【作者】 张超；

【导师】 吴铭方；

【作者基本信息】 江苏科技大学 ， 材料科学与工程， 2023， 博士

【摘要】 铝合金具有密度小、比强度高、导电导热性好及耐腐蚀性能好等优点,以铝代钢的结构件可大幅度减轻重量和拥有较好的综合性能。为了响应全球提出的“节能减排,保护环境”的号召,汽车、船舶等运输工具生产以铝代钢实现轻量化必将成为发展趋势。然而,铝合金和钢因物理及化学性能的差异,在制备铝合金/钢结构件时存在界面易生成脆性Fe-Al金属间化合物和焊接接头残余应力大等问题,进而导致焊缝成型和焊接接头性能差。CMT熔钎焊技术因热输入量可控被认为是实现铝合金和钢连接的最有效焊接方法之一。但较小的热输入量使铝合金焊丝在钢表面润湿铺展性能不佳。因此,为了改善焊丝的润湿铺展性能和控制铝合金/钢界面脆性金属间化合物的生成,本文选用Al Si5和Al Si12药芯焊丝作为填充金属,在Q235B低碳钢表面分别镀铜和镀镍,进行6082铝合金和低碳钢、镀铜钢及镀镍钢的CMT熔钎焊工艺研究,分析不同药芯焊丝,不同镀层对铝合金在钢表面润湿铺展行为的影响,探讨Si、Cu、Ni合金元素作用下铝合金/钢CMT熔钎焊接头的宏观形貌、微观形貌、力学性能和耐腐蚀性能,揭示合金元素对铝合金/钢CMT熔钎焊接头界面反应区金属间化合物种类及厚度的控制机理,主要研究内容和结果如下:(1)采用Al Si5药芯焊丝和Al Si12药芯焊丝进行铝合金/钢CMT熔钎焊的润湿铺展试验研究,在CMT熔钎焊工艺参数不变的条件下,Al Si12药芯焊丝在低碳钢表面的润湿铺展面积略大于Al Si5药芯钎料。随后,选定Al Si12药芯焊丝作为填充金属,在钢表面镀铜层和镀镍层均能改善Al Si12药芯焊丝在钢表面的润湿铺展。这主要是因为镀铜层和镀镍层的表面张力大于钢的表面张力,加之Cu与Al和Ni与Al之间的互溶降低了熔融铝合金的界面张力,从而使焊丝与母材之间的接触角变小,润湿铺展性能提高。最后,增加CMT电弧的燃弧时间和提高焊接时的送丝速度均能提高Al Si12焊丝在钢表面的润湿铺展面积。(2)选用Al Si5和Al Si12药芯焊丝对铝合金和低碳钢进行CMT熔钎焊工艺研究。结果表明:Al Si12药芯焊丝作填充金属时铝合金/钢焊接接头的正反面成形更好。药芯焊丝中Si含量的增加,不改变铝合金/钢焊接接头组织构成,尤其是不改变界面金属间化合物的种类,但可以减小金属间化合物层厚度。铝合金/钢焊接接头钎焊界面区主要是由θ-Fe(Al,Si)3和τ5-Al7.2Fe1.8Si构成,熔焊区由Al-Si共晶组织和α-Al构成。Al Si12药芯焊丝所得铝合金/钢CMT熔钎焊接头的拉伸强度更大,达到140.29MPa。对Al Si5药芯焊丝所得焊接接头在300℃、0.5h的热处理工艺下,铝合金/钢CMT熔钎焊接头的抗拉强度提高到127.92MPa。Al Si5药芯焊丝和Al Si12药芯焊丝所得铝合金/钢CMT熔钎焊接头的耐腐蚀性能相差不大。(3)在钢表面镀铜,用Al Si12药芯焊丝CMT熔钎焊铝合金和钢。结果表明:适当的镀铜层,一方面在界面反应层阻碍Al、Fe之间反应,使接头界面反应层厚度减小;另一方面Cu参与Al-Fe-Si之间的反应,促进τ5-Al7.2(Fe,Cu)1.8Si相的生成,同时抑制脆性θ-Fe(Al,Si)3相的生成,改善了金属间化合物层韧性。钢表面镀10μm Cu层,铝合金/钢CMT熔钎焊接头的抗拉强度达到最大值178.7 MPa,接头的耐腐蚀性能最好。但镀铜层厚度过大,增加至20μm时,虽然接头界面反应层厚度最小,但接头中有脆性的Al2Cu相生成,熔钎焊接头的性能反而下降。(4)选用Al Si12药芯焊丝作为填充材料,对铝合金/镀镍钢进行CMT熔钎焊工艺研究。结果表明:镀镍层提高了Al Si12药芯焊丝在钢上的的润湿铺展性能,使铝合金/钢CMT熔钎焊接头成形良好。与钢表面未镀镍相比,钢表面镀镍使得铝合金/钢CMT接头熔焊区有Al3Ni生成,界面反应层由Fe Al3和θ-Fe(Si,Al)3转变成(Fe,Ni)Al3和(Fe,Ni)2Al3。钢表面镀镍提高了铝合金/钢CMT熔钎焊接头的抗拉强度,接头均断裂于铝合金处,镀镍层厚度为5μm时,接头抗拉强度达到最大值为202.5 MPa。钢表面镀镍提高了铝合金/钢CMT熔钎焊接头的耐腐蚀性能。这主要是因为一方面镀镍层阻碍了Al和Fe之间的化学冶金反应,抑制了脆性的Fe-Al金属间化合物生成,减小了金属间化合物层的厚度。另一方面Ni参与了Fe、Al之间的化学冶金反应,与Al反应生成Ni3Al相,与Fe、Al反应生成(Fe,Ni)2Al3相,提高了铝合金/钢接头的抗拉强度和耐腐蚀性能。(5)通过热力学计算,合金元素Si、Cu、Ni调控铝合金/钢界面组织构成,Si容易偏聚在铝合金和钢界面处,与Fe、Al之间发生化学冶金反应形成τ5-Al7.2Fe1.8Si和θ-Fe(Al,Si)3;钢表面的镀铜层参与了Fe、Al之间化学冶金反应,形成了τ5-Al7.2Fe1.8Si相、τ5-Al7.2(Fe,Cu)1.8Si和Al2Cu相;钢表面的镀镍层改变了铝合金/钢之间脆性金属间化合物的相组成,界面反应层化合物由Fe Al3和θ-Fe(Si,Al)3转变成(Fe,Ni)Al3、θ-Fe(Si,Al)3和(Fe,Ni)2Al3。另外,Si、Cu、Ni控制了铝合金/钢界面金属间化合物层厚度,最终调控铝合金/钢CMT熔钎焊接头力学性能。更多还原

【Abstract】 In response to the global call for“energy conservation,emission reduction and environmental protection”,Al alloy,as a substitution of parts of steels,can effectively achieve a structural lightweight in vehicles,ships and other means of transportation because of its low density,high specific strength,good conductivity and corrosion resistance.However,due to the great difference in physical and chemical properties between Al alloy and steel,in the connection process of aluminum alloy/steel structural components,the brittle Fe-Al intermetallic compounds formed at the interface of joints and high residual stress occurred in welded joints,which can lead to the poor formation and the low property of joints.CMT welding brazing technology is considered as one of the most effective welding methods to join aluminum alloys and steel because of its controllable heat input.However,the smaller heat input results in poor