Study On Preparation And Properties Of Skin And Loading Frame Aluminum Alloy Profiles For Railway Transportation

This paper is a part of work of Project-Processing of "Ultrathin and Ultrawide Aluminium Alloys Structural Member with High Quality"（code:2013AA032402）supported by the National High-tech Research and Development Program of China（863）,aiming at processing a new generation structural profiles used in railway transportation field,such as fast train bottom loading frame and skin.A hollow rectangular tube of bottom loading frame with 7N01 alloy designed by Sifang Qingdao Company is the first product of this work.A set of producing technology of 7N01 aluminum bottom loading frame profile for railway transportation was finished by confirming technical parameters of DC casting process by numerical simulation based on ANSYS finite element software,designing and modifying the die design with the help of Hyperxtrude software and adjusting welding process to decrease the sensitive of stress-corrosion and increase the fatigue property.The producing technology and properties of 7N01 alloy bottom loading frame are as followings:The four distribution ports were used in the die to avoid the coarse grain size on the profile surface,to improve the exit temperature and temperature homogeneity at the exit,to improve the metal welding quality in the die chamber and also increase the die life.The best extrusion process for this profile is that billet temperature 510-485℃ at the billet head end,extrusion speed 1.4-1.5m/min with fan quenching.The properties of the bottom loading frame profile is R_p0.2=370MPa,R_m=406MPa,A=15.6%after aging at 95℃×8h+150℃×8h.And the strength at the welding joint area of the extrusion profiles is higher than 70%of profile matrix.The fatigue property also meets the requirement.The reasonable welding process for 7N01 loading frame is that current,voltage,welding speed and heat input is 210～220A,23.8V,8.5～10.2mm/s and 0.401～0.493KJ/mm respectively.The tensile strength at the welding joint of the 7N01-T6 profile increases a little with the decrease of circumstance temperature.The property of the profile at the welding joint area is R_p0.2=279MPa,R_m=300MPa,A=17.5%and the medium fatigue durability value is 119MPa at-35℃.There is little change in the bending property with the decrease of circumstance temperature.The impact absorbing energy of profile along the traversal and longitude direction with U shape samples is 43.9J and 76.9J respectively,which is better than that of the same kind of profile made in Japan.6N01 alloy skin profile of the fast train is difficult to fabricate because it is an ultrathin and ultrawide profile with big width/thickness ratio.Hyperxtrude software was used to design and adjust the die configuration and extrusion technology to solve the problem of waving or big grain size at some specified positions.Finally,welding process was modified as well to decrease the sensitive of the stress corrosion and to improve the fatigue strength of the profiles.Then the fabrication process of 6N01 skin profile was achieved as followings:The bearing zone with various lengths was designed and strengthening ribs&anti-flexibility slot was added in the die.In addition,dummy block dimension was changed for this case to make metal flow well-distributed so that the profile dimension accuracy can be ensured.The extrusion process for 6N01-T6 profile is that billet temperature 520-500℃ for the billet head end,extrusion speed 2.0-2.2m/min with fan quenching.The properties of skin profile is R_p0.2=255MPa,R_m=300MPa,A=13%after aging at 175℃×8h.And the mechanical property of the skin profile is up to R_p0.2=268MPa,R_m=331MPa,A=14%and the medium fatigue durability value is 78MPa at-35℃.All the testing results of mechanical property,bending capability and micro structures can meet the requirement at room temperature and low temperature.The impact testing results showed that the impact absorbing along the traversal direction with U shape samples energy is 24.6J.And the strength of 6N01-T6 skin profile at the welding joint area is higher than 60%matrix materials.The reasonable welding parameters are that current,voltage,welding speed and heat input is 130-150A,19.2～21.1V,9.7mm/s and 0.29～0.35KJ/mm respectively.The mechanical property,fatigue property at different circumstance temperatures,such as room temperature,-20℃、-35℃ and-50℃ were measured.The mechanical property of 6N01 profile at the welding joint at-35℃ is R_p0.2=160.5MPa,R_m=197MPa,A=14%and the the medium fatigue durability value is 78MPa.The tensile at the welding joint area of 6N01-T6 skin profile increased and fatigue property didn’t change with the decrease of circumstance temperature.Because of low temperature in winter at northeast region of China,it’s more critical for the high speed railway train material serviced at this climate condition,which required some special properties such as strength fatigue and impact toughness at low-temperature.These requirements were met by optimization of the chemical composition of the alloys,improvements of extrusion and heat treatment technology of the profiles.