積層製造是一種新的生產工藝，而這種新的生產工藝為製造業帶來傳統工藝所無法達到的設計與製造的自由度，也大幅降低構造複雜零件的生產成本與時間，在各項金屬積層製技術中造目前以粉末床的選擇性雷射燒熔技術所製出產品能夠達到最佳的品質，但從過去的研究中發現選擇性雷射燒熔的金屬粉末加熱，溶解與冷卻的過程(稱謂熱循環)受到多個因子的影響。複雜的熱循環造成金屬積層製造的過程中積層與積層間產生缺陷與孔洞，並且產生複雜的金屬微結構，這些現象造成金屬積層製造的成品品質不穩定。NASA Marshall Space Flight Center(MSFC)進行了品質管理的相關研究，MSFC建議要確認質量上達到生產後就需要使用非破檢測來確認這些產品的缺陷是否會影響性能。傳統的非破檢測規範不能完全套用在積層製造上，主要原因是積層製造的缺陷種類與大小都與傳統製造不同。積層製造檢測方法中以X光顯微斷層掃描可偵測到最小的缺陷，但因費用與檢查所需的時間，和游離輻射讓這個方法不適合用在大量檢查或是產線監控。在既有的非破檢測方發中超音波具有彌補X光顯微斷層掃描不足的潛力，但是在過去的應用中因為傳統金屬製造中的缺陷多數集中在表面或連接處，並且缺陷大小多在數個mm以上這與金屬積層製造不同。金屬積層製造上小於mm的缺陷很常見並且缺陷散佈在成品的各處難以預測，這造成己有的超音波檢測方法上常在缺陷與微結構之間造成誤判。為了減少誤判並且增加超音波檢測的敏感度，了解金屬微結構所造成的散射統計分布可以更好的區分缺陷和背景區分散射。目前現有的研究金屬中粒徑(grain size)所造成的散射是呈現瑞利分佈(Rayleigh distribution)，但是金屬積層製造的微結構複雜在多篇研究都顯示在後處理前與晶粒的分佈一般鍛鐵不同，這篇研究採用Nakgami distribution與IB來調查實際上微結構所造成的散射分佈。在與熱壓鋼板做比較後發現透過熱處理後IB與SNR的數值會下降而整體的Nakagami-m 參數也會下降，但是仍然高於熱壓剛辦所製成的校正塊。這現象顯示Nakagami-m 與IB 的數值上升是與微結構的不均勻程度有關，而在Nakagami影像上小區域的m下降與缺陷有相關性，而較高的m值區域應該是由金屬微結構不均勻所造成。

Additive manufacture (AM) is a new manufacturing process, and this new process bring design and manufacture freedom unmatched by conventional manufacture process, at same time lowering the cost and time to manufacturing parts with complex geometry. Out of all metal additive manufacturing techniques, currently selective laser melting (SLM) produces part with highest quality, but passed researches show the heating ,melting and cooling (also known as heat cycle) of SLM is affect by many factors. The complex heat cycle during the manufacture creates defect and pores between layers, at same time the microstructure of the metal become very complicated, which cause the quality of the SLM product to be unstable.
NASA Marschall Space Flight Center (MFSC) has conduct several research on the quality management, and MFSC suggest that in order to confirm the SLM product comply with required quality the nondestructive testing (NDT) shall be used to verify the existing defect do not hinder the performance to the produced products. The type and size of defect exist in AM are different with conventional manufacture; therefore, existing NDT standards cannot be used for AM. In current NDT researches on AM, Micro-X ray computed tomography (X ray CT) can detect the smallest defect in all NDT technique, however, the cost, scan time and ionizing radiation make X ray CT unsuited for in-situ and mass production monitoring. In the existing NDT methods, ultrasound has the highest potential to make up area where X ray CT is lacking; however, in past application defect of conventional product concentrate at surface or connection area with a defect size of several mm. In AM sub mm defect are very common, and the defect distribute throughout the whole part, which increase the chance of false positive defect detection. Understand the distribution of the background scattering signal can increase the sensitivity and reduce the chance of false detection, but current research of metal background scattering signal all focused on grain size and indicating the distribution of grain size. Many researches show the microstructure for AM product is very different with wrought steel before post processing. This research attended to use Nakagami-m parameter and IB to investigate the scattering signal behavior of the metal microstructure. Comparing the result from SLM sample with result of calibration block, which was made hot rolling steel, shows the IB and SNR were decreasing after annealing heat treatment, and Nkagami-m was decreasing as well. However, the value of IB, SNR and Nakagami-m were still high than hot rolling steel, and this shows higher IB, SNR and Nakagami-m were correlate with non-uniformity of the microstructure. On Nakagami image a local small area of lower Nakagami-m is correlate with presents of a defect, and area with higher Nakagami-m value may suggest high degree of non-uniformity in the microstructure.

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