both steels the microstructure varied with the distance from the fusion line. The tensile strengths of welded joints were Rm=850 MPa (S690QL) and 1074MPa (S960QL) and corresponded the tensile strengths of the base materials.

Electron microscopy investigation of a cast AlSi9Mg aluminum alloy subjected to friction stir processing with overlapping passes

Abstract The effect on microstructure of overlapping multiple pass friction stir processing (FSP) of an AlSi9Mg cast aluminum alloy was investigated. The microstructure was examined by means of scanning and transmission electron microscopy. The studies show that the multiple pass FSP resulted in the elimination of porosity, the refinement of second phase particles and the decrease of grain size within the processed material surface. Electron back scattering diffraction analysis showed that high angle grain boundaries prevailed in the processed material. However, relatively high dislocation density revealed by transmission electron microscopy indicated that the recrystallization was not completed. The investigation showed that the FSP technique may be successfully used for producing a homogeneous near-surface layer in a cast aluminum alloy.

Microstructure of Friction Stir Welded AlSi9Mg Cast with 5083 and 2017A Wrought Aluminum Alloys

Wrought aluminum alloys 5083 and 2017A were each joined with cast aluminum alloy AlSi9Mg through friction stir welding in butt weld configurations. For each material system, the wrought and cast alloy positions, i.e., the advancing side or the retreating side, were exchanged between welding trials. The produced weldments were free from cracks and discontinuities. For each alloy configuration, a well-defined nugget comprised of alternating bands of the welded alloys characterized the microstructure. The degree of mixing, however, strongly depended on which wrought alloy was present and on its position during processing. In all cases, the cast AlSi9Mg alloy dominated the weld center regardless of its position during welding. Electron backscattered diffraction analysis showed that the grain size in both alloys (bands) constituting the nugget was similar and that the majority of grain boundaries exhibited a high angle character (20°-60°). Regardless of the alloy, however, all grains were elongated along the direction of the material plastic flow during welding. A numerical simulation of the joining process visualized the material flow patterns and temperature distribution and helped to rationalize the microstructural observations. The hardness profiles across the weld reflected the microstructure formed during welding and correlated well with the temperature changes predicted by the numerical model. Tensile specimens consistently fractured in the cast alloy near the weld nugget.

The Influence of Microstructure of Medium Carbon Heat-Treatable Steel on its Tribological Properties

The hereby work presents the influence of microstructure on the tribological properties of medium carbon heat-treatable steel. These steel is used for various elements of which in addition to high wear resistance a high plasticity is required. For tested steel there different heat treatments were performed in order to obtain the different microstructure and properties. Each sample was hardened from the same temperature, which has been selected on the basis of standards for these steel and it was equal 890 °C. Two samples were quenched after austenitising in water. After quenching one of the samples was cooled in liquid nitrogen for 30 minutes (sub-zero treatment). After that these two samples were tempered in the preheated furnace at 200 °C for 2 hours. The third sample immediately after austenitising was quenched in mixture of molten salts heated to 200 °C (fused salt quenching). After these three carried out heat treatments the various microstructures were obtained with preserved high hardness. After heat treatment samples for the tribological test were prepared. The test was performed using the T-05 type roll-block test machine. This type of test machine is used to obtain the resistance to friction wear of metals and others. Thanks to these tribological tests and mass loss measurements the best heat treatment, which significantly increase of the wear resistance for tested steels was indicated.