Fahrettin Ozturk

Tensile and spring-back behavior of DP600 advanced high strength steel at warm temperatures

In recent years, the use of advanced high strength steels in automotive industry has been increased remarkably. Among advanced high strength steels, dual phase (DP) steels have gained a great attention owing to a combination of high strength and good formability. However, high strength usually increases the spring-back behavior of the material, which creates problems for the parts during the assembly. Thus, the uniaxial tensile deformation and spring-back behaviors of DP600 advanced high strength steel were investigated in rolling (0°), diagonal (45°), and transverse (90°) directions in the temperature range from room temperature (RT) to 300 °C. All tests were performed at a deformation speed of 25 mm/min. A V-shaped die (60°) was used for the spring-back measurements. The results indicated that the formability and spring-back of the material were decreased with increasing the temperatures. The material showed complex behaviors in different directions and at different temperatures.

Flow curve prediction of Al-Mg alloys under warm forming conditions at various strain rates by ANN

This paper describes an approach based on artificial neural network (ANN) to identify the material flow curves of strain hardened 5083-H111 and 5754-O Al-Mg alloys at the temperature ranges from room temperature (RT) to 300^oC and a strain rate of 0.0016-0.16s^-^1. The tensile tests were performed to determine the material responses at various temperatures and strain rates. An ANN model was developed to predict the flow curves of the materials in terms of experimental data. The input parameters of the model are strain rate, temperature, and strain while tensile flow stress is the output. A three layer feed-forward network was trained with BFGS (Broyden, Fletcher, Goldfarb, and Shanno) algorithm. The amount of the neurons in the hidden layer was determined by determining of the root mean square error (RMSE) values for each material. Results reveal that the predicted values in the ANN model are in very good agreement with the experimental data. The ANN model, described in this paper, is an efficient quantitative tool to evaluate and predict the deformation behavior of 5083-H111 and 5754-O Al-Mg alloys for tensile test at prescribed deformation conditions.

Strain Hardening and Strain Rate Sensitivity Behaviors of Advanced High Strength Steels

The mechanical properties of commercial dual phase (DP), transformation induced plasticity (TRIP), and high strength low alloy (HSLA-340) steel sheets are investigated and compared at various strain rates ranging from 0.0017 to 0.17 s−1 at ambient temperature. TRIP steel outperforms the other two materials, having comparable ductility and twice as large strength relative to DP steel. TRIP has larger strength and much larger ductility than HSLA-340. The exceuent ductility of TRIP800 is due to its high strain hardening capability, which promotes stable plastic deformation. It is observed that the strain hardening rate in TRIP800 does not decrease to zero at failure, as common in most materials in which failure is preceded by necking.

Effect of Warm Temperature on Springback Compensation of Titanium Sheet

In the present study, the effect of warm temperature on springback compensation of commercially pure grade 2 (CP2) titanium sheet is investigated. The results reveal that the springback is substantially reduced with increasing temperature. The elastic energy and the ratio of tensile strength/yield strength (TS/YS) of the material are measured at various temperatures under tensile loading condition. A gradual decrease of elastic energy is observed with increasing temperature. The change in the ratio of TS/YS is left diminutive with increasing temperature.

Detailed Investigation of Forming Limit Determination Standards for Aluminum Alloys

In this study, experimental studies were conducted to evaluate the differences between the ASTM E2218-02 and ISO 12004-2 standards that are used for construction of the forming limit curve (FLC) and that made various assumptions, which create dissimilar FLCs for the same material. The comparison was made for two materials which have moderate brittle and ductile characteristics, AA2024-T4 and AA5754-O alloys, respectively. The effects of a specimen’s geometry, lubrication condition, and determination methods of limit strains on FLCs were considered and compared. Because the same strain evaluation method should be used for the standards, so as to be able to investigate the effect of only standards, a simple method in the computer grid analysis system was used. To test the validity and the reliability of the method, limit strains on the same specimens were also determined with using a real-time measurement method for the ISO experiments, and the results reveal that the method is reliable. Failure mechanisms were inspected for further investigation. The Nakajima specimens formed with the two standards showed different failure mechanisms. Finally, conducting the case studies, it was concluded that ISO 12004-2 yields more reliable and reproducible results than the ASTM standard.

