The investigation on friction stir process applicability of automotive grade steel sheets: deformation behaviour, microstructural and mechanical properties

Today, environmental awareness and sustainability, which develops day by day, direct car manufacturers to develop vehicles that are lighter, thus consuming less fuel. To this end, weight reduction of structural elements in modern automobile chassis has become one of the important design goals. Achieving this goal requires manufacturing of the body components from materials with both high strength and high ductility and toughness. To meet these requirements, a new generation advanced high strength steels (AHSS) was developed and introduced to the service of automotive manufacturers. Since the specific strength values of these steels are higher than that of the conventional ones, they provide effective solutions for reducing the weight of the body components. On the other hand, the increasing expectations of high performance and high safety from modern automobiles require higher levels of mechanical behavior to be exhibited by the materials used. One of the important approaches that can contribute to the mechanical performance of AHSS can be regarded as application of innovative strength enhancing processes. In this thesis, friction stir processing (FSP) was applied to Dual Phase (DP) 600 and Transform Induced Plasticity (TRIP) 780 which represents frequently used steel grades in the automotive industry in recent years. Effects of FSP on the microstructural properties of steel sheets and the deformation behaviors under static and cyclic loading were investigated. Within the scope of the structural analyzes, the phase transformations, grain refinement mechanism, material flow characteristics of the deformation zones formed by the thermo-mechanical effects during the application of the FSP were evaluated. Hardness measurements were carried out to examine the effects of variations in structural properties on mechanical properties. After FSP, room temperature tensile tests were performed to determine the deformation behavior, strength and ductility properties of the steels. The uniaxial fatigue tests were used to determine the crack initiation and crack propagation behaviors of FSPed steel under the cyclic loads. Specimens in the form of 1.1 mm thick sheet made of DP 600 steel were subjected to FSP using a WC tool having a diameter of 14 mm and a conical pin with a diameter and length of 5 mm and 0.8 mm, respectively. Tool rotation speed and processing speed was set at 1000 rpm and 1.6 mm/s, respectively. Specimens in the form of 1.95 mm thick sheet made of TRIP 780 steel were subjected to FSP using a WC tool having a diameter of 16 mm and a conical pin with a diameter and length of 6 mm and 0.8 mm, respectively. Tool rotation speed and processing speed was set at 1000 rpm and 1.3 mm/s, respectively. For both specimens the shoulder tilt angle was 3°, and the tool plunge depth were kept constant at 0.1 mm. Processing temperature was determined with a thermal camera system. Scanning electron microscope (SEM) and optical microscope (OM) were used to observe the microstructure of steel samples before and after FSP. Hardness measurements were performed using a Vickers micro-hardness tester under a load of 500 g and for 10 s dwell time. Tensile properties were determined on dog bone-shaped specimens with gauge section of 6 mm 12 mm at a strain rate of 0.001 s-1. The tensile axis of the samples was oriented parallel to the processing direction. Also, with tensile test data, strain hardening coefficient, strain hardening exponent and anisotropy coefficient at ultimate tensile strength was calculated. Fatigue tests were performed on servo-hydraulic machine at a frequency of 15 Hz during repeated tension at a cycle asymmetry R=0 and 106 loading cycles. As a result of the experimental work carried out, it has been understood that the FSP process can be applied to DP 600 and TRIP 780 steels without causing macro damage, cracking or deformation discontinuities. FSP process increased initial hardness of the DP 600 and TRIP 780 steel from 178 Hv and 250 Hv to 315 Hv and 490 Hv respectively. The DP 600 and TRIP 780 steels are one of the members of a steel grade that mainly designed to balance of high strength and good formability properties. DP 600 steel reflected yield strength of about 300 MPa and reached to UTS of about 620 MPa with uniform elongation of 21 % and fractured after a total elongation of 34 % elongation in its as-received condition. After FSP, it was observed that the yield strength increased to about 811 MPa and the tensile strength value reached to 1053 MPa. This effective strength enhancement brought about an acceptable decrease in ductility values of the DP 600 steel resulting in uniform elongation and elongation to failure of 6.3 % and 13.0 %, respectively. Based ductility values obtained, it can be considered that, FSPed DP 600 shows a deformation behavior that mostly dominated by the strain hardening. FSP also strongly enhanced steel yield strength and tensile strength of TRIP 780 steel from 420 MPa and 820 MPa to 1120 MPa to 1470 MPa, respectively. However, uniform elongation and elongation to failure values contracted from 24 % and 36 % to 10.1 % and 21.8 %, respectively. Static strength enhancement obtained by FSP of DP 600 steel also yielded a favorable effect on the fatigue behavior and stress level leading to transition to the infinite life. As a result of the fatigue tests, it was determined that the fatigue limit of the as-received DP 600 steel increased from 350 MPa to 480 MPa after the applied FSP process. Based on SEM examinations of the fracture surfaces that obtained after fatigue testing applied at different levels, FSP was found ineffective on the morphological properties of crack propagation and sudden break regions. FSP process applied to the TRIP 780 steel slightly decreased as-received fatigue limit of 420 MPa to 320 MPa. Such a decrease in fatigue life mainly attributed to low crack initiation/propagation resistance of the martesitic structure that formed via triggering of transformation of retained austenite during plastic deformation imposed by FSP. Experimental results obtained in the study mainly indicate that, friction stir processing is an easy to apply and practical procedure which provides significant enhancement on the mechanical performance of automotive high strength steels under both static and cyclic loading conditions.