Study On The Welding Microstructure And Mechanical Property Of 6063 Aluminum Alloy Porthole Die Extrusion

Currently,most of research works mainly focused on the prediction of the bonding degree of welding seams.In fact,even though the atomic bonding is realized on a bonding interface,the metal in the welding zone usually has the particular microstructure different from the other zones since it experiences the special thermal and mechanical history.The mechanical properties of the metal in the welding zone are usually different from those in the other zones.Therefore,further investigation is of great significance.In addition,many researchers has been focused on the effect of aging process on precipitation hardening of Al-Mg-Si alloy and achieved significant research results.However,research about the effect of aging process on the welding strength of porthole die extrusions is not enough.Based on the above two points,the present study is mainly focused on the research of the effect of ram speed and aging time on the microstructures and mechanical properties of 6063 aluminum alloy porthole die extrusions.The dynamic recrystallization behaviors under different ram speeds and precipitation hardening during artificial aging were investigated,which provides a theoretical instruction for practical production.High temperature flow behavior of 6063 aluminum alloy was investigated employing uniaxial compression test on a Gleeble-1500 machine.The expressions of adiabatic correction factor and mechanical work partitioning factor were obtained.An expression for the temperature rise involving these two variable factors was optimized.Based on these results,a constitutive model for 6063 aluminum alloy was derived with strain compensation.And the numerical model was established using the constitutive model with the software of Deform-3D.The results showed that adiabatic correction factor decreased with increasing strain and decreasing strain rate,but it was relatively not sensitive to the temperature.Mechanical work partitioning factor was a function of strain,strain rate and temperature.The exit temperatures simulated with corrected stresses using the optimized expression were more accordant with the experimental data.The maximum temperature rise in the thermal simulation was 44 ?C,resulting in the softening flow stress of 40 MPa.The extrusion pressure and welding pressure firstly increased with the ram speed increasing from 3 mm/s to 9 mm/s and then decreased with the ram speed further increase.The exit temperature increased with the increase of ram speed.Comparisons between the simulated and experimental results confirmed the accuracy and reliability of the finite element simulation model.The effect of ram speed on the welding strength was investigated by means of OM observation,EBSD technology,mechanical testing.Experiments and simulation were conducted to analyze the recrystallization behavior of 6063 aluminum alloy porthole die extrusions.The results showed that the fraction of recrystallization firstly increased and then decreased with the increase ram speed.Increasing the ram speed from 3 mm/s to 7 mm/s caused a decrease in grain size due to dynamic recrystallization.In contrast,at high ram speed,further increased to ram speed had much less effect on the strain rate,leading to an increase in grain size as the increased extrusion temperature dominated the mechanics of grain growth.The dynamic recrystallization fraction in the matrix zone was higher than that in welding zone,leading to the smaller grain size.The larger grain size in the welding zone caused that the mechanical properties was lower than those in the matrix zone.The difference of mechanical property would lead to the local stain concentration during expansion,which was adverse for the improvement of welding quality.And the larger the difference was,the greater local strain concentration was.The expansion ratio firstly decreased and then increased with the increase ram speed.The influence of aging time on the microstructure characteristic and mechanical behavior of 6063 aluminum alloy porthole die extrusion welds was investigated by mechanical testing and TEM.It was found that aging time had a significant effect on the size,morphology and distribution of precipitates and welding strength.The hardness of the extrusion profile increased with the increase aging time due to precipitation strengthening.The hardness in the welding zone was lower than that in the matrix zone.This was attributed to the lower precipitation rate and non-uniform of size and distribution of precipitates,which was influenced by grain size.With the increase of aging time,the expansion ratio decreased sharply due to precipitates in grain interiors,precipitate free zones and coarse precipitations at grain boundaries.After aging 4h,expansion ratio remained almost unchanged and the crack propagated along the weld seam until the profile was broken,indicating a low welding strength.