Assessment of Electrical Resistance Heating for Hot Formability of Ti-6Al-4V Alloy Sheet

Ti-6Al-4V (Ti64) is the most commercially used heat treatable high strength/weight ratio, high corrosion, and thermal resistance alloy in titanium alloys. However, room temperature (RT) formability of this alloy is very poor and springback after forming is very severe due to the high yield strength and low elasticity modulus. In this research, the applicability of electrical resistance heating process which is a new and rapid heating process for hot forming application is investigated in order to improve formability and eliminate springback. The electrical resistance heating method is found to be effective for T64 alloy. Results reveal that the changes in hardness and grain size of the alloy have been found inconsiderable when the method is used. Springback compensation is achieved at high temperatures and springback free part is almost produced.

Evaluation of tensile behaviour of 5754 aluminium–magnesium alloy at cold and warm temperatures

Abstract Cost effective lightweight vehicles have recently become a key target for automotive manufactures due to increasing concerns about minimising environmental impact and maximising fuel economy without sacrificing the vehicle performance and comfort. Aluminium–magnesium alloys are the most important candidate materials due to their excellent high strength to weight ratio, good formability, good corrosion resistance and recycling potential. Although their deformations leave undesirable traces on the surface of the final product and their formability are not very favorable at room temperatures, the formability can be improved by changing the forming temperature to either cryogenic or warm. In this study, uniaxial tensile deformation behaviour of 5754 aluminium–magnesium alloy sheet was extensively studied at cold (–60 to 0°C) and warm (room to 250°C) forming temperatures and a strain rate range of 0·0016–0·042 s–1. Results indicate that the formability of this material at cold and warm temperatures is better than at room temperature. Stretching lines were eliminated at both cold and warm temperatures especially above 175°C and 0·0016 s–1 strain rate. The most suitable forming conditions were obtained at 250°C and 0·0083 s–1 or 175°C and 0·0016 s–1.

Effects of Pre-strain and Temperature on Bake Hardening of TWIP900CR Steel

The effects of pre-strain and baking temperature on bake hardening behaviour of TWIP900CR steel were investigated. The results reveal that the bake hardening process contributes to an increase in yield strength up to 65 MPa at the baking temperature of 200 °C. The difference in yield strength between baking temperatures of 170 and 200 °C is almost insignificant. It is clearly observed that baking at a high temperature does not result in a significant increase in yield strength. For a reasonable bake hardening, a good combination of pre-strain and baking temperature is necessary. Besides, the toughness of the material is found to decrease with increasing pre-strain.

Investigation of tensile strength of hydroxyapatite with various porosities by diametral strength test

Abstract It is appropriate to administer the diametral test to biomedical materials used in dental applications because stresses formed on dental implants are similar to those that formed in this test. To show this similarity, an experimental study of diametral strength testing of hydroxyapatite was performed. The influence of porosity on hydroxyapatite was investigated experimentally to determine how the diametral strength was affected. Hydroxyapatite was air sintered at 1100°C for 1 h with porosities ranging from 1 to 32%. The results indicated that hydroxyapatite with improved densification had higher diametral strength values. X-ray diffraction analysis showed that sintered samples were pure hydroxyapatite.

Tensile deformation behavior of AA5083-H111 at cold and warm temperatures

Abstract The effects of strain rate and temperature on the deformation behavior of hardened 5083-H111 aluminum magnesium alloy sheet were investigated by performing uniaxial tensile tests at various strain rates from 0.0083 to 0.16 s−1 and temperatures from – 100 to 300 °C. Results from the prescribed test ranges indicate that the formability of this material at cold and warm temperatures is better than at room temperature. The improvement in formability at cold temperatures is principally due to the strain hardening of the material. However, the improvement at warm temperature and low strain rate is specifically due to the high strain rate sensitivity characteristic of the material. Results indicate that this alloy should be formed at temperatures higher than 200 °C and at low strain rates.