wetting and spreading performance of aluminum alloy on the steel surface.Therefore,in order to improve the wetting and spreading performance of Al alloy and control the generation of brittle intermetallic compounds at the aluminum alloy/steel interface,the CMT process of 6082 aluminum alloy and Q235B low carbon steel has been conducted by using Al Si5 and Al Si12 flux-cored wire as the filler metals.Meanwhile,the surface of steel was coated by Ni layer and Cu layer.The influence of different kinds of Al-Si filler metals and different coatings on the wetting and spreading behavior of molten aluminum alloy on the steel surface was analyzed.The macro morphology,micro morphology,mechanical properties and corrosion resistance of Al alloy/steel CMT joints under the action of different alloying elements were discussed.The controlling mechanism of alloying elements on the type and thickness of IMCs in the interface reaction zone of Al alloy/steel CMT welding-brazing joint has been revealed.The main research contents and results are as follows:（1）Al Si5 flux-cored wires and Al Si12 flux-cored wires were first used to study the wetting and spreading of CMT welding-brazing of Al alloy/steel.Under the condition of constant CMT welding-brazing process parameters,the wetting and spreading area of Al Si12 flux-cored wire on the surface of low carbon steel was slightly larger than that of Al Si5 flux-cored filler metal.Subsequently,Al Si12 flux-cored wire was selected as filler metal.Both the Cu layer and the Ni layer on the steel surface can improve the wetting and spreading of Al Si12 flux-cored wire on the steel surface.This is mainly because the surface tension of the Cu coating and the Ni coating is greater than the surface tension of the low-carbon steel,and the mutual solubility between Cu and Al and the mutual solubility between Ni and Al reduces the surface tension of the molten aluminum alloy,thereby reducing the contact angle between the welding wire and the base metal and improving the wetting and spreading properties.Finally,increasing the arcing time or the wire feeding speed during welding can increase the wetting spreading area of Al Si12 flux-cored wire on the steel surface.（2）The CMT welding-brazing process of Al alloy and low carbon steel was studied by using Al Si5 and Al Si12 flux-cored welding wires.The results showed that the formation of Al alloy/steel CMT welding-brazing joint with the Al Si12 flux-cored welding wire was better.With the increase of Si content in the welding wire,the microstructure of the Al alloy/steel welding joint was not changed,especially the type of intermetallic compounds at the interface was not changed,but the thickness of the intermetallic compound layer can be reduced.Al alloy/steel welding brazing joint was composed ofθ-Fe（Al,Si）₃andτ₅-Al_7.2Fe_1.8Si at the brazing interface zone and consisted of Al-Si eutectic structure and Al solid solution at the fusion zone.The tensile strength of Al alloy/steel CMT welding-brazing joint obtained by Al Si12 flux-cored welding wire was greater and reached up to the value of 140.29MPa.After heat treatment process of 300℃and 0.5h,the tensile strength of Al alloy/steel CMT welding brazing joint with Al Si5 flux-cored wire increased to127.92MPa.The corrosion resistance of Al alloy/steel joints obtained by the two welding wires were similar.