ABSTRACT
There are multiple routes to prepare semi-solid slurries with a globular microstructure for semi-solid forming. The variations in the microstructure of semi-solid slurries prepared using different routes may lead to significant differences in the flow behavior and mechanical properties of rheo-diecasting parts. Therefore, it is crucial to have a comprehensive understanding of the microstructure evolution associated with different slurry preparation routes and their resulting effects. In this study, the gas-induced semi-solid process (GISS) and the swirl enthalpy equilibrium device (SEED) routes were employed to prepare semi-solid Al-Si-Mg slurries for their simplicity and productivity in potential industrial applications. The prepared slurries were then injected into the shoot sleeves of a high-pressure die casting (HPDC) machine to produce tensile test bars. Subsequently, the bars underwent T6 treatment to enhance their mechanical properties. The microstructure, segregation, and mechanical properties of the samples were investigated and compared with those of conventional HPDC. The results indicated that the GISS and SEED can produce semi-solid slurries containing a spherical α-Al primary phase, as opposed to the dendritic structure commonly found in conventional castings. The liquid fraction had a significant effect on the flow behavior, resulting in variations in liquid segregation and mechanical properties. It was observed that a higher solid fraction (>75%) had a suppressing effect on surface liquid segregation. In addition, the tendency for liquid segregation gradually increased along the filling direction due to the special flow behavior of the semi-solid slurry with a low solid fraction. Furthermore, under the same die-casting process parameters, the conventional HPDC samples exhibit higher yield stress (139 ± 3 MPa) compared to SEED-HPDC and GISS-HPDC samples, which may be attributed to the small grain size and the distribution of eutectic phases. After undergoing the T6 treatment, both SEED-HPDC and GISS-HPDC samples showed a significant improvement in yield and tensile strength. These improvements are a result of solution and precipitation strengthening effects as well as the spheroidization of the eutectic Si phase. Moreover, the heat-treated SEED-HPDC samples demonstrate higher ultimate strength (336 ± 5 MPa) and elongation (13.7 ± 0.3%) in comparison to the GISS-HPDC samples (307 ± 4 MPa, 8.8 ± 0.2%) after heat treatment, mainly due to their low porosity density. These findings suggest that both GISS-HPDC and SEED-HPDC processes can be utilized to produce parts with favorable mechanical properties by implementing appropriate heat treatments. However, further investigation is required to control the porosities of GISS-HPDC samples during heat treatment.
ABSTRACT
OBJECTIVES: To investigate the differences in non-suicidal self-injury (NSSI) behaviors between only-child and non-only-child adolescents with mood disorders. METHODS: A three-stage sampling method was used to perform a cross-sectional survey of 529 adolescents, aged 12-18 years, who had mood disorders and NSSI behaviors. These adolescents were sampled from the outpatient service of 20 mental hospitals in 9 provinces of China from August to November 2020. A self-made questionnaire was used to collect general demographic data. The Functional Assessment of Self-Mutilation, Beck Scale for Suicide Ideation, Kessler Psychological Distress Scale, Stress Mindset Measure-General, Multidimensional Scale of Perceived Social Support, Multidimensional Students' Life Satisfaction Scales, and Rosenberg Self-Esteem Scale were used to collect the information on self-injury behaviors and psychological factors in these adolescents. RESULTS: A total of 529 adolescents with mood disorders and NSSI behaviors were surveyed, among whom 375 were only-child adolescents and 154 were non-only-child adolescents. Compared with the non-only-child group, the only-child group had a significantly higher total score of Functional Assessment of Self-Mutilation (P<0.05) .The type and frequency of self-injury in the only-child group were significantly higher than those in the non-only-child group (P<0.05). Psychological analysis showed that compared with the non-only-child group, the only-child group had a significantly lower score of self-esteem (P<0.05) and significantly higher scores of psychological distress and depressive symptoms (P<0.05). The multiple linear regression analysis showed that the score of suicidal ideation was positively correlated with the frequency of NSSI behaviors in both only-child and non-only-child adolescents with mood disorders (P<0.05); in the only-child adolescents, the level of self-esteem was negatively correlated with the frequency of NSSI behaviors (P<0.05), and the score of stress perception was positively correlated with the frequency of NSSI behaviors (P<0.05); in the non-only-child adolescents, the score of anxious emotion was positively correlated with the frequency of NSSI behaviors (P<0.05). CONCLUSIONS: Among the adolescents with mood disorders and NSSI behaviors, the only-child adolescents tend to have a higher frequency of self-injury and poorer mental health, and therefore, the only-child adolescents with mood disorders and NSSI behaviors need more attention.
