Research On The Forming Processes And Properties In Selective Laser Melting Of N-type ZrNiSn Thermoelectric Materials

Thermoelectric materials can realize the direct reversible conversion between thermal and electrical energy via the Seebeck and Peltier effects.Thermoelectric energy conversion shows many advantages,such as no moving components,high reliability,and noiseless and pollution-free operation.ZrNiSn and other half-Heusler alloys are among the best thermoelectric materials for the mid-to-high temperature range operation.Moreover,their superior mechanical properties and thermal stability are also advantageous for commercial applications in the field of thermoelectric power generation.The traditional preparation methods of ZrNiSn include arc melting,induction melting,and solid-state reactions,all of which require long time post annealing to obtain the desired homogenous composition and microstructure.Therefore,these fabrication methods are time consuming,waste a lot of energy,and are not well suited for large-scale industrial production.In addition,traditional manufacturing of thermoelectric modules includes multi-step processes,such as slicing,surface metallization,dicing,and assembling and soldering that results in a low material utilization rate and high cost of thermoelectric modules.Hence,the fabrication technology of thermoelectric modules that relies on simple processes,low energy consumption,and high material utilization is critically important for making ZrNiSn-based thermoelectric devices economically viable.Selective Laser Melting?SLM?,as a new additive manufacturing technology,uses a laser beam to melt a single layer powder bed that rapidly solidifies once the laser moves away.A layer upon a layer,a three-dimensional object can be formed using highly automated processing steps and achieving high utilization of raw materials as mechanical cutting is avoided.The SLM processing technology is being widely used in aviation,automotive and other industrial manufacturing fields.If this technology could be applied in the synthesis of TE materials,fabrication of n-and p-type TE legs,and the assembly of modules,the entire manufacturing of TE modules would be fundamentally transformed,significant cost savings achieved,and greatly expanded large-scale applications of thermoelectricity would materialize.The thermoelectric conversion technology would then play an increasingly important role in relieving energy shortages and mitigating environmental pollution.In this study,the traditional ZrNiSn thermoelectric material was used as the research object,the powder prepared by the ultra-fast and low-cost laser self-propagating combustion synthesis technology developed in our laboratory,and the n-type ZrNiSn samples was successfully printed on the Ti substrate by combining the SLM technology.The phase,structure and properties of the bulk materials were characterized,and the interface layer between ZrNiSn bulk and Ti was characterized.The main work of this paper is as follows:The powder materials with different size and morphology were prepared by various grinding methods and printed by single window.The more suitable grinding powder was selected as raw material for SLM processing.By adjusting laser power,laser scanning rate,laser scanning distance and so on,a more suitable process window was prepared.When the thickness of pawnshop powder is 30?m,the laser power P is16-20 W,the scanning rate v is 80-120 mm/s,and the scanning distance H is 50-80?m,we can prepare a better shaping surface.When the coating thickness is increased to 50?m,the laser power P is 16-22 W,the scanning rate v is 60-100 mm/s,and the scanning distance H is 50-80?m,we can obtain a better shaping surface.We selected a better surface with a thickness of 30?m for chemical phase analysis and found that there was a small amount of phase separation.After laser treatment,Zr,Sn and ZrNi₂Sn will be slightly separated into ZrNiSn,and then a small amount of Zr will react with oxygen in vacuum cavity to form ZrO₂,which will flow to the bulk surface during SLM process.However,according to the volatilization rate of the elements,we found that Sn has a small volatilization rate,and the SLM process is an extremely fast preparation process,that is,the melting time is very short,there is no element missing in the SLM process.We selected laser processing parameters such as P=18 W,v=80 mm/s,H=50?m,d=30?m)to fabricate ZrNiSn blocks with 1.7 mm thick on homogeneous ZrNiSn 1.7 mm thick substrate,and to characterize the properties of the blocks.We select different metal materials as heterogeneous substrate after screening,and try to print ZrNiSn on heterogeneous substrate.It is found that only Ti substrate can be well combined with ZrNiSn substrate by laser.Then,we increase the thickness of powder coating by 50?m,and select laser processing parameters such as?P=18 W,v=80 mm/s,H=50?m,d=30?m?to prepare non-oriented blocks with thickness of about 1.4 mm on ZrNiSn homogeneous substrate and Ti heterostructure substrate.Compared with the bulk material prepared by SPS,the bulk has higher carrier concentration,lower Seebeck coefficient and lower thermal conductivity.The maximum ZT value of the bulk prepared by SLM is only 0.39,which is 59%of the prepared sample.At the same time,we also characterize the ZrNiSn-Ti heterojunction formed in the SLM process of ZrNiSn and Ti substrate.The heterojunction is Ti rich,and the thickness is about 5?m.After annealing at 500?for 24 h,the structure and composition of the heterojunction have no obvious change.At the same time,the contact resistance of the heterojunction is characterized by the contact resistivity of27.25 m?cm²,which is close to the contact resistance of the thermoelectrical device used in industry.