（3）The CMT brazing welding process of Al alloy and Cu-coated steel was studied by using Al Si12 flux cored wire.The results showed that the appropriate Cu could effectively inhibit the reaction of Al and Fe and reduce the thickness of interfacial brittle intermetallic compound（IMCs）.And Cu could react with Al,Fe and Si to promote the formation ofτ₅-Al_7.2（Fe,Cu）_1.8Si phase and control the formation of the brittleθ-Fe（Al,Si）₃phase,with improved the toughness of IMCs.When the thickness of Cu coating on the surface of steel was 10μm,the tensile strength of Al/steel CMT welding-brazing joint reached the maximum value of 178.7 MPa and the corrosion resistance of the joint reached the optimal.However,the thickness of Cu coating increased to 20μm,the IMCs thickness at the interface of the joint was up to the minimum,but Cu and Al reacted with each other to form brittle Al₂Cu phase under the higher Cu content,which deteriorated the tensile strength and corrosion resistance of joints.This work will provide experimental and theoretical basis for developing excellent properties of Al/Cu-coating/steel welding joints.（4）The cold metal transfer（CMT）welding-brazing process was chosen to join Al alloy and Ni-coated steel using Al Si12 as the filler wire.The results showed that Ni coating could improve the wettability and spread ability of molten Al Si12 filler metal on the steel surface,resulting in a good appearance for the Al alloy/steel joint.Ni coating could hinder the chemical metallurgical reaction between Al atom and Fe atoms to inhibit the formation of brittle Fe-Al intermetallic compounds（IMCs）and reduce the thickness of the IMCs layer.Meanwhile,the Ni atom reacted with the Fe and Al atoms to form Al₃Ni₂,（Fe,Ni）Al₃and（Fe,Ni）₂Al₃,which improved the tensile strength of the joints.All joints with Ni coating cracked near the Al alloy.When the Ni-coating thickness was 5μm,the tensile strength of the joint reached a maximum of 202.5 MPa.The addition of Ni could also improve the corrosion resistance of the joints.Significantly,when the Ni-coating thickness was 10μm,most of the Ni coating was still solid,and the interface reaction layer was mainly composed ofα-Ni solid solution and some（Fe,Ni）₂Al₃.（5）Based on thermodynamic calculations,elements Si,Cu and Ni regulate and control the microstructure composition at the interface of Al alloy and steel,and element Si is prone to segregate at the interface,and take part in a chemical metallurgical reaction with Fe and Al,to formτ₅-Al_7.2Fe_1.8Si andθ-Fe（Al,Si）_3.Cu plating layer on the surface of the steel participates in the chemical metallurgical reaction between Fe and Al to formingτ₅-Al_7.2Fe_1.8Si、τ₅-Al_7.2（Fe,Cu）_1.8Si and Al₂Cu.Ni plating layer on the surface of steel changes the phase composition of brittle intermetallic compounds and IMCs was composed from Fe Al₃andθ-Fe（Si,Al）₃intoθ-Fe（Si,Al）_3、（Fe,Ni）Al₃and（Fe,Ni）₂Al_3.In addition,elements Si,Cu,and Ni control the thickness of the intermetallic compound layer,and control the mechanical properties of the Al alloy/steel CMT welding-brazing joints.更多还原