8.5 Dynamic Dilution Behavior of Moving Weld Pools …
231
Figure 8.25 shows the calculated flow and heat transfer characteristics of quasisteady moving weld pool. From Fig. 8.25a, it can be seen that after the moving
weld pool reaches the quasi-steady state, the width of the moving weld pool is about
1.8 mm and the length of the weld pool is about 4.5 mm. The length–width ratio
is in good agreement with the length–width ratio, observed in general tests, of the
weld pool obtained by low-speed deep penetration laser welding; in addition, the
temperature distribution on the upper surface of the weld pool is close to the motion
Gaussian distribution, which is consistent with the usual welding experience. As can
be seen from the results of Fig. 8.25c–f, the absolute value of velocity near both
sides of the keyhole in the weld pool is large. In addition, the pressure gradient near
the front and the rear of the keyhole is large, and thus the value of velocity is also
large. Although it is very difficult to directly verify these theoretical results, the above
results are basically reasonable from practical experience. Figure 8.25g and h show
the results of the flow field inside the weld joint and inside the longitudinal section
of the weld pool, respectively. By comparing the simulation results based on the
coupling of the transient keyhole and the weld pool in Chap. 4, it can be seen that
the results of the flow field in the upper part of the moving weld pool are basically
consistent with the transient simulation results.
8.5.2.2 Dynamic Dilution Process of Wire Compositions
in Quasi-Steady Moving Weld Pool
According to the analysis results in Sect. 8.5.2.1, based on the velocity trend of the
fluid in the weld pool obtained from the quasi-steady-state model established in this
chapter, the results of the flow field in the upper part of the quasi-steady moving weld
pool are of practical significance. Therefore, in this section, the dynamic dilution
behavior of the wire compositions in the upper part of the quasi-steady moving weld
pool is mainly studied.
In order to study the dynamic dilution behavior of chemical compositions of the
wire in the upper part of the quasi-steady moving weld pool, in this section, the
wire alloy, Si element, Ti element and Al element are respectively used as research
objects, and their specific percentage content and characteristics are shown in Table
8.12 (where being diluted by parent metal indicates that the chemical composition of
an element in the wire is higher than that in the parent metal, so the wire is diluted by
parent metal; that the parent metal is diluted indicates that the chemical composition
of an element in the wire is lower than that in the parent metal, so the parent metal
is diluted).
According to the boundary conditions in Sect. 8.5.1, the wire initial boundary
conditions shown in Fig. 8.26a are adopted in the study: in the red zone, the concentration of the wire composition is set to be 1.0 (red zone), while that in other positions
is 0. After the mathematical model built in the chapter is used to figure out the quasisteady moving weld pool, Eq. (8.17) is solved to calculate the dynamic dilution
process of wire composition to the moving weld pool.
231
Figure 8.25 shows the calculated flow and heat transfer characteristics of quasisteady moving weld pool. From Fig. 8.25a, it can be seen that after the moving
weld pool reaches the quasi-steady state, the width of the moving weld pool is about
1.8 mm and the length of the weld pool is about 4.5 mm. The length–width ratio
is in good agreement with the length–width ratio, observed in general tests, of the
weld pool obtained by low-speed deep penetration laser welding; in addition, the
temperature distribution on the upper surface of the weld pool is close to the motion
Gaussian distribution, which is consistent with the usual welding experience. As can
be seen from the results of Fig. 8.25c–f, the absolute value of velocity near both
sides of the keyhole in the weld pool is large. In addition, the pressure gradient near
the front and the rear of the keyhole is large, and thus the value of velocity is also
large. Although it is very difficult to directly verify these theoretical results, the above
results are basically reasonable from practical experience. Figure 8.25g and h show
the results of the flow field inside the weld joint and inside the longitudinal section
of the weld pool, respectively. By comparing the simulation results based on the
coupling of the transient keyhole and the weld pool in Chap. 4, it can be seen that
the results of the flow field in the upper part of the moving weld pool are basically
consistent with the transient simulation results.
8.5.2.2 Dynamic Dilution Process of Wire Compositions
in Quasi-Steady Moving Weld Pool
According to the analysis results in Sect. 8.5.2.1, based on the velocity trend of the
fluid in the weld pool obtained from the quasi-steady-state model established in this
chapter, the results of the flow field in the upper part of the quasi-steady moving weld
pool are of practical significance. Therefore, in this section, the dynamic dilution
behavior of the wire compositions in the upper part of the quasi-steady moving weld
pool is mainly studied.
In order to study the dynamic dilution behavior of chemical compositions of the
wire in the upper part of the quasi-steady moving weld pool, in this section, the
wire alloy, Si element, Ti element and Al element are respectively used as research
objects, and their specific percentage content and characteristics are shown in Table
8.12 (where being diluted by parent metal indicates that the chemical composition of
an element in the wire is higher than that in the parent metal, so the wire is diluted by
parent metal; that the parent metal is diluted indicates that the chemical composition
of an element in the wire is lower than that in the parent metal, so the parent metal
is diluted).
According to the boundary conditions in Sect. 8.5.1, the wire initial boundary
conditions shown in Fig. 8.26a are adopted in the study: in the red zone, the concentration of the wire composition is set to be 1.0 (red zone), while that in other positions
is 0. After the mathematical model built in the chapter is used to figure out the quasisteady moving weld pool, Eq. (8.17) is solved to calculate the dynamic dilution
process of wire composition to the moving weld pool.