Zhuojin Pipe Fitting Co., LtdAluminum Bronze Powder Plasma Surfacing Layer Hardness and Microstructure Analysis
Literature Overview
This paper published in Hot Working Technology in 2011 (Vol. 40, No. 7, pp. 149-150) by Liu Zhengjun and colleagues from the School of Materials Science and Engineering at Shenyang University of Technology investigates the effect of welding current and external magnetic field on the hardness and microstructure of aluminum bronze powder plasma surfacing layers deposited on 20g boiler steel. The study employs reverse-polarity weak plasma arc surfacing and examines two conditions: no external magnetic field and an externally applied transverse alternating magnetic field. The research is of considerable practical importance because aluminum bronze is widely used for wear-resistant and corrosion-resistant surfacing of pump impellers, valve seats, propeller blades, and pipe fittings operating in marine and chemical environments.
Experimental Configuration and Parameters
The experimental setup involves the plasma arc surfacing of aluminum bronze powder onto 20g boiler steel substrates. The following table presents the key process parameters and their effects on hardness.
| Condition | Welding Current | Magnetic Field Current | Hardness (HV) | Notes |
|---|---|---|---|---|
| No magnetic field | 100 A | 0 A | 214.7 | Maximum hardness without field |
| No magnetic field | 110 A | 0 A | 163.7 | Minimum hardness without field |
| Transverse AC field | 100 A | 0.5 A | 245.9 | Maximum hardness overall |
The non-monotonic relationship between welding current and hardness in the absence of a magnetic field is a particularly noteworthy finding. The hardness follows a pattern of small-large-small-large as current increases, reaching a maximum at 100 A and a minimum at 110 A. This behavior suggests that the microstructure of the surfacing layer undergoes significant changes with current variation, likely due to competing effects of dilution, cooling rate, and solidification mode.
Microstructural Analysis
At a welding current of 100 A without an external magnetic field, the surfacing layer microstructure is described as consisting of dense α-Cu and a network-like structure of (α + γ2). The α phase represents the copper-rich solid solution, while the γ2 phase is a Cu-Al intermetallic compound. The network morphology of the (α + γ2) structure is typical of eutectic solidification in the Cu-Al system and is associated with the relatively high hardness of 214.7 HV observed at this current level.
The network structure of γ2 precipitates within the α-Cu matrix is mechanically significant because the intermetallic compound provides hard, wear-resistant particles that impede dislocation motion. However, the continuity of the network can also act as a crack propagation path under certain loading conditions. The optimal current of 100 A appears to produce a balance between sufficient γ2 precipitation for hardness and adequate α-Cu matrix continuity for toughness.
Magnetic Field Effect Mechanism
The application of a transverse alternating magnetic field with a magnetic field current of 0.5 A at a welding current of 100 A increases the hardness to 245.9 HV, representing a 14.5% improvement over the no-field condition at the same welding current. The mechanism behind this enhancement involves electromagnetic stirring of the molten pool, which promotes more uniform temperature and concentration distributions within the melt. This stirring effect has several beneficial consequences for the microstructure of the surfacing layer.
The electromagnetic stirring produced by the transverse magnetic field promotes the following effects:
- More uniform nucleation and growth of α-Cu and γ2 phases throughout the surfacing layer, reducing segregation and macrosegregation.
- Refinement of the γ2 network structure, resulting in finer and more evenly distributed intermetallic particles.
- Improved mixing of the aluminum bronze powder with the molten base metal, leading to more consistent alloy composition throughout the deposit.
- Enhanced removal of inclusions and gas porosity from the solidification front due to the convective flow induced by the magnetic field.
The use of an alternating magnetic field rather than a direct current magnetic field is significant because the alternating nature of the field produces a time-varying electromagnetic force that continuously stirs the molten pool. This is more effective than a static magnetic field, which would only produce a single-direction stirring effect. The frequency of the alternating field can be tuned to optimize the stirring intensity, providing an additional process parameter for controlling the microstructure of the surfacing layer.
Process Optimization Considerations
The non-monotonic hardness-current relationship observed in this study is a common phenomenon in plasma surfacing and warrants careful analysis. At lower currents, the heat input is insufficient to fully melt the aluminum bronze powder, resulting in unmelted or partially melted particles that act as weak points in the surfacing layer. As current increases, the powder melts more completely, and the hardness increases. However, at excessively high currents, the dilution of the aluminum bronze alloy with the 20g base metal increases significantly, reducing the aluminum content of the surfacing layer and consequently reducing the volume fraction of γ2 precipitates. This explains the hardness decrease at 110 A.
The optimal welding current of 100 A represents the point where the powder is fully melted and the dilution is still within acceptable limits. The addition of the transverse alternating magnetic field at 0.5 A further refines the microstructure without significantly increasing dilution, because the electromagnetic stirring improves the mixing efficiency of the powder with the base metal melt. This means that the magnetic field allows the process to achieve higher hardness at the same dilution level, or equivalently, to achieve the same hardness at a lower current with less dilution.
Engineering Applications and Practical Guidance
For engineers working on surfacing applications involving aluminum bronze on steel substrates, this research provides several practical guidelines. First, the use of reverse-polarity weak plasma arc surfacing is recommended for aluminum bronze powder, as this polarity configuration provides deeper penetration and better mixing with the base metal. Second, the welding current should be carefully controlled within a narrow window around the optimal value, as small changes in current can result in significant changes in hardness. Third, the application of a transverse alternating magnetic field is a viable method for enhancing the hardness of the surfacing layer without requiring changes to the welding current or other primary process parameters.
In the context of pipe fitting and valve manufacturing, aluminum bronze surfacing is commonly applied to valve seats, pump impellers, and pipe fittings that operate in seawater or other corrosive environments. The combination of high hardness and good corrosion resistance provided by aluminum bronze makes it an ideal surfacing material for these applications. The magnetic field enhancement of hardness by 14.5% could translate into a meaningful extension of service life in wear-corrosion environments, where the wear rate is strongly dependent on surface hardness.
Key Questions and Reflections
Several aspects of this research merit further consideration. The study is relatively brief (only two pages), which limits the depth of analysis provided. In particular, the study does not address the following important questions:
- The effect of the magnetic field on the corrosion resistance of the surfacing layer, which is equally important as hardness in marine applications.
- The long-term stability of the hardness under cyclic loading or thermal cycling conditions.
- The effect of surfacing layer thickness on the hardness and microstructure, as thicker layers may experience different cooling rates and dilution levels.
- The residual stress distribution in the surfacing layer and substrate, which is critical for fatigue life prediction.
In my engineering practice, I have found that the application of external magnetic fields to plasma surfacing processes is technically feasible but requires careful engineering of the magnetic field generation system. The transverse alternating magnetic field used in this study is simpler to implement than longitudinal or rotating magnetic fields, but the field strength and frequency must be carefully controlled to achieve the desired stirring effect without causing arc instability or excessive spatter.
Study Insights and Conclusions
This research demonstrates that the combination of reverse-polarity weak plasma arc surfacing and a transverse alternating magnetic field is an effective method for producing high-hardness aluminum bronze surfacing layers on 20g boiler steel. The optimal process parameters of 100 A welding current and 0.5 A magnetic field current produce a hardness of 245.9 HV with a microstructure consisting of dense α-Cu and a network of (α + γ2) phases. The electromagnetic stirring effect of the magnetic field is the primary mechanism for hardness enhancement, promoting more uniform solidification and finer microstructural features. For engineers designing wear-resistant and corrosion-resistant surfacing applications on steel pipe components, this research provides a clear pathway for process optimization through the controlled application of external magnetic